- Email: [email protected]
rill and gully erosion to soil loss from a small cultivated catchment in Sicily
Soil & Tillage Research 135 (2014) 18–27
Contents lists available at ScienceDirect
Soil & Tillage Research journal homepage: www.elsevier.com/locate/still
Using 137Cs and 210Pbex measurements and conventional surveys to investigate the relative contributions of interrill/rill and gully erosion to soil loss from a small cultivated catchment in Sicily Paolo Porto a,b,*, Desmond E. Walling a, Antonina Capra b a
Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, UK Dipartimento di Agraria, Universita` degli Studi Mediterranea di Reggio Calabria, Agro-Forest and Environmental Sciences and Technologies, Contrada Feo di Vito, Reggio Calabria 89122, Italy
b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 March 2013 Received in revised form 21 August 2013 Accepted 31 August 2013
In many cultivated areas in semiarid Mediterranean regions, soil erosion is responsible for problems related to both on-site and off-site impacts, including reduced crop productivity, water quality, and degradation of freshwater ecosystems. In some areas of Sicily, where intense short duration rainfall events are common, soil erosion is a very serious problem, especially on land subjected to continuous tillage operations. The rates of soil loss in these areas and their impact differ according to the dominant type of erosion. Several existing studies have focused on the impacts of either linear (gully- or ephemeral gully-erosion) or interrill–rill erosion, but to date the relative magnitude of these two different types of erosion, has rarely been assessed. This paper reports the results of a study aimed at comparing the relative contribution of interrill–rill erosion and gully erosion to soil loss from a small cultivated catchment located in Sicily (Italy). Surveys of ephemeral gullies (EG) in the study catchment carried out at the event scale since 1999 are used to quantify soil loss attributable to EG erosion. 137Cs and 210Pbex measurements are used to quantify the net soil loss from the catchment attributable to interrill–rill (IRR) erosion. The study demonstrates that EG formation occurred 7 years out of 10, with a mean soil loss averaged over a 10-year period equal to 26.5 t ha1 yr1. The rates of IRR erosion estimated using 137Cs and 210Pbex measurements provided values of mean annual net soil loss of 38.8 t ha1 yr1 and 34.2 t ha1 yr1, respectively. The resulting ratios of soil loss attributable to EG to total soil loss (IRR + EG) were 0.41 and 0.44 for the 137Cs and 210Pbex measurements, respectively. The results suggest that the contributions of EG and IRR erosion are of a similar magnitude in semiarid regions of Sicily, although the precise value of the ratio is likely to vary both spatially and temporally in response to catchment morphology, soil erodibility and land use and inter-annual variability of rainfall magnitude and erosivity. The findings are consistent with those of other studies that have attempted to compare the relative efficacy of the two erosion types. The use of 137Cs and 210Pbex measurements in the study area provided important insights into the relative importance of IRR and EG erosion and the same approach could be employed in other locations where both forms of erosion occur and there is a need to quantify their relative importance. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Cs ;210Pbex Interrill and rill erosion Ephemeral gully erosion Sediment redistribution Sicily 137
1. Introduction In recent decades, land degradation and soil erosion have been increasingly recognised as a serious environmental problem in semiarid Mediterranean regions. Recent studies carried out in southern Italy have documented rates of soil erosion ranging from 10–85 t ha1 yr1, on cultivated land (Porto and Walling, 2012a,b)
* Corresponding author. Tel.: +39096557481. E-mail addresses: [email protected], [email protected] (P. Porto), [email protected] (D.E. Walling), [email protected] (A. Capra). 0167-1987/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.still.2013.08.013
and from 100–150 t ha1 yr1, in areas covered by forests (Porto et al., 2011). These high erosion rates reflect both the nature of the environment and the long-term impact of human activity. The rainfall regime of Mediterranean areas, which is characterised by events of extremely short duration and very high intensity followed by long dry periods, is particularly conducive to erosion. Human activity further increases the erosion risk through, for example, creating bare surfaces on cultivated land after tillage operations and land abandonment in marginal and improductive areas. In Sicily, soil loss from cultivated lands reflects the general situation in Southern Italy, but rates of soil loss differ according to erosion type and land degradation processes. Erosion due to
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
concentrated flow is very severe on many unprotected farm fields and the presence of various gully types can be observed in many areas of the region. The initiation and development of channels routinely obliterated by tillage and other farm operations, commonly referred to as ephemeral gullies (EG), constitutes a severe problem (Capra, 2013). Growing crops can be removed by scour as these small gullies develop, the crops at the lower end of the gully can be buried by the sediment discharged from the ephemeral gully and deposited in an alluvial fan. Furthermore, filling operations reduce the long-term productivity of the farmland. Although the importance of EG erosion is well recognised, even at the local scale (Capra and Scicolone, 2002; Capra et al., 2005) little research has focused on this erosion type and most soil erosion prediction studies have relied on generalised empirical models (Capra et al., 2005; Capra et al., 2009a,b; Di Stefano et al., 2013). In contrast, IRR erosion rates are better understood in Sicily and over the last 50 years many different approaches have been employed to predict IRR erosion, in order to assess soil erosion risk and develop effective strategies to control erosion and sedimentation in these areas. These different approaches are based on different types of models that range from empirical-parametric approaches such as SEDD (Ferro, 1997; Ferro and Porto, 2000), through conceptual models, which correlate soil loss to physical parameters depending on soil erodibility and land use (Novara et al., 2011), topography (Bagarello et al., 2011) or rainfall erosivity (Agnese et al., 2006), to recent physically-based models, such as WEPP (Nearing et al., 1989), which aim to simulate both the detachment and transport of soil particles (Amore et al., 2004). The results provided by these studies demonstrate that IRR erosion is also an important problem in Sicily, although there is a need for further calibration and validation of the models employed for local conditions, in order to increase confidence in their output. The use of fallout radionuclides and more particularly caesium137 (137Cs) and excess lead-210 (210Pbex) to document rates of soil and sediment redistribution in the landscape has attracted increasing attention in recent years (Mabit et al., 2008; Ritchie and Ritchie, 2007; Walling, 2010; Zapata, 2002) and the approach is now being successfully employed in Mediterranean areas (e.g. Benmansour et al., 2013; Estrany et al., 2010; Gaspar et al., 2013a,b; Navas et al., 2013; Porto et al., 2006, 2013). This approach is able to overcome several of the limitations associated with more traditional methods of documenting erosion and soil redistribution. Of particular importance is its potential to provide retrospective information on medium-term average rates of soil redistribution on the basis of a single site visit and representative distributed data for fields and larger areas, without the need to disturb the system by installing measuring equipment. For some applications, particularly those requiring spatially distributed information on soil redistribution rates, fallout radionuclides arguably provide an essentially unique means of assembling data that cannot be obtained using alternative approaches. By virtue of their different half-lives and fallout origins, 137Cs and 210 Pbex provide information relating to different periods of time. 137 Cs measurements are primarily used to generate information on mean annual erosion rates over the past ca. 50 years and 210 Pbex measurements are able to provide information relating
19
to a longer period of up to ca. 100 years (Walling and He, 1999a,b). The study reported here aims to quantify EG and IRR erosion rates in a small cultivated catchment located in Sicily (Italy), to compare their relative contribution to the total soil loss from the catchment and thereby provide important information on soil erosion rates in the study region. An empirical approach, based on field measurements was used to quantify the erosion rates associated with EG erosion, while the rates IRR erosion were estimated using 137Cs and 210Pbex measurements. To the authors’ knowledge this is the first attempt to undertake such a comparison in Italy. 2. The study area The study area (Fig. 1) comprises a small 0.86 ha catchment located in Sicily, Italy. This catchment is part of a larger drainage area (80 ha in size) belonging to a national network of experimental catchments for erosion studies. The data-set of EG measurements in the study area extends over 18 years (from 1995 to 2013). The study catchment is characterised by an altitudinal range of 325 to 355 m above sea level, a mean slope of 28% and a single main drainage line with a NW–SE orientation (Capra and Scicolone, 2005) (see Table 1). The catchment is a tributary (W-side) of the middle reaches of the Simeto river, and is developed on the oldest allochthonous units of the Apennines– Maghrebian Chain (Imerese Unit, Upper Trias–Middle Miocene), which underlie the syn- and post-orogenic units (Burdigalian and lower Tortonian) (Longhitano and Colella, 2007). The dominant soil type is Vertic Xerocrept, which is very common in Sicily. Particle size analysis carried out for 10 soil samples collected within the catchment, indicated the presence of silty clay loam and silt loam textural classes (see Table 1 for details) with negligible stone content. The catchment has been cultivated since the early 1950s. The main crop is durum wheat that requires two tillage operations in summer or early autumn with a cultivator and one ploughing operation carried out every few years (generally a minimum of 3 years). The climate of the study area is typically Mediterranean with a mild wet winter and a warm dry summer. The mean annual rainfall for the period 1971–2007 is ca. 500 mm, with a coefficient of variation of ca. 40%. More than 80% of the rainfall occurs during the period extending from October to May. An active ephemeral gully (EG) develops along the main swale in the centre of the catchment during the rainy season in most years (Fig. 1). As is usually the case for cultivated land, the EG is obliterated by infilling with soil from areas immediately adjacent to the channel, using ordinary tillage equipment. However, the EG frequently recurs in the same place during the next rainy season. The extent and development of the EG system in the catchment has been documented since 1995, but measurements at the event level did not commence until 1999 (Capra and Scicolone, 2002, 2005). Until 1998, rainfall data were collected using an autographic rain gauge equipped with a chart recorder, located about 600 m away from the study area. In 1999, a digital recording rain gauge was installed in the basin. A comparison of the rainfall measured by both gauges over a period of 4 months showed good agreement between the two gauges (Capra et al., 2009b).
Table 1 Characteristics of the study catchment. Drainage area (ha)
Min altitude (m asl)
Max altitude (m asl)
Mean slope (%)
Sand (%)
Silt (%)
Clay (%)
Organic matter (%)
Bulk density (g cm3)
0.86
325
355
28
4–15
62–79
17–28
0.98–1.13
1.14–1.17
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
20
Fig. 1. The study area and the study catchment, showing the ephemeral gully system that developed in 2008.
3. The measurement programme 3.1. The ephemeral gully system The measurement programme for the ephemeral gully system involved field surveys undertaken after each erosive event. When the time interval between two erosive events was too short to visit the site, a single measurement subsumed both events. The surveys involved surveying the main branch of the EG (see Fig. 1) and its tributaries (if present). A post processing differential GPS with a planimetric accuracy of 12 cm (Capra and Scicolone, 2002) was used to establish the spatial co-ordinates of points located along the channel at about 5 m intervals in the longitudinal direction. Cross sections were measured at about every 5 m of channel, or whenever a change in the EG cross section or the entry of tributaries was observed. As it was possible to treat the cross section of the EG as a trapezium (or a rectangle) (Capra et al., 2011), the channel widths (upper and lower) and depths were measured with a steel tape graduated every 5 mm. All the measures were made by the same expert operator. The length of the EG was computed from the co-ordinates of the survey points. The volumes of material removed by erosion to create each channel segment were calculated using the end area method (i.e. the product of the mean area of two successive cross sections and the distance between them). The total volume of soil eroded from the EG was calculated as: V¼
n n X X Ai1 þ Ai Li Vi ¼ 2 i¼1 i¼1
(1)
where V is the total volume of soil eroded from the EG (m3); n is the number of segments; Vi is the volume of eroded soil from each
segment (m3); Ai1 is the downstream cross sectional area of the segment (m2); Ai is the upstream cross sectional area of the segment (m2); and Li is the distance between adjacent cross sections (m). The data relating to the volume of soil removed from the gulley system were converted to values of mass using representative values of in situ bulk density for the material removed. 3.2. Soil sampling of
137
Cs and
210
Pbex
In order to use 137Cs and 210Pbex measurement to estimate rates of interrill–rill erosion within the study catchment, two separate soil sampling programmes were undertaken. The first aimed to establish the magnitude and spatial distribution of soil redistribution rates within the catchment and involved two sampling campaigns. During the first campaign, undertaken in 2009, replicate bulk soil cores were collected at 30 sites, using an 11 cm diameter steel core tube inserted to depth of 45 cm. Deeper cores were collected from sites where there was the possibility of deposition. These soil cores, which were collected at the intersections of an approximate 20 m 20 m grid, were supplemented by a further 52 bulk cores collected in the same way from sites selected to improve the coverage of topographic variability, during a second sampling campaign in 2010 (see Fig. 1). In all cases the sampling points were selected to avoid the zone occupied by the EG and the zone from which soil was moved for infilling the gully, and are therefore seen as being representative of the soil redistribution occurring on the slopes of the catchment beyond the EG. The second sampling programme, undertaken in 2010, aimed to obtain information on the local reference inventory and the depth distribution of 137Cs and 210Pbex both at the reference site and in the cultivated soil profile of the
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
catchment. Since it was not possible to identify a site that was both undisturbed and unaffected by soil redistribution within the study catchment, the samples used to establish the reference inventory were collected from an area of permanent pasture with minimal slope adjacent to the study catchment. In this case, eight separate cores were collected from an area of ca. 25 m2 using an 11 cm diameter steel core tube inserted to depth of 60 cm, in order to take account of micro-scale variability in the reference inventory (cf. Owens and Walling, 1996). Each core was sectioned using the same depth increments, which ranged from 1 to 4 cm, and the individual depth increments from the eight cores were bulked. Bulking was undertaken to reduce the mass of material that needed to be transported to the laboratory in the UK, where the samples were assayed for 137Cs and 210Pbex, and because of limitations on the total number of samples that could be assayed. Additional sectioned cores were also obtained from two sampling sites within the catchment selected to be representative of an eroding and a depositional site (see Fig. 1), using the same procedure as employed at the reference site. 3.3. Laboratory analyses for
137
Cs and
210
Pbex
All bulk core and depth incremental samples collected from the catchment and from the reference area were oven dried at 105 8C for 48 h, disaggregated and dry sieved to separate the <2 mm fraction. A representative sub-sample of this fraction was packed into a 330 cm3 cylindrical plastic pot for determination of its 137Cs and 210Pbex activity by gamma spectroscopy in the radiometry laboratory of the Department of Geography at the University of Exeter. The samples were sealed for 21 days prior to assay, in order to achieve equilibrium between 226Ra and its daughter 214Pb. Activities of both 137Cs and 210Pb in the soil and sediment samples were measured simultaneously by gamma-ray spectrometry, using a high-resolution low energy LOAX coaxial HPGe detector (relative efficiency 30%) coupled to an amplifier and PC-based data collection system. Count times were typically ca. 90000 s, providing results with an analytical precision of 10% at the 95% level of confidence. Detection limits for 137Cs and 210Pb were ca. 0.5 and 5.0 Bq kg1, respectively. The efficiency of the detection system was calibrated using standard samples prepared by adding known amounts of certified 137Cs, 210Pb and multi-element standards to a soil/sediment matrix representative of the samples to be analysed. The 137Cs activities in the samples were obtained from the counts at 662 keV. The total 210Pb activity of the samples was measured at 46.5 keV, and the 226Ra activity was obtained by measuring the activity of 214Pb, a short-lived daughter of 226Ra, at 351.9 keV. No self absorption correction was applied to the 210Pb measurements, since the detector was calibrated with representative soil/sediment standards. The in
21
226 situ Ra-supported 210Pb concentration, associated with individual soil and sediment samples, was derived from the measured 226Ra concentration. In most terrestrial environments, the supported 210Pb will not be in equilibrium with the 226Ra, since some 222Rn will diffuse upwards through the soil or rock and escape to the overlying atmosphere. This loss is commonly accounted using a reduction factor based on the average ratio of the measured total 210Pb and 226Ra concentrations for samples collected from the lower part of the soil profile, where fallout 210 Pb or 210Pbex can be assumed to be absent (cf. Graustein and Turekian, 1986; Wallbrink and Murray, 1996). A value of 0.8 was obtained for the study site. Excess 210Pb concentrations associated with the samples were calculated by subtracting the 226Ra-supported 210Pb concentrations from the total 210Pb concentrations (cf. Joshi, 1987).
4. Results 4.1. Soil loss from the ephemeral gully system The event-based monitoring of the ephemeral gully system spanned 9 years and extended from August 1999 to November 2008 (see Table 2 for details). As observed in many other studies, EG formation and expansion commonly occurs as a result of only a very limited number of precipitation events during a given year (e.g. Casalı´ et al., 1999, 2008). In the study area, the mean number of rainy days per year is about 50, but EG formation and development generally occurs during only a single erosive event. In the study reported, the impact of 13 erosive events responsible for EG formation and development were documented and these represent effectively all of the erosive events resulting in EG development that occurred during the period covered by the detailed EG surveys (1999–2008). The rainfall totals associated with the different erosive events ranged from a minimum of 17.7 mm to a maximum of 193.4 mm, with a mean of 66.6 mm. The minimum EG erosion was associated with the events of 29/8/1999 and 3/9/1999, when the soil loss from the catchment was only 0.03 t ha1 and the maximum was associated with the last event that occurred in November 2008, when a soil loss of 82.1 t ha1 was observed. Based on the 18 years of observation of EG development in the study catchment, EG formation occurred in 7 years out of 10, with a frequency corresponding to 70% of the years covered by the survey. The cumulative soil loss for the period covered by the detailed EG surveys that commenced in 1999 was calculated, in order to quantify the total EG erosion for the period. The mean annual soil loss from the catchment from the EG, for the seven years when the EG was active, was 37.9 t ha1 yr1. Averaging the results over the nine years, including the years during which no EG erosion
Table 2 Characteristics of the erosive events responsible for EG formation and development during the study period (H = precipitation depth; Imax = maximum 30 min intensity for the precipitation event; R = R-factor; A = mean EG cross-sectional area; D = mean EG depth). Date
H (mm)
Imax (mm h1)
R (MJ mm1 ha1 h1)
A (m2)
D (m)
EG erosion (t ha1)
29/08/1999 03/09/1999 07/09/1999 09/09–13/11/1999 28/11–1/12/99 12–14/1/2000 15/10/2003 03/03/2005 22/10/2005 25/12/2006 3–4/11/2007 13/11/2008 28/11/2008
24.6 17.7 34.9 80.5 193.4 75.3 104.2 65.6 60.4 82.2 51.6 34.0 41.0
20.0 13.2 20.9 20.1 19.9 12.3 43.0 8.6 60.4 16.0 33.2 9.6 14.4
111.7 46.2 157.1 302.4 663.6 376.8 1130.5 536.4 1701.2 876.0 3372.2 410.8 1539.5
0.01 0.01 0.02 0.16 0.03 0.37 0.18 0.11 0.05 0.16 0.10 0.04 0.18
0.06 0.05 0.07 0.47 0.06 0.35 0.54 0.17 0.28 0.29 0.24 0.15 0.31
0.03 0.03 5.34 18.97 4.35 16.19 21.47 10.82 15.86 67.15 19.38 5.56 82.98
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
22
occurred, the mean annual soil loss from the catchment associated with the development of the EG was ca. 26.5 t ha1 yr1. 4.2. 137Cs and 210Pbex inventories and depth distributions at the reference site The estimates of the 137Cs and 210Pbex reference inventories for the study area are based on assay of the composited sections from eight cores. These are seen as providing representative values for the local reference inventories, although their precision or uncertainty is not explicitly quantified, due to the bulking of the samples. The uncertainty introduced by measurement precision (i.e. ca. 10%) can be expected to be considerably reduced, relative to that associated with an individual measurement of a bulk core, because the value represents the sum of the values of areal activity density obtained for the individual slices and positive and negative precision errors associated with those values are likely to cancel out to some degree. Equally, the averaging of the results from 8 cores through the use of composite samples means that the uncertainty associated with micro-scale, and sampling variability as well as local variability of the reference inventory will also be significantly reduced. Based on previous experience in the wider region, an uncertainty of 10% at the 95% level of confidence has been assumed for the estimates of the 137Cs and 210Pbex reference inventories obtained (Porto et al., 2011). The representative depth distributions of 137Cs and 210Pbex documented for the reference site based on the composited slices from the eight cores are presented in Fig. 2. These are typical of an undisturbed site (Walling and Quine, 1992; Porto et al., 2001, 2003), with a well defined exponential reduction in activity with depth and with 90% of the total inventory existing in the top
Fig. 2. The depth distribution of
137
15–20 cm. Values of 432 and 2800 Bq m2 were obtained for the 137 Cs and 210Pbex reference inventories. The reference inventory values obtained for both 137Cs and 210 Pbex must be seen as relatively low when compared with corresponding inventories found at other sites in southern Italy (see Porto et al., 2001, 2006). However, they can be accounted for by the lower mean annual rainfall (ca. 500 mm) in this area. Similarly low values for the 137Cs reference inventory have also been documented for another area of Sicily, where the mean annual rainfall is ca. 700 mm (Di Stefano et al., 2000). In this case, a reference value of 944 Bq m2 was reported (Di Stefano et al., 1999). Correction of this value to the same year as the measurements made in the current study provides a value of 717 Bq m2, and this can be seen as consistent with the value reported for the current study site. 4.3.
137
Cs and 210Pbex inventories on the slopes of the study catchment
The values of 137Cs inventory associated with the 82 sampling points in the study catchment ranged from 0.25 to 1192 Bq m2, with a mean value of 255 Bq m2 (see Table 3 for details). In the case of 210Pbex, the equivalent inventory values obtained for the same sampling points ranged from 0.6 to 14443 Bq m2, with a mean value of 2285 Bq m2. Taking account of the 10% uncertainty associated with the reference inventories indicated above, comparison of the inventory values for the individual sampling points with the reference values indicated that 67% of the 137Cs inventories were significantly lower than the reference value, indicating erosion, and 18% were significantly greater, indicating deposition. The results indicated also that 15% of the 137Cs inventory values were not significantly different
Cs (a) and
210
Pbex (b) at the reference site.
Table 3 The range of 137Cs and 210Pbex inventories associated with the sampling points in the study catchment and the number of points showing evidence of erosion, deposition or stable conditions.
137 210
Cs Pbex
Min (Bq m2)
Max (Bq m2)
Mean (Bq m2)
SD (Bq m2)
Reference value (Bq m2)
Nerod
Ndep
Nstable
0.25 0.6
1192 14443
255 2285
294 2646
432 2800
55 62
15 16
12 4
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
Fig. 3. The
137
Cs (a) and
210
Pbex (b) depth distributions documented for a representative eroding site within the study catchment.
from the reference value, indicating that the sampling points were essentially stable, experiencing neither erosion nor deposition. Similar results were obtained for 210Pbex inventories, where 76% of the measured values provide evidence of erosion, 19% indicated deposition and 5% experienced neither erosion nor deposition. It is clear that considerable soil redistribution has occurred within the study basin since the commencement of 137Cs fallout in the mid 1950s, but erosion has dominated soil redistribution within the catchment. In the case of 210Pbex, the period reflected by the reduced and increased inventories is less easy to define, since, unlike 137Cs, the fallout is essentially continuous from year to year. However, although 210 Pbex inventories may be sensitive to erosion occurring during the past ca. 100 years, they will be particularly sensitive to soil
Fig. 4. The
137
Cs (a) and
210
23
redistribution occurring in the past 20 years, due to the relatively short half-life of 210Pb (22 years). Fig. 3 presents depth profiles of the two radionuclides typical of an eroding site. The total inventories of 349 Bq m2 for 137Cs and 1823 Bq m2 for 210Pbex, are considerably lower than the reference inventories. This sampling site is located in the upper part of the catchment where erosion processes are expected to dominate (see Fig. 1). The depth profiles shown in Fig. 4 are characterised by inventory values of 4219 Bq m2 for 137Cs and 7792 Bq m2 for 210 Pbex, which are considerably higher than the reference values. These are typical of a depositional site. In this case, the sectioned cores were collected in the middle part of the catchment close to the confluence of the main EG and one of its tributaries (see Fig. 1).
Pbex (b) depth distributions documented for a representative depositional site within the study catchment.
24
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
4.4. Using 137Cs and 210Pbex to estimate soil redistribution by interrill– rill erosion on the catchment slopes Estimation of rates of erosion and deposition from 137Cs and Pbex measurements is generally based on the degree of reduction or increase of the measured inventory, relative to the local reference inventory. For cultivated soils, the calibration relationship (or conversion model) required to convert the magnitude of the reduction in the radionuclide inventory to an estimate of the rate of soil loss commonly employs a mass balance model (e.g. Kachanoski and de Jong, 1984; Walling and He, 1999a,b). Such models are based on the assumption that a sampling point with a total radionuclide inventory A (Bq m2) less than the local reference inventory Aref (Bq m2) represents an eroding site, whereas a point with a total radionuclide inventory greater than the local reference inventory is assumed to be a depositional site. Following Walling and He (1999a,b), the activity of accumulated 210Pbex or 137Cs A(t) (Bq m2) per unit area with time t (yr) at an eroding site can be represented as: 210
Zt ðPR=DþlÞ dt 0
t0
AðtÞ ¼ Aðt 0 Þe
þ
Zt
0
ð1 G ÞIðt 0 ÞeðPR=DþlÞðtt Þ dt 0
(2)
t0
where R is the erosion rate (kg m2 yr1); D is the cumulative mass depth representing the average plough depth (kg m2); l is the decay constant for 137Cs or 210Pbex (yr1); I(t) is the annual 137Cs or 210 Pbex deposition flux (Bq m2 yr1); G is the percentage of the freshly deposited 137Cs or 210Pbex fallout removed by erosion before being mixed into the plough layer; P is the particle size correction factor; t0 (yr) is the year when cultivation started; A(t0) (Bq m2) = 210Pbex or 137Cs inventory at t0; A(t) is greater than the local reference inventory Aref at a sampling point, deposition may be assumed. In this case, the mean soil deposition rate R0 can be calculated from the following equation: Rt 0
R ¼
PP 0
Rt
t0
t0
dt0
R
0
R0 C d ðt 0 Þelðtt Þ dt0
R
S
RdS
0 R=H Þ=R þ Aðt 0 Þ=DÞ dS S ðIðt Þg ð1 e
(3)
where H is the relaxation mass depth of the initial depth distribution of the fallout input. This represents the depth to which the fresh fallout input penetrates the soil. Assuming that the depth distribution is exponential, H is defined as the mass depth (kg m2) at which the radionuclide concentration reduces to 1/e of the surface concentration (see He and Walling, 1997). Cd(t0 ) reflects the radionuclide content of sediment mobilised from all the eroding areas that converge on the aggrading point. Generally, Cd(t0 ) can be assumed to be represented by the weighted mean 137Cs or 210Pbex activity of the sediment mobilised from the upslope contributing area S (m2); P0 is a further particle size correction factor reflecting differences in grain size composition between mobilised and deposited sediment; g is the proportion of the annual fallout susceptible to be removed by erosion prior to incorporation into the soil profile by tillage. 4.5. Soil redistribution rates on the catchment slopes The conversion model described above was used to derive estimates of erosion and deposition rates for the sampling points within the study catchment that showed inventories significantly different from the reference inventory (67 and 78, respectively for 137 Cs and 210Pbex). A computer-based routine which converts the percentage loss or gain in the 137Cs or 210Pbex inventory, relative to the local
reference value, to an equivalent rate of soil loss or deposition was used to solve Eqs. (2) and (3), and to estimate the erosion or deposition rates associated with the individual sampling points within the study catchment. An average plough depth D of 200 (kg m2) was selected as being representative of cultivation in the catchment, and an average value of 4 (kg m2) was used to describe the relaxation mass depth H of the initial fallout input. A value of 1 was assumed both for g, based on the relationship between the timing of cultivation and the rainfall regime, and for the particle size correction factor P, based on the lack of an appreciable difference between the grain size composition of the soil and of samples of transported sediment collected during the study period. The magnitude and spatial variability of the erosion and deposition rates estimated for the individual sampling points within the study catchment are presented in Fig. 5a, for 137Cs and in Fig. 5b for 210Pbex. 5. Discussion 5.1. Interrill/rill erosion on the slopes of the catchment The results presented in Fig. 5 emphasise that the slopes of the study catchment are characterised by appreciable rates of soil redistribution, with a clear dominance of eroding sites. Estimates of the gross erosion rate (t ha1 yr1) for the catchment associated with the 82 sampling points identified in Fig. 5 have been derived as the product of the mean erosion rate for the points indicated by the 137Cs or 210Pbex measurements to be characterised by erosion and the proportion of the catchment subject to erosion, as represented by the proportion of the sampled points that documented erosion. The same approach was applied to estimate the total deposition on the slopes of the catchment that was derived as the product of the mean deposition rate for the sampled points in the catchment demonstrating deposition and the proportion of the catchment subject to deposition. Subtraction of the total deposition within the catchment from the gross erosion provides an estimate of the net erosion, which is here interpreted to represent the sediment delivered to the channel system. Based on the 82 sampling points, the gross erosion from the slopes was estimated to be 49 and 60 t ha1 yr1, based on the 137 Cs and 210Pbex measurements, respectively. The net soil loss from the slopes of the study catchment estimated from the 137Cs and 210Pbex measurements was 38.8 and 34.2 t ha1 yr1, respectively. These values provide a sediment delivery ratio of ca. 79% and 57% for 137Cs and 210Pbex, respectively. Net soil loss rates of this magnitude, although very high when compared to those documented for other regions of Italy, are relatively common in Sicily. For example, Bagarello et al. (2010) working on experimental plots ranging in length from 11 to 44 m, documented values of mean soil loss ranging from ca. 22 to 57 t ha1 yr1 in a similar location. There are, however, a number of important differences between the estimates of soil redistribution rate provided by the 137Cs and 210 Pbex measurements. The first is that the estimates of gross erosion rate provided by the 210Pbex measurements are higher than those provided by the 137Cs measurements. Secondly, the net erosion rate estimated from the 137Cs measurements is slightly higher than that obtained from the 210Pbex measurements. This latter contrast reflects the higher estimates of deposition rates provided by the 210Pbex measurements and the associated reduction in the sediment delivery ratio. Although it is difficult to provide a definitive explanation for the different values of gross and net erosion provided by the two radionuclides, it is important to recognise their different temporal sensitivities. In the case of 137 Cs, the measurements provide a time-integrated estimate of soil redistribution rates for the period extending from the
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
Fig. 5. The soil redistribution rates estimated from the measurements of
commencement of 137Cs fallout in the mid 1950s to the present. Fallout was minimal during much of this period (i.e. since the mid 1970s) and the 137Cs inventories primarily reflect the longer-term influence of soil redistribution processes on a relatively short-term input of fallout in the late 1950s and 1960s, which has remained in the soil. In contrast, the annual fallout of 210Pbex can be seen as essentially constant and, in view of its shorter half life (22.3 yr), current inventories are likely to be more sensitive to erosion and soil redistribution occurring during the past 20 years. Where erosion and deposition rates derived using the 210Pbex measurements are higher than those obtained using 137Cs measurements, this could reflect increased erosion in recent years, due, for example, to an increased incidence of high magnitude events during this shorter time window, than during the longer timewindow reflected by the 137Cs measurements. This was the case for the areas in Calabria (southern Italy) investigated by Porto et al. (2009), Porto and Walling (2012a,b). However, D’Asaro et al. (2007) investigated trends in annual rainfall erosivity in Sicily and found that in general the rainfall erosivity factor R (Wischmeier and Smith, 1978) has not shown a significant increase in Sicily during the twentieth century. A further difference relates to the spatial patterns of soil redistribution demonstrated by the two radionuclides (see Fig. 5). This is not unexpected considering the different temporal sensitivity of the two radionuclides noted above and both this and the reduced sediment delivery ratio associated with the 210Pbex measurements may reflect progressive changes in soil properties caused by the high rates of soil loss from the field. A gross erosion rate of 50–60 t ha1 year equates to an average surface lowering of ca. 20–25 cm over the 50 year period. The rate of surface lowering in the main eroding areas within the field would be significantly greater. This progressive loss of surface soil caused by erosion and the associated incorporation into the plough horizon of soil from beneath the original plough depth, characterised by different texture, can be expected to have caused changes in the texture and organic matter content of the surface
137
Cs (a) and
25
210
Pbex (b) inventory.
soil. Such changes are likely to be reflected by changes in both the magnitude and spatial distribution of runoff and erosion. 5.2. The relative efficacy of interrill/rill erosion and gully erosion The estimates of net soil loss provided by the 137Cs and 210Pbex measurements can be compared with those obtained for the EG. The different time windows represented by the different sets of measurements necessarily introduce some uncertainty, but the comparison is, nevertheless, seen as meaningful. Combining the 137 Cs and 210Pbex estimates (IRR) with the EG measurements reported above, provides estimates of the total annual soil loss (TSL = EG + IRR) of 65.3 t ha1 yr1, and 60.7 t ha1 yr1 and of the ratio EG/TSL equal to 0.41 and 0.44 based on the estimates if IRR derived from the 137Cs and 210Pbex measurements, respectively. The estimate of soil loss provided by the 210Pbex measurements is arguably more representative of the 9 year period 1999–2008 covered by the gully surveys than that provided by the 137Cs measurements, which relates more clearly to the period extending from the mid 1950s to the time of sampling. Therefore, the estimates of TSL of 60.7 t ha1 yr1 and of the ratio EG/TSL of 0.44 could be seen as more reliable. These results suggest that in the study catchment the contributions of EG and IRR erosion to the total net soil loss are of similar magnitude. However, the magnitude of the EG/TSL ratio can be expected to vary both through time, in response to changing land use and hydrometeorological conditions, and between different catchments in response to variations in catchment morphology, soil properties and land use. Further work is clearly required to establish if the value for the EG/TSL ratio obtained for the study catchment is representative of the local region in Sicily. However, studies in other parts of Europe have reported similar results. For example, Casalı´ et al. (1999), investigated some actively eroding areas in Navarra (Spain), and reported that EG typically contributes about 30% to the total soil loss, although it can reach as high as 100%.
26
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
Vandaele et al. (1996) working in central Belgium document values of the EG/TSL ratio ranging from 0.29 to 0.7. Cerdan et al. (2002), following two extreme rainfall events that occurred in Normandy (France), calculated that the EG/TSL ratios for the two events were 0.21 and 0.56. Looking more generally, Poesen et al. (2003) recognised that the rates of soil loss rates associated with different kinds of gully could vary considerably, resulting in gully contributions to the total sediment export from a catchment ranging from 10% to up 94% (i.e. a G/TSL ratio of 0.1–0.94). 6. Conclusions Prediction of soil erosion is an important requirement for managing land degradation processes in semiarid Mediterranean areas. However, most prediction techniques require calibration and/or validation, if they are to produce reliable results. The study reported here has provided empirical confirmation of the magnitude of both EG and IRR erosion from a small cultivated catchment located in Sicily. The results obtained have permitted the relative contribution of EG and IRR erosion to the total sediment output from the catchment to be quantified and have shown that the two erosion processes are of similar importance. The latter finding is consistent with results of other studies that have attempted to establish the relative importance of EG and IRR erosion and emphasises that both forms of erosion need to be considered when planning catchment scale soil conservation and sediment control measures within the study region. The study also demonstrates the potential for using 137Cs and 210 Pbex measurements to obtain information on medium-term soil erosion and soil redistribution rates within small catchments. By virtue of their different half-lives and fallout origins, 137Cs and 210 Pbex can provide information on land degradation processes relating to different time windows. In this study the information on IRR erosion rates generated by the 137Cs and 210Pbex measurements was combined with the results of a more traditional measurement programme aimed at assessing soil loss associated with EG erosion to assess the relative importance of the two erosion types to the total soil loss from the study catchment. As such 137Cs and 210Pbex measurements should be seen as potentially providing an important complement to more traditional measurements, rather than an alternative. Acknowledgements The study reported in this paper was supported by grants from MIUR PRIN 2010–2011, and the IAEA (Technical Contract 15478). The assistance of Sue Rouillard in producing the figures and of Jim Grapes in undertaking the gamma spectrometry measurements are gratefully acknowledged. References Agnese, C., Bagarello, V., Corrao, C., D’Agostino, L., D’Asaro, F., 2006. Influence of the rainfall measurement interval on the erosivity determinations in the Mediterranean area. Journal of Hydrology 329, 39–48. Amore, E., Modica, C., Nearing, M.A., Santoro, V.C., 2004. Scale effect in USLE and WEPP application for soil erosion computation from three Sicilian basins. Journal of Hydrology 293, 100–114. Bagarello, V., Ferro, V., Giordano, G., 2010. Misura dell’erodibilita` del suolo nelle parcelle sperimentali di Sparacia, in Sicilia. In: XXXII Convegno Nazionale di Idraulica e Costruzioni Idrauliche. Palermo, (in Italian), pp. 1–10. Bagarello, V., Di Stefano, C., Ferro, V., Kinnel, P.I.A., Pampalone, V., Porto, P., Todisco, F., 2011. Predicting soil loss on moderate slopes using an empirical model for sediment concentration. Journal of hydrology 400, 267–273. Benmansour, M., Mabit, L., Nouira, A., Moussadek, R., Bouksirate, H., Duchemin, M., Benkdad, A., 2013. Assessment of soil erosion and deposition rates in a Moroccan agricultural field using fallout 137Cs and 210Pbex. Journal of Environmental Radioactivity 115, 97–106. Capra, A., Scicolone, B., 2002. Ephemeral gully erosion in a wheat-cultivated area in Sicily (Italy). Biosystems Engineering 83 (1) 119–126.
Capra, A., Scicolone, B., 2005. Monitoraggio dell’erosione ephemeral gully in un piccolo bacino siciliano. In: Atti del Convegno dell’Associazione Italiana di Ingegneria Agraria (AIIA) 2005. Catania cod. 6010, (in Italian), pp. 1–12. Capra, A., Mazzara, L.M., Scicolone, B., 2005. Application of the EGEM model to predict ephemeral gully erosion in Sicily (Italy). Catena 59, 133–146. Capra, A., Di Stefano, C., Ferro, V., Scicolone, B., 2009. Similarity between morphological characteristics of rills and ephemeral gullies in Sicily, Italy. Hydrological Processes 23, 3334–3341. Capra, A., Porto, P., Scicolone, B., 2009. Relationships between rainfall characteristics and ephemeral gully erosion in a cultivated catchment in Sicily (Italy). Soil & Tillage Research 105, 77–87. Capra, A., Di Stefano, C., Ferro, V., Scicolone, B., 2011. Morphological characteristics of ephemeral gullies in Sicily, South Italy. Landform Analysis 17, 27–32. Capra, A., 2013. Ephemeral gully and gully erosion in cultivated land: a review. In: Lannon, E.C. (Ed.), Drainage Basins and Catchment Management: Classification, Modelling and Environmental Assessment. Nova Science Publishers, Hauppauge, NY. Casalı´, J., Gastesi, R., A´lvarez-Mozos, J., De Santisteban, L.M., Lersundi, J., Del Valle de ˇ aga, M., Gon ˇ i, U., Agirre, M.A., Campo, J.J., Lersundi, R., Gime´nez, A., Larran Lo`pez, M., Done´zar, M., 2008. Runoff, erosion, and water quality of agricultural watersheds in central Navarra (Spain). Agricultural Water Management 95, 1111–1128. Casalı´, J., Lo´pez, J.J., Gira´ldez, J.V., 1999. Ephemeral gully erosion in Southern Navarra (Spain). Catena 36, 65–84. Cerdan, O., Le Bissonnais, Y., Couturier, A., Bourennane, H., Souche`re, V., 2002. Rill erosion on cultivated hill slopes during two extreme rainfall events in Normandy, France. Soil & Tillage Research 67, 99–108. D’Asaro, F., D’Agostino, L., Bagarello, V., 2007. Assessing changes in rainfall erosivity in Sicily during the twentieth century. Hydrological Processes 21, 2862–2871. Di Stefano, C., Ferro, V., Porto, P., 1999. Linking sediment yield and caesium-137 spatial distribution at basin scale. Journal of Agricultural Engineering Research 74 (1) 41–62. Di Stefano, C., Ferro, V., Porto, P., 2000. Length slope factors for applying the revised universal soil loss equation at basin scale in southern Italy. Journal of Agricultural Engineering Research 75 (4) 349–364. Di Stefano, C., Ferro, V., Pampalone, V., Sanzone, F., 2013. Field investigation of rill and ephemeral gully erosion in the Sparacia experimental area, South Italy. Catena 101, 226–234. Estrany, J., Garcia, C., Walling, D.E., 2010. An investigation of soil erosion and redistribution in a Mediterranean lowland agricultural catchment using caesium-137. International Journal of Sediment Research 25, 1–16. Ferro, V., 1997. Further remarks on a distributed approach to sediment delivery. Hydrological Sciences Journal 42 (5) 633–648. Ferro, V., Porto, P., 2000. Sediment Delivery Distributed (SEDD) Model. Journal of Hydrologic Engineering 5 (4) 411–422. Gaspar, L., Navas, A., Walling, D.E., Machı´n, J., Go´mez Arozamena, J., 2013. Using 137 Cs and 210Pbex to assess soil redistribution on slopes at different temporal scales. Catena 102, 46–54. Gaspar, L., Navas, A., Machı´n, J., Walling, D.E., 2013. Using 210Pbex measurements to quantify soil redistribution along two complex toposequences in Mediterranean agroecosystems, northern Spain. Soil & Tillage Research 130, 81–90. Graustein, W.C., Turekian, K.K., 1986. 210Pb and 137Cs in air and soils measure the rate and vertical profile of aerosol scavenging. Journal of Geophysical Research 91 (D13) 14355–14366. He, Q., Walling, D.E., 1997. The distribution of fallout 137Cs and 210Pb in undisturbed and cultivated soils. Applied Radiation and Isotopes 48, 677–690. Joshi, S.R., 1987. Nondestructive determination of Lead-210 and Radium-226 in sediments by direct photon analysis. Journal of Radioanalytical and Nuclear Chemistry 116, 169–212. Kachanoski, R.J., de Jong, E., 1984. Predicting the temporal relationship between soil cesium-137 and erosion rate. Journal of Environmental Quality 13, 301–304. Longhitano, S., Colella, A., 2007. Geomorphology, sedimentology and recent evolution of the anthropogenically modified Simeto River delta system (eastern Sicily, Italy). Sedimentary Geology 194, 195–221. Mabit, L., Benmansour, M., Walling, D.E., 2008. Comparative advantages and limitations of the fallout radionuclides 137Cs, 210Pbex and 7Be for assessing soil erosion and sedimentation. Journal of Environmental Radioactivity 99, 1799–1807. Nearing, M.A., Foster, G.R., Lane, L.J., Finkner, S.C., 1989. A process based soil erosion model for USDA-water erosion prediction project technology. Transactions of the ASAE 32 (5) 1587–1593. Navas, A., Lo´pez-Vicente, M., Gaspar, L., Machı´n, J., 2013. Assessing soil redistribution in a complex karst catchment using fallout 137Cs and GIS. Geomorphology 196, 231–241. Novara, A., Gristina, L., Saladino, S.S., Santoro, A.C., Cerda`, A., 2011. Soil erosion assessment on tillage and alternative soil managements in a Sicilian vineyard. Soil & Tillage Research 117, 140–147. Owens, P.N., Walling, D.E., 1996. Spatial variability of caesium-137 inventories at reference sites: an example from two contrasting sites in England and Zimbabwe. Applied Radiation and Isotopes 47, 699–707. Poesen, J., Nachtergaele, J., Verstraeten, G., Valentin, C., 2003. Gully erosion and environmental change: importance and research needs. Catena 50 (2– 4) 91–133. Porto, P., Walling, D.E., 2012a. Validating the use of 137Cs and 210Pbex measurements to estimate rates of soil loss from cultivated land in southern Italy. Journal of Environmental Radioactivity 106, 47–57.
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27 Porto, P., Walling, D.E., 2012b. Using plot experiments to test the validity of mass balance models employed to estimate soil redistribution rates from 137Cs and 210Pbex measurements. Applied Radiation and Isotopes 70, 2451–2459. Porto, P., Walling, D.E., Callegari, G., 2011. Using 137Cs measurements to establish catchment sediment budgets and explore scale effects. Hydrological Processes 25, 886–900. Porto, P., Walling, D.E., Callegari, G., 2013. Using 137Cs and 210Pbex measurements to investigate the sediment budget of a small forested catchment in southern Italy. Hydrological Processes 27, 795–806. Porto, P., Walling, D.E., Callegari, G., Capra, A., 2009. Using caesium-137 and unsupported lead-210 measurements to explore the relationship between sediment mobilisation, sediment delivery and sediment yield for a Calabrian catchment. Marine and Freshwater Research 60, 680–689. Porto, P., Walling, D.E., Callegari, G., Catona, F., 2006. Using fallout lead-210 measurements to estimate soil erosion in three small catchments in southern Italy. Water, Air, & Soil Pollution: Focus 6, 657–667. Porto, P., Walling, D.E., Ferro, V., 2001. Validating the use of caesium-137 measurements to estimate soil erosion rates in a small drainage basin in Calabria, southern Italy. Journal of Hydrology 248, 93–108. Porto, P., Walling, D.E., Ferro, V., Di Stefano, C., 2003. Validating erosion rate estimates by caesium-137 measurements for two small forested catchments in Calabria, Southern Italy. Land Degradation and Development 14, 389–408.
27
Ritchie, J.C., Ritchie, C.A., 2007. Bibliography of publications of 137Cs studies related to erosion and sediment deposition. Agricultural Research Service. US Dept. Agriculture, Washington, DC. Vandaele, K., Poesen, J., Marques de Silva, J.R., Desmet, P., 1996. Rates and predictability of ephemeral gully erosion in two contrasting environments. Ge´omorphologie: Relief, Processes Environment 2 (2) 83–95. Wallbrink, P.J., Murray, A.S., 1996. Determining soil loss using the inventory ratio of excess lead-210 to cesium-137. Soil Science Society of America Journal 60, 1201–1208. Walling, D.E., 2010. Using fallout radionuclides to investigate erosion and sediment delivery: some recent advances. In: IAHS Publ. 337. IAHS Press, Wallingford, pp. 3–16. Walling, D.E., He, Q., 1999a. Improved models for estimating soil erosion rates from cesium-137 measurements. Journal of Environmental Quality 28 (2) 611–622. Walling, D.E., He, Q., 1999b. Using fallout lead-210 measurements to estimate soil erosion on cultivated land. Soil Science Society of America Journal 63, 1404–1412. Walling, D.E., Quine, T.A., 1992. The use of caesium-137 measurements in soil erosion surveys. In: IAHS Publ 210. IAHS Press, Wallingford, pp. 143–152. Wischmeier, W.H., Smith, D.D., 1978. Predicting rainfall erosion losses: a guide to conservation planning. In: Agricultural Research Service Handbook 537. US Dept. Agriculture, Washington, DC. Zapata, F. (Ed.), 2002. Handbook for the Assessment of Soil Erosion and Sedimentation Using Environmental Radionuclides. Kluwer, Dordrecht, Netherland, 219 pp.
Contents lists available at ScienceDirect
Soil & Tillage Research journal homepage: www.elsevier.com/locate/still
Using 137Cs and 210Pbex measurements and conventional surveys to investigate the relative contributions of interrill/rill and gully erosion to soil loss from a small cultivated catchment in Sicily Paolo Porto a,b,*, Desmond E. Walling a, Antonina Capra b a
Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, UK Dipartimento di Agraria, Universita` degli Studi Mediterranea di Reggio Calabria, Agro-Forest and Environmental Sciences and Technologies, Contrada Feo di Vito, Reggio Calabria 89122, Italy
b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 March 2013 Received in revised form 21 August 2013 Accepted 31 August 2013
In many cultivated areas in semiarid Mediterranean regions, soil erosion is responsible for problems related to both on-site and off-site impacts, including reduced crop productivity, water quality, and degradation of freshwater ecosystems. In some areas of Sicily, where intense short duration rainfall events are common, soil erosion is a very serious problem, especially on land subjected to continuous tillage operations. The rates of soil loss in these areas and their impact differ according to the dominant type of erosion. Several existing studies have focused on the impacts of either linear (gully- or ephemeral gully-erosion) or interrill–rill erosion, but to date the relative magnitude of these two different types of erosion, has rarely been assessed. This paper reports the results of a study aimed at comparing the relative contribution of interrill–rill erosion and gully erosion to soil loss from a small cultivated catchment located in Sicily (Italy). Surveys of ephemeral gullies (EG) in the study catchment carried out at the event scale since 1999 are used to quantify soil loss attributable to EG erosion. 137Cs and 210Pbex measurements are used to quantify the net soil loss from the catchment attributable to interrill–rill (IRR) erosion. The study demonstrates that EG formation occurred 7 years out of 10, with a mean soil loss averaged over a 10-year period equal to 26.5 t ha1 yr1. The rates of IRR erosion estimated using 137Cs and 210Pbex measurements provided values of mean annual net soil loss of 38.8 t ha1 yr1 and 34.2 t ha1 yr1, respectively. The resulting ratios of soil loss attributable to EG to total soil loss (IRR + EG) were 0.41 and 0.44 for the 137Cs and 210Pbex measurements, respectively. The results suggest that the contributions of EG and IRR erosion are of a similar magnitude in semiarid regions of Sicily, although the precise value of the ratio is likely to vary both spatially and temporally in response to catchment morphology, soil erodibility and land use and inter-annual variability of rainfall magnitude and erosivity. The findings are consistent with those of other studies that have attempted to compare the relative efficacy of the two erosion types. The use of 137Cs and 210Pbex measurements in the study area provided important insights into the relative importance of IRR and EG erosion and the same approach could be employed in other locations where both forms of erosion occur and there is a need to quantify their relative importance. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Cs ;210Pbex Interrill and rill erosion Ephemeral gully erosion Sediment redistribution Sicily 137
1. Introduction In recent decades, land degradation and soil erosion have been increasingly recognised as a serious environmental problem in semiarid Mediterranean regions. Recent studies carried out in southern Italy have documented rates of soil erosion ranging from 10–85 t ha1 yr1, on cultivated land (Porto and Walling, 2012a,b)
* Corresponding author. Tel.: +39096557481. E-mail addresses: [email protected], [email protected] (P. Porto), [email protected] (D.E. Walling), [email protected] (A. Capra). 0167-1987/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.still.2013.08.013
and from 100–150 t ha1 yr1, in areas covered by forests (Porto et al., 2011). These high erosion rates reflect both the nature of the environment and the long-term impact of human activity. The rainfall regime of Mediterranean areas, which is characterised by events of extremely short duration and very high intensity followed by long dry periods, is particularly conducive to erosion. Human activity further increases the erosion risk through, for example, creating bare surfaces on cultivated land after tillage operations and land abandonment in marginal and improductive areas. In Sicily, soil loss from cultivated lands reflects the general situation in Southern Italy, but rates of soil loss differ according to erosion type and land degradation processes. Erosion due to
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
concentrated flow is very severe on many unprotected farm fields and the presence of various gully types can be observed in many areas of the region. The initiation and development of channels routinely obliterated by tillage and other farm operations, commonly referred to as ephemeral gullies (EG), constitutes a severe problem (Capra, 2013). Growing crops can be removed by scour as these small gullies develop, the crops at the lower end of the gully can be buried by the sediment discharged from the ephemeral gully and deposited in an alluvial fan. Furthermore, filling operations reduce the long-term productivity of the farmland. Although the importance of EG erosion is well recognised, even at the local scale (Capra and Scicolone, 2002; Capra et al., 2005) little research has focused on this erosion type and most soil erosion prediction studies have relied on generalised empirical models (Capra et al., 2005; Capra et al., 2009a,b; Di Stefano et al., 2013). In contrast, IRR erosion rates are better understood in Sicily and over the last 50 years many different approaches have been employed to predict IRR erosion, in order to assess soil erosion risk and develop effective strategies to control erosion and sedimentation in these areas. These different approaches are based on different types of models that range from empirical-parametric approaches such as SEDD (Ferro, 1997; Ferro and Porto, 2000), through conceptual models, which correlate soil loss to physical parameters depending on soil erodibility and land use (Novara et al., 2011), topography (Bagarello et al., 2011) or rainfall erosivity (Agnese et al., 2006), to recent physically-based models, such as WEPP (Nearing et al., 1989), which aim to simulate both the detachment and transport of soil particles (Amore et al., 2004). The results provided by these studies demonstrate that IRR erosion is also an important problem in Sicily, although there is a need for further calibration and validation of the models employed for local conditions, in order to increase confidence in their output. The use of fallout radionuclides and more particularly caesium137 (137Cs) and excess lead-210 (210Pbex) to document rates of soil and sediment redistribution in the landscape has attracted increasing attention in recent years (Mabit et al., 2008; Ritchie and Ritchie, 2007; Walling, 2010; Zapata, 2002) and the approach is now being successfully employed in Mediterranean areas (e.g. Benmansour et al., 2013; Estrany et al., 2010; Gaspar et al., 2013a,b; Navas et al., 2013; Porto et al., 2006, 2013). This approach is able to overcome several of the limitations associated with more traditional methods of documenting erosion and soil redistribution. Of particular importance is its potential to provide retrospective information on medium-term average rates of soil redistribution on the basis of a single site visit and representative distributed data for fields and larger areas, without the need to disturb the system by installing measuring equipment. For some applications, particularly those requiring spatially distributed information on soil redistribution rates, fallout radionuclides arguably provide an essentially unique means of assembling data that cannot be obtained using alternative approaches. By virtue of their different half-lives and fallout origins, 137Cs and 210 Pbex provide information relating to different periods of time. 137 Cs measurements are primarily used to generate information on mean annual erosion rates over the past ca. 50 years and 210 Pbex measurements are able to provide information relating
19
to a longer period of up to ca. 100 years (Walling and He, 1999a,b). The study reported here aims to quantify EG and IRR erosion rates in a small cultivated catchment located in Sicily (Italy), to compare their relative contribution to the total soil loss from the catchment and thereby provide important information on soil erosion rates in the study region. An empirical approach, based on field measurements was used to quantify the erosion rates associated with EG erosion, while the rates IRR erosion were estimated using 137Cs and 210Pbex measurements. To the authors’ knowledge this is the first attempt to undertake such a comparison in Italy. 2. The study area The study area (Fig. 1) comprises a small 0.86 ha catchment located in Sicily, Italy. This catchment is part of a larger drainage area (80 ha in size) belonging to a national network of experimental catchments for erosion studies. The data-set of EG measurements in the study area extends over 18 years (from 1995 to 2013). The study catchment is characterised by an altitudinal range of 325 to 355 m above sea level, a mean slope of 28% and a single main drainage line with a NW–SE orientation (Capra and Scicolone, 2005) (see Table 1). The catchment is a tributary (W-side) of the middle reaches of the Simeto river, and is developed on the oldest allochthonous units of the Apennines– Maghrebian Chain (Imerese Unit, Upper Trias–Middle Miocene), which underlie the syn- and post-orogenic units (Burdigalian and lower Tortonian) (Longhitano and Colella, 2007). The dominant soil type is Vertic Xerocrept, which is very common in Sicily. Particle size analysis carried out for 10 soil samples collected within the catchment, indicated the presence of silty clay loam and silt loam textural classes (see Table 1 for details) with negligible stone content. The catchment has been cultivated since the early 1950s. The main crop is durum wheat that requires two tillage operations in summer or early autumn with a cultivator and one ploughing operation carried out every few years (generally a minimum of 3 years). The climate of the study area is typically Mediterranean with a mild wet winter and a warm dry summer. The mean annual rainfall for the period 1971–2007 is ca. 500 mm, with a coefficient of variation of ca. 40%. More than 80% of the rainfall occurs during the period extending from October to May. An active ephemeral gully (EG) develops along the main swale in the centre of the catchment during the rainy season in most years (Fig. 1). As is usually the case for cultivated land, the EG is obliterated by infilling with soil from areas immediately adjacent to the channel, using ordinary tillage equipment. However, the EG frequently recurs in the same place during the next rainy season. The extent and development of the EG system in the catchment has been documented since 1995, but measurements at the event level did not commence until 1999 (Capra and Scicolone, 2002, 2005). Until 1998, rainfall data were collected using an autographic rain gauge equipped with a chart recorder, located about 600 m away from the study area. In 1999, a digital recording rain gauge was installed in the basin. A comparison of the rainfall measured by both gauges over a period of 4 months showed good agreement between the two gauges (Capra et al., 2009b).
Table 1 Characteristics of the study catchment. Drainage area (ha)
Min altitude (m asl)
Max altitude (m asl)
Mean slope (%)
Sand (%)
Silt (%)
Clay (%)
Organic matter (%)
Bulk density (g cm3)
0.86
325
355
28
4–15
62–79
17–28
0.98–1.13
1.14–1.17
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
20
Fig. 1. The study area and the study catchment, showing the ephemeral gully system that developed in 2008.
3. The measurement programme 3.1. The ephemeral gully system The measurement programme for the ephemeral gully system involved field surveys undertaken after each erosive event. When the time interval between two erosive events was too short to visit the site, a single measurement subsumed both events. The surveys involved surveying the main branch of the EG (see Fig. 1) and its tributaries (if present). A post processing differential GPS with a planimetric accuracy of 12 cm (Capra and Scicolone, 2002) was used to establish the spatial co-ordinates of points located along the channel at about 5 m intervals in the longitudinal direction. Cross sections were measured at about every 5 m of channel, or whenever a change in the EG cross section or the entry of tributaries was observed. As it was possible to treat the cross section of the EG as a trapezium (or a rectangle) (Capra et al., 2011), the channel widths (upper and lower) and depths were measured with a steel tape graduated every 5 mm. All the measures were made by the same expert operator. The length of the EG was computed from the co-ordinates of the survey points. The volumes of material removed by erosion to create each channel segment were calculated using the end area method (i.e. the product of the mean area of two successive cross sections and the distance between them). The total volume of soil eroded from the EG was calculated as: V¼
n n X X Ai1 þ Ai Li Vi ¼ 2 i¼1 i¼1
(1)
where V is the total volume of soil eroded from the EG (m3); n is the number of segments; Vi is the volume of eroded soil from each
segment (m3); Ai1 is the downstream cross sectional area of the segment (m2); Ai is the upstream cross sectional area of the segment (m2); and Li is the distance between adjacent cross sections (m). The data relating to the volume of soil removed from the gulley system were converted to values of mass using representative values of in situ bulk density for the material removed. 3.2. Soil sampling of
137
Cs and
210
Pbex
In order to use 137Cs and 210Pbex measurement to estimate rates of interrill–rill erosion within the study catchment, two separate soil sampling programmes were undertaken. The first aimed to establish the magnitude and spatial distribution of soil redistribution rates within the catchment and involved two sampling campaigns. During the first campaign, undertaken in 2009, replicate bulk soil cores were collected at 30 sites, using an 11 cm diameter steel core tube inserted to depth of 45 cm. Deeper cores were collected from sites where there was the possibility of deposition. These soil cores, which were collected at the intersections of an approximate 20 m 20 m grid, were supplemented by a further 52 bulk cores collected in the same way from sites selected to improve the coverage of topographic variability, during a second sampling campaign in 2010 (see Fig. 1). In all cases the sampling points were selected to avoid the zone occupied by the EG and the zone from which soil was moved for infilling the gully, and are therefore seen as being representative of the soil redistribution occurring on the slopes of the catchment beyond the EG. The second sampling programme, undertaken in 2010, aimed to obtain information on the local reference inventory and the depth distribution of 137Cs and 210Pbex both at the reference site and in the cultivated soil profile of the
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
catchment. Since it was not possible to identify a site that was both undisturbed and unaffected by soil redistribution within the study catchment, the samples used to establish the reference inventory were collected from an area of permanent pasture with minimal slope adjacent to the study catchment. In this case, eight separate cores were collected from an area of ca. 25 m2 using an 11 cm diameter steel core tube inserted to depth of 60 cm, in order to take account of micro-scale variability in the reference inventory (cf. Owens and Walling, 1996). Each core was sectioned using the same depth increments, which ranged from 1 to 4 cm, and the individual depth increments from the eight cores were bulked. Bulking was undertaken to reduce the mass of material that needed to be transported to the laboratory in the UK, where the samples were assayed for 137Cs and 210Pbex, and because of limitations on the total number of samples that could be assayed. Additional sectioned cores were also obtained from two sampling sites within the catchment selected to be representative of an eroding and a depositional site (see Fig. 1), using the same procedure as employed at the reference site. 3.3. Laboratory analyses for
137
Cs and
210
Pbex
All bulk core and depth incremental samples collected from the catchment and from the reference area were oven dried at 105 8C for 48 h, disaggregated and dry sieved to separate the <2 mm fraction. A representative sub-sample of this fraction was packed into a 330 cm3 cylindrical plastic pot for determination of its 137Cs and 210Pbex activity by gamma spectroscopy in the radiometry laboratory of the Department of Geography at the University of Exeter. The samples were sealed for 21 days prior to assay, in order to achieve equilibrium between 226Ra and its daughter 214Pb. Activities of both 137Cs and 210Pb in the soil and sediment samples were measured simultaneously by gamma-ray spectrometry, using a high-resolution low energy LOAX coaxial HPGe detector (relative efficiency 30%) coupled to an amplifier and PC-based data collection system. Count times were typically ca. 90000 s, providing results with an analytical precision of 10% at the 95% level of confidence. Detection limits for 137Cs and 210Pb were ca. 0.5 and 5.0 Bq kg1, respectively. The efficiency of the detection system was calibrated using standard samples prepared by adding known amounts of certified 137Cs, 210Pb and multi-element standards to a soil/sediment matrix representative of the samples to be analysed. The 137Cs activities in the samples were obtained from the counts at 662 keV. The total 210Pb activity of the samples was measured at 46.5 keV, and the 226Ra activity was obtained by measuring the activity of 214Pb, a short-lived daughter of 226Ra, at 351.9 keV. No self absorption correction was applied to the 210Pb measurements, since the detector was calibrated with representative soil/sediment standards. The in
21
226 situ Ra-supported 210Pb concentration, associated with individual soil and sediment samples, was derived from the measured 226Ra concentration. In most terrestrial environments, the supported 210Pb will not be in equilibrium with the 226Ra, since some 222Rn will diffuse upwards through the soil or rock and escape to the overlying atmosphere. This loss is commonly accounted using a reduction factor based on the average ratio of the measured total 210Pb and 226Ra concentrations for samples collected from the lower part of the soil profile, where fallout 210 Pb or 210Pbex can be assumed to be absent (cf. Graustein and Turekian, 1986; Wallbrink and Murray, 1996). A value of 0.8 was obtained for the study site. Excess 210Pb concentrations associated with the samples were calculated by subtracting the 226Ra-supported 210Pb concentrations from the total 210Pb concentrations (cf. Joshi, 1987).
4. Results 4.1. Soil loss from the ephemeral gully system The event-based monitoring of the ephemeral gully system spanned 9 years and extended from August 1999 to November 2008 (see Table 2 for details). As observed in many other studies, EG formation and expansion commonly occurs as a result of only a very limited number of precipitation events during a given year (e.g. Casalı´ et al., 1999, 2008). In the study area, the mean number of rainy days per year is about 50, but EG formation and development generally occurs during only a single erosive event. In the study reported, the impact of 13 erosive events responsible for EG formation and development were documented and these represent effectively all of the erosive events resulting in EG development that occurred during the period covered by the detailed EG surveys (1999–2008). The rainfall totals associated with the different erosive events ranged from a minimum of 17.7 mm to a maximum of 193.4 mm, with a mean of 66.6 mm. The minimum EG erosion was associated with the events of 29/8/1999 and 3/9/1999, when the soil loss from the catchment was only 0.03 t ha1 and the maximum was associated with the last event that occurred in November 2008, when a soil loss of 82.1 t ha1 was observed. Based on the 18 years of observation of EG development in the study catchment, EG formation occurred in 7 years out of 10, with a frequency corresponding to 70% of the years covered by the survey. The cumulative soil loss for the period covered by the detailed EG surveys that commenced in 1999 was calculated, in order to quantify the total EG erosion for the period. The mean annual soil loss from the catchment from the EG, for the seven years when the EG was active, was 37.9 t ha1 yr1. Averaging the results over the nine years, including the years during which no EG erosion
Table 2 Characteristics of the erosive events responsible for EG formation and development during the study period (H = precipitation depth; Imax = maximum 30 min intensity for the precipitation event; R = R-factor; A = mean EG cross-sectional area; D = mean EG depth). Date
H (mm)
Imax (mm h1)
R (MJ mm1 ha1 h1)
A (m2)
D (m)
EG erosion (t ha1)
29/08/1999 03/09/1999 07/09/1999 09/09–13/11/1999 28/11–1/12/99 12–14/1/2000 15/10/2003 03/03/2005 22/10/2005 25/12/2006 3–4/11/2007 13/11/2008 28/11/2008
24.6 17.7 34.9 80.5 193.4 75.3 104.2 65.6 60.4 82.2 51.6 34.0 41.0
20.0 13.2 20.9 20.1 19.9 12.3 43.0 8.6 60.4 16.0 33.2 9.6 14.4
111.7 46.2 157.1 302.4 663.6 376.8 1130.5 536.4 1701.2 876.0 3372.2 410.8 1539.5
0.01 0.01 0.02 0.16 0.03 0.37 0.18 0.11 0.05 0.16 0.10 0.04 0.18
0.06 0.05 0.07 0.47 0.06 0.35 0.54 0.17 0.28 0.29 0.24 0.15 0.31
0.03 0.03 5.34 18.97 4.35 16.19 21.47 10.82 15.86 67.15 19.38 5.56 82.98
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
22
occurred, the mean annual soil loss from the catchment associated with the development of the EG was ca. 26.5 t ha1 yr1. 4.2. 137Cs and 210Pbex inventories and depth distributions at the reference site The estimates of the 137Cs and 210Pbex reference inventories for the study area are based on assay of the composited sections from eight cores. These are seen as providing representative values for the local reference inventories, although their precision or uncertainty is not explicitly quantified, due to the bulking of the samples. The uncertainty introduced by measurement precision (i.e. ca. 10%) can be expected to be considerably reduced, relative to that associated with an individual measurement of a bulk core, because the value represents the sum of the values of areal activity density obtained for the individual slices and positive and negative precision errors associated with those values are likely to cancel out to some degree. Equally, the averaging of the results from 8 cores through the use of composite samples means that the uncertainty associated with micro-scale, and sampling variability as well as local variability of the reference inventory will also be significantly reduced. Based on previous experience in the wider region, an uncertainty of 10% at the 95% level of confidence has been assumed for the estimates of the 137Cs and 210Pbex reference inventories obtained (Porto et al., 2011). The representative depth distributions of 137Cs and 210Pbex documented for the reference site based on the composited slices from the eight cores are presented in Fig. 2. These are typical of an undisturbed site (Walling and Quine, 1992; Porto et al., 2001, 2003), with a well defined exponential reduction in activity with depth and with 90% of the total inventory existing in the top
Fig. 2. The depth distribution of
137
15–20 cm. Values of 432 and 2800 Bq m2 were obtained for the 137 Cs and 210Pbex reference inventories. The reference inventory values obtained for both 137Cs and 210 Pbex must be seen as relatively low when compared with corresponding inventories found at other sites in southern Italy (see Porto et al., 2001, 2006). However, they can be accounted for by the lower mean annual rainfall (ca. 500 mm) in this area. Similarly low values for the 137Cs reference inventory have also been documented for another area of Sicily, where the mean annual rainfall is ca. 700 mm (Di Stefano et al., 2000). In this case, a reference value of 944 Bq m2 was reported (Di Stefano et al., 1999). Correction of this value to the same year as the measurements made in the current study provides a value of 717 Bq m2, and this can be seen as consistent with the value reported for the current study site. 4.3.
137
Cs and 210Pbex inventories on the slopes of the study catchment
The values of 137Cs inventory associated with the 82 sampling points in the study catchment ranged from 0.25 to 1192 Bq m2, with a mean value of 255 Bq m2 (see Table 3 for details). In the case of 210Pbex, the equivalent inventory values obtained for the same sampling points ranged from 0.6 to 14443 Bq m2, with a mean value of 2285 Bq m2. Taking account of the 10% uncertainty associated with the reference inventories indicated above, comparison of the inventory values for the individual sampling points with the reference values indicated that 67% of the 137Cs inventories were significantly lower than the reference value, indicating erosion, and 18% were significantly greater, indicating deposition. The results indicated also that 15% of the 137Cs inventory values were not significantly different
Cs (a) and
210
Pbex (b) at the reference site.
Table 3 The range of 137Cs and 210Pbex inventories associated with the sampling points in the study catchment and the number of points showing evidence of erosion, deposition or stable conditions.
137 210
Cs Pbex
Min (Bq m2)
Max (Bq m2)
Mean (Bq m2)
SD (Bq m2)
Reference value (Bq m2)
Nerod
Ndep
Nstable
0.25 0.6
1192 14443
255 2285
294 2646
432 2800
55 62
15 16
12 4
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
Fig. 3. The
137
Cs (a) and
210
Pbex (b) depth distributions documented for a representative eroding site within the study catchment.
from the reference value, indicating that the sampling points were essentially stable, experiencing neither erosion nor deposition. Similar results were obtained for 210Pbex inventories, where 76% of the measured values provide evidence of erosion, 19% indicated deposition and 5% experienced neither erosion nor deposition. It is clear that considerable soil redistribution has occurred within the study basin since the commencement of 137Cs fallout in the mid 1950s, but erosion has dominated soil redistribution within the catchment. In the case of 210Pbex, the period reflected by the reduced and increased inventories is less easy to define, since, unlike 137Cs, the fallout is essentially continuous from year to year. However, although 210 Pbex inventories may be sensitive to erosion occurring during the past ca. 100 years, they will be particularly sensitive to soil
Fig. 4. The
137
Cs (a) and
210
23
redistribution occurring in the past 20 years, due to the relatively short half-life of 210Pb (22 years). Fig. 3 presents depth profiles of the two radionuclides typical of an eroding site. The total inventories of 349 Bq m2 for 137Cs and 1823 Bq m2 for 210Pbex, are considerably lower than the reference inventories. This sampling site is located in the upper part of the catchment where erosion processes are expected to dominate (see Fig. 1). The depth profiles shown in Fig. 4 are characterised by inventory values of 4219 Bq m2 for 137Cs and 7792 Bq m2 for 210 Pbex, which are considerably higher than the reference values. These are typical of a depositional site. In this case, the sectioned cores were collected in the middle part of the catchment close to the confluence of the main EG and one of its tributaries (see Fig. 1).
Pbex (b) depth distributions documented for a representative depositional site within the study catchment.
24
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
4.4. Using 137Cs and 210Pbex to estimate soil redistribution by interrill– rill erosion on the catchment slopes Estimation of rates of erosion and deposition from 137Cs and Pbex measurements is generally based on the degree of reduction or increase of the measured inventory, relative to the local reference inventory. For cultivated soils, the calibration relationship (or conversion model) required to convert the magnitude of the reduction in the radionuclide inventory to an estimate of the rate of soil loss commonly employs a mass balance model (e.g. Kachanoski and de Jong, 1984; Walling and He, 1999a,b). Such models are based on the assumption that a sampling point with a total radionuclide inventory A (Bq m2) less than the local reference inventory Aref (Bq m2) represents an eroding site, whereas a point with a total radionuclide inventory greater than the local reference inventory is assumed to be a depositional site. Following Walling and He (1999a,b), the activity of accumulated 210Pbex or 137Cs A(t) (Bq m2) per unit area with time t (yr) at an eroding site can be represented as: 210
Zt ðPR=DþlÞ dt 0
t0
AðtÞ ¼ Aðt 0 Þe
þ
Zt
0
ð1 G ÞIðt 0 ÞeðPR=DþlÞðtt Þ dt 0
(2)
t0
where R is the erosion rate (kg m2 yr1); D is the cumulative mass depth representing the average plough depth (kg m2); l is the decay constant for 137Cs or 210Pbex (yr1); I(t) is the annual 137Cs or 210 Pbex deposition flux (Bq m2 yr1); G is the percentage of the freshly deposited 137Cs or 210Pbex fallout removed by erosion before being mixed into the plough layer; P is the particle size correction factor; t0 (yr) is the year when cultivation started; A(t0) (Bq m2) = 210Pbex or 137Cs inventory at t0; A(t) is greater than the local reference inventory Aref at a sampling point, deposition may be assumed. In this case, the mean soil deposition rate R0 can be calculated from the following equation: Rt 0
R ¼
PP 0
Rt
t0
t0
dt0
R
0
R0 C d ðt 0 Þelðtt Þ dt0
R
S
RdS
0 R=H Þ=R þ Aðt 0 Þ=DÞ dS S ðIðt Þg ð1 e
(3)
where H is the relaxation mass depth of the initial depth distribution of the fallout input. This represents the depth to which the fresh fallout input penetrates the soil. Assuming that the depth distribution is exponential, H is defined as the mass depth (kg m2) at which the radionuclide concentration reduces to 1/e of the surface concentration (see He and Walling, 1997). Cd(t0 ) reflects the radionuclide content of sediment mobilised from all the eroding areas that converge on the aggrading point. Generally, Cd(t0 ) can be assumed to be represented by the weighted mean 137Cs or 210Pbex activity of the sediment mobilised from the upslope contributing area S (m2); P0 is a further particle size correction factor reflecting differences in grain size composition between mobilised and deposited sediment; g is the proportion of the annual fallout susceptible to be removed by erosion prior to incorporation into the soil profile by tillage. 4.5. Soil redistribution rates on the catchment slopes The conversion model described above was used to derive estimates of erosion and deposition rates for the sampling points within the study catchment that showed inventories significantly different from the reference inventory (67 and 78, respectively for 137 Cs and 210Pbex). A computer-based routine which converts the percentage loss or gain in the 137Cs or 210Pbex inventory, relative to the local
reference value, to an equivalent rate of soil loss or deposition was used to solve Eqs. (2) and (3), and to estimate the erosion or deposition rates associated with the individual sampling points within the study catchment. An average plough depth D of 200 (kg m2) was selected as being representative of cultivation in the catchment, and an average value of 4 (kg m2) was used to describe the relaxation mass depth H of the initial fallout input. A value of 1 was assumed both for g, based on the relationship between the timing of cultivation and the rainfall regime, and for the particle size correction factor P, based on the lack of an appreciable difference between the grain size composition of the soil and of samples of transported sediment collected during the study period. The magnitude and spatial variability of the erosion and deposition rates estimated for the individual sampling points within the study catchment are presented in Fig. 5a, for 137Cs and in Fig. 5b for 210Pbex. 5. Discussion 5.1. Interrill/rill erosion on the slopes of the catchment The results presented in Fig. 5 emphasise that the slopes of the study catchment are characterised by appreciable rates of soil redistribution, with a clear dominance of eroding sites. Estimates of the gross erosion rate (t ha1 yr1) for the catchment associated with the 82 sampling points identified in Fig. 5 have been derived as the product of the mean erosion rate for the points indicated by the 137Cs or 210Pbex measurements to be characterised by erosion and the proportion of the catchment subject to erosion, as represented by the proportion of the sampled points that documented erosion. The same approach was applied to estimate the total deposition on the slopes of the catchment that was derived as the product of the mean deposition rate for the sampled points in the catchment demonstrating deposition and the proportion of the catchment subject to deposition. Subtraction of the total deposition within the catchment from the gross erosion provides an estimate of the net erosion, which is here interpreted to represent the sediment delivered to the channel system. Based on the 82 sampling points, the gross erosion from the slopes was estimated to be 49 and 60 t ha1 yr1, based on the 137 Cs and 210Pbex measurements, respectively. The net soil loss from the slopes of the study catchment estimated from the 137Cs and 210Pbex measurements was 38.8 and 34.2 t ha1 yr1, respectively. These values provide a sediment delivery ratio of ca. 79% and 57% for 137Cs and 210Pbex, respectively. Net soil loss rates of this magnitude, although very high when compared to those documented for other regions of Italy, are relatively common in Sicily. For example, Bagarello et al. (2010) working on experimental plots ranging in length from 11 to 44 m, documented values of mean soil loss ranging from ca. 22 to 57 t ha1 yr1 in a similar location. There are, however, a number of important differences between the estimates of soil redistribution rate provided by the 137Cs and 210 Pbex measurements. The first is that the estimates of gross erosion rate provided by the 210Pbex measurements are higher than those provided by the 137Cs measurements. Secondly, the net erosion rate estimated from the 137Cs measurements is slightly higher than that obtained from the 210Pbex measurements. This latter contrast reflects the higher estimates of deposition rates provided by the 210Pbex measurements and the associated reduction in the sediment delivery ratio. Although it is difficult to provide a definitive explanation for the different values of gross and net erosion provided by the two radionuclides, it is important to recognise their different temporal sensitivities. In the case of 137 Cs, the measurements provide a time-integrated estimate of soil redistribution rates for the period extending from the
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
Fig. 5. The soil redistribution rates estimated from the measurements of
commencement of 137Cs fallout in the mid 1950s to the present. Fallout was minimal during much of this period (i.e. since the mid 1970s) and the 137Cs inventories primarily reflect the longer-term influence of soil redistribution processes on a relatively short-term input of fallout in the late 1950s and 1960s, which has remained in the soil. In contrast, the annual fallout of 210Pbex can be seen as essentially constant and, in view of its shorter half life (22.3 yr), current inventories are likely to be more sensitive to erosion and soil redistribution occurring during the past 20 years. Where erosion and deposition rates derived using the 210Pbex measurements are higher than those obtained using 137Cs measurements, this could reflect increased erosion in recent years, due, for example, to an increased incidence of high magnitude events during this shorter time window, than during the longer timewindow reflected by the 137Cs measurements. This was the case for the areas in Calabria (southern Italy) investigated by Porto et al. (2009), Porto and Walling (2012a,b). However, D’Asaro et al. (2007) investigated trends in annual rainfall erosivity in Sicily and found that in general the rainfall erosivity factor R (Wischmeier and Smith, 1978) has not shown a significant increase in Sicily during the twentieth century. A further difference relates to the spatial patterns of soil redistribution demonstrated by the two radionuclides (see Fig. 5). This is not unexpected considering the different temporal sensitivity of the two radionuclides noted above and both this and the reduced sediment delivery ratio associated with the 210Pbex measurements may reflect progressive changes in soil properties caused by the high rates of soil loss from the field. A gross erosion rate of 50–60 t ha1 year equates to an average surface lowering of ca. 20–25 cm over the 50 year period. The rate of surface lowering in the main eroding areas within the field would be significantly greater. This progressive loss of surface soil caused by erosion and the associated incorporation into the plough horizon of soil from beneath the original plough depth, characterised by different texture, can be expected to have caused changes in the texture and organic matter content of the surface
137
Cs (a) and
25
210
Pbex (b) inventory.
soil. Such changes are likely to be reflected by changes in both the magnitude and spatial distribution of runoff and erosion. 5.2. The relative efficacy of interrill/rill erosion and gully erosion The estimates of net soil loss provided by the 137Cs and 210Pbex measurements can be compared with those obtained for the EG. The different time windows represented by the different sets of measurements necessarily introduce some uncertainty, but the comparison is, nevertheless, seen as meaningful. Combining the 137 Cs and 210Pbex estimates (IRR) with the EG measurements reported above, provides estimates of the total annual soil loss (TSL = EG + IRR) of 65.3 t ha1 yr1, and 60.7 t ha1 yr1 and of the ratio EG/TSL equal to 0.41 and 0.44 based on the estimates if IRR derived from the 137Cs and 210Pbex measurements, respectively. The estimate of soil loss provided by the 210Pbex measurements is arguably more representative of the 9 year period 1999–2008 covered by the gully surveys than that provided by the 137Cs measurements, which relates more clearly to the period extending from the mid 1950s to the time of sampling. Therefore, the estimates of TSL of 60.7 t ha1 yr1 and of the ratio EG/TSL of 0.44 could be seen as more reliable. These results suggest that in the study catchment the contributions of EG and IRR erosion to the total net soil loss are of similar magnitude. However, the magnitude of the EG/TSL ratio can be expected to vary both through time, in response to changing land use and hydrometeorological conditions, and between different catchments in response to variations in catchment morphology, soil properties and land use. Further work is clearly required to establish if the value for the EG/TSL ratio obtained for the study catchment is representative of the local region in Sicily. However, studies in other parts of Europe have reported similar results. For example, Casalı´ et al. (1999), investigated some actively eroding areas in Navarra (Spain), and reported that EG typically contributes about 30% to the total soil loss, although it can reach as high as 100%.
26
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27
Vandaele et al. (1996) working in central Belgium document values of the EG/TSL ratio ranging from 0.29 to 0.7. Cerdan et al. (2002), following two extreme rainfall events that occurred in Normandy (France), calculated that the EG/TSL ratios for the two events were 0.21 and 0.56. Looking more generally, Poesen et al. (2003) recognised that the rates of soil loss rates associated with different kinds of gully could vary considerably, resulting in gully contributions to the total sediment export from a catchment ranging from 10% to up 94% (i.e. a G/TSL ratio of 0.1–0.94). 6. Conclusions Prediction of soil erosion is an important requirement for managing land degradation processes in semiarid Mediterranean areas. However, most prediction techniques require calibration and/or validation, if they are to produce reliable results. The study reported here has provided empirical confirmation of the magnitude of both EG and IRR erosion from a small cultivated catchment located in Sicily. The results obtained have permitted the relative contribution of EG and IRR erosion to the total sediment output from the catchment to be quantified and have shown that the two erosion processes are of similar importance. The latter finding is consistent with results of other studies that have attempted to establish the relative importance of EG and IRR erosion and emphasises that both forms of erosion need to be considered when planning catchment scale soil conservation and sediment control measures within the study region. The study also demonstrates the potential for using 137Cs and 210 Pbex measurements to obtain information on medium-term soil erosion and soil redistribution rates within small catchments. By virtue of their different half-lives and fallout origins, 137Cs and 210 Pbex can provide information on land degradation processes relating to different time windows. In this study the information on IRR erosion rates generated by the 137Cs and 210Pbex measurements was combined with the results of a more traditional measurement programme aimed at assessing soil loss associated with EG erosion to assess the relative importance of the two erosion types to the total soil loss from the study catchment. As such 137Cs and 210Pbex measurements should be seen as potentially providing an important complement to more traditional measurements, rather than an alternative. Acknowledgements The study reported in this paper was supported by grants from MIUR PRIN 2010–2011, and the IAEA (Technical Contract 15478). The assistance of Sue Rouillard in producing the figures and of Jim Grapes in undertaking the gamma spectrometry measurements are gratefully acknowledged. References Agnese, C., Bagarello, V., Corrao, C., D’Agostino, L., D’Asaro, F., 2006. Influence of the rainfall measurement interval on the erosivity determinations in the Mediterranean area. Journal of Hydrology 329, 39–48. Amore, E., Modica, C., Nearing, M.A., Santoro, V.C., 2004. Scale effect in USLE and WEPP application for soil erosion computation from three Sicilian basins. Journal of Hydrology 293, 100–114. Bagarello, V., Ferro, V., Giordano, G., 2010. Misura dell’erodibilita` del suolo nelle parcelle sperimentali di Sparacia, in Sicilia. In: XXXII Convegno Nazionale di Idraulica e Costruzioni Idrauliche. Palermo, (in Italian), pp. 1–10. Bagarello, V., Di Stefano, C., Ferro, V., Kinnel, P.I.A., Pampalone, V., Porto, P., Todisco, F., 2011. Predicting soil loss on moderate slopes using an empirical model for sediment concentration. Journal of hydrology 400, 267–273. Benmansour, M., Mabit, L., Nouira, A., Moussadek, R., Bouksirate, H., Duchemin, M., Benkdad, A., 2013. Assessment of soil erosion and deposition rates in a Moroccan agricultural field using fallout 137Cs and 210Pbex. Journal of Environmental Radioactivity 115, 97–106. Capra, A., Scicolone, B., 2002. Ephemeral gully erosion in a wheat-cultivated area in Sicily (Italy). Biosystems Engineering 83 (1) 119–126.
Capra, A., Scicolone, B., 2005. Monitoraggio dell’erosione ephemeral gully in un piccolo bacino siciliano. In: Atti del Convegno dell’Associazione Italiana di Ingegneria Agraria (AIIA) 2005. Catania cod. 6010, (in Italian), pp. 1–12. Capra, A., Mazzara, L.M., Scicolone, B., 2005. Application of the EGEM model to predict ephemeral gully erosion in Sicily (Italy). Catena 59, 133–146. Capra, A., Di Stefano, C., Ferro, V., Scicolone, B., 2009. Similarity between morphological characteristics of rills and ephemeral gullies in Sicily, Italy. Hydrological Processes 23, 3334–3341. Capra, A., Porto, P., Scicolone, B., 2009. Relationships between rainfall characteristics and ephemeral gully erosion in a cultivated catchment in Sicily (Italy). Soil & Tillage Research 105, 77–87. Capra, A., Di Stefano, C., Ferro, V., Scicolone, B., 2011. Morphological characteristics of ephemeral gullies in Sicily, South Italy. Landform Analysis 17, 27–32. Capra, A., 2013. Ephemeral gully and gully erosion in cultivated land: a review. In: Lannon, E.C. (Ed.), Drainage Basins and Catchment Management: Classification, Modelling and Environmental Assessment. Nova Science Publishers, Hauppauge, NY. Casalı´, J., Gastesi, R., A´lvarez-Mozos, J., De Santisteban, L.M., Lersundi, J., Del Valle de ˇ aga, M., Gon ˇ i, U., Agirre, M.A., Campo, J.J., Lersundi, R., Gime´nez, A., Larran Lo`pez, M., Done´zar, M., 2008. Runoff, erosion, and water quality of agricultural watersheds in central Navarra (Spain). Agricultural Water Management 95, 1111–1128. Casalı´, J., Lo´pez, J.J., Gira´ldez, J.V., 1999. Ephemeral gully erosion in Southern Navarra (Spain). Catena 36, 65–84. Cerdan, O., Le Bissonnais, Y., Couturier, A., Bourennane, H., Souche`re, V., 2002. Rill erosion on cultivated hill slopes during two extreme rainfall events in Normandy, France. Soil & Tillage Research 67, 99–108. D’Asaro, F., D’Agostino, L., Bagarello, V., 2007. Assessing changes in rainfall erosivity in Sicily during the twentieth century. Hydrological Processes 21, 2862–2871. Di Stefano, C., Ferro, V., Porto, P., 1999. Linking sediment yield and caesium-137 spatial distribution at basin scale. Journal of Agricultural Engineering Research 74 (1) 41–62. Di Stefano, C., Ferro, V., Porto, P., 2000. Length slope factors for applying the revised universal soil loss equation at basin scale in southern Italy. Journal of Agricultural Engineering Research 75 (4) 349–364. Di Stefano, C., Ferro, V., Pampalone, V., Sanzone, F., 2013. Field investigation of rill and ephemeral gully erosion in the Sparacia experimental area, South Italy. Catena 101, 226–234. Estrany, J., Garcia, C., Walling, D.E., 2010. An investigation of soil erosion and redistribution in a Mediterranean lowland agricultural catchment using caesium-137. International Journal of Sediment Research 25, 1–16. Ferro, V., 1997. Further remarks on a distributed approach to sediment delivery. Hydrological Sciences Journal 42 (5) 633–648. Ferro, V., Porto, P., 2000. Sediment Delivery Distributed (SEDD) Model. Journal of Hydrologic Engineering 5 (4) 411–422. Gaspar, L., Navas, A., Walling, D.E., Machı´n, J., Go´mez Arozamena, J., 2013. Using 137 Cs and 210Pbex to assess soil redistribution on slopes at different temporal scales. Catena 102, 46–54. Gaspar, L., Navas, A., Machı´n, J., Walling, D.E., 2013. Using 210Pbex measurements to quantify soil redistribution along two complex toposequences in Mediterranean agroecosystems, northern Spain. Soil & Tillage Research 130, 81–90. Graustein, W.C., Turekian, K.K., 1986. 210Pb and 137Cs in air and soils measure the rate and vertical profile of aerosol scavenging. Journal of Geophysical Research 91 (D13) 14355–14366. He, Q., Walling, D.E., 1997. The distribution of fallout 137Cs and 210Pb in undisturbed and cultivated soils. Applied Radiation and Isotopes 48, 677–690. Joshi, S.R., 1987. Nondestructive determination of Lead-210 and Radium-226 in sediments by direct photon analysis. Journal of Radioanalytical and Nuclear Chemistry 116, 169–212. Kachanoski, R.J., de Jong, E., 1984. Predicting the temporal relationship between soil cesium-137 and erosion rate. Journal of Environmental Quality 13, 301–304. Longhitano, S., Colella, A., 2007. Geomorphology, sedimentology and recent evolution of the anthropogenically modified Simeto River delta system (eastern Sicily, Italy). Sedimentary Geology 194, 195–221. Mabit, L., Benmansour, M., Walling, D.E., 2008. Comparative advantages and limitations of the fallout radionuclides 137Cs, 210Pbex and 7Be for assessing soil erosion and sedimentation. Journal of Environmental Radioactivity 99, 1799–1807. Nearing, M.A., Foster, G.R., Lane, L.J., Finkner, S.C., 1989. A process based soil erosion model for USDA-water erosion prediction project technology. Transactions of the ASAE 32 (5) 1587–1593. Navas, A., Lo´pez-Vicente, M., Gaspar, L., Machı´n, J., 2013. Assessing soil redistribution in a complex karst catchment using fallout 137Cs and GIS. Geomorphology 196, 231–241. Novara, A., Gristina, L., Saladino, S.S., Santoro, A.C., Cerda`, A., 2011. Soil erosion assessment on tillage and alternative soil managements in a Sicilian vineyard. Soil & Tillage Research 117, 140–147. Owens, P.N., Walling, D.E., 1996. Spatial variability of caesium-137 inventories at reference sites: an example from two contrasting sites in England and Zimbabwe. Applied Radiation and Isotopes 47, 699–707. Poesen, J., Nachtergaele, J., Verstraeten, G., Valentin, C., 2003. Gully erosion and environmental change: importance and research needs. Catena 50 (2– 4) 91–133. Porto, P., Walling, D.E., 2012a. Validating the use of 137Cs and 210Pbex measurements to estimate rates of soil loss from cultivated land in southern Italy. Journal of Environmental Radioactivity 106, 47–57.
P. Porto et al. / Soil & Tillage Research 135 (2014) 18–27 Porto, P., Walling, D.E., 2012b. Using plot experiments to test the validity of mass balance models employed to estimate soil redistribution rates from 137Cs and 210Pbex measurements. Applied Radiation and Isotopes 70, 2451–2459. Porto, P., Walling, D.E., Callegari, G., 2011. Using 137Cs measurements to establish catchment sediment budgets and explore scale effects. Hydrological Processes 25, 886–900. Porto, P., Walling, D.E., Callegari, G., 2013. Using 137Cs and 210Pbex measurements to investigate the sediment budget of a small forested catchment in southern Italy. Hydrological Processes 27, 795–806. Porto, P., Walling, D.E., Callegari, G., Capra, A., 2009. Using caesium-137 and unsupported lead-210 measurements to explore the relationship between sediment mobilisation, sediment delivery and sediment yield for a Calabrian catchment. Marine and Freshwater Research 60, 680–689. Porto, P., Walling, D.E., Callegari, G., Catona, F., 2006. Using fallout lead-210 measurements to estimate soil erosion in three small catchments in southern Italy. Water, Air, & Soil Pollution: Focus 6, 657–667. Porto, P., Walling, D.E., Ferro, V., 2001. Validating the use of caesium-137 measurements to estimate soil erosion rates in a small drainage basin in Calabria, southern Italy. Journal of Hydrology 248, 93–108. Porto, P., Walling, D.E., Ferro, V., Di Stefano, C., 2003. Validating erosion rate estimates by caesium-137 measurements for two small forested catchments in Calabria, Southern Italy. Land Degradation and Development 14, 389–408.
27
Ritchie, J.C., Ritchie, C.A., 2007. Bibliography of publications of 137Cs studies related to erosion and sediment deposition. Agricultural Research Service. US Dept. Agriculture, Washington, DC. Vandaele, K., Poesen, J., Marques de Silva, J.R., Desmet, P., 1996. Rates and predictability of ephemeral gully erosion in two contrasting environments. Ge´omorphologie: Relief, Processes Environment 2 (2) 83–95. Wallbrink, P.J., Murray, A.S., 1996. Determining soil loss using the inventory ratio of excess lead-210 to cesium-137. Soil Science Society of America Journal 60, 1201–1208. Walling, D.E., 2010. Using fallout radionuclides to investigate erosion and sediment delivery: some recent advances. In: IAHS Publ. 337. IAHS Press, Wallingford, pp. 3–16. Walling, D.E., He, Q., 1999a. Improved models for estimating soil erosion rates from cesium-137 measurements. Journal of Environmental Quality 28 (2) 611–622. Walling, D.E., He, Q., 1999b. Using fallout lead-210 measurements to estimate soil erosion on cultivated land. Soil Science Society of America Journal 63, 1404–1412. Walling, D.E., Quine, T.A., 1992. The use of caesium-137 measurements in soil erosion surveys. In: IAHS Publ 210. IAHS Press, Wallingford, pp. 143–152. Wischmeier, W.H., Smith, D.D., 1978. Predicting rainfall erosion losses: a guide to conservation planning. In: Agricultural Research Service Handbook 537. US Dept. Agriculture, Washington, DC. Zapata, F. (Ed.), 2002. Handbook for the Assessment of Soil Erosion and Sedimentation Using Environmental Radionuclides. Kluwer, Dordrecht, Netherland, 219 pp.