Erosion rates in badland areas of the central Ebro Basin (NE-Spain)

CATENA

vol. 19, p. 269-286

Cremlingen 1992

Erosion Rates in Badland Areas of the Central Ebro Basin (NE-Spain) G. Benito, M. Guti~rrez & C. Sancho Summary Erosion rates have been estimated for two badland areas in the Ebro Basin ("El Barranco" and "La Charca" plots) using direct measurement techniques. These measurements were made by means of erosion pins and a microtopographic profile gauge. Measurements with erosion pins at the "El Barranco" plot indicate highest erosion rates around rills and at rill junctions. In contrast, at the "La Charca" plot, only small differences between erosional rates for rill and interrill areas are observed. Microtopographic profiles carried out in "El Barranco" show irregular ground lowering, greater in rill than in interrill areas. At "La Charca", similar denudation rates are observed in rill and interrill areas. Erosion rates determined with the microtopographic profile gauge were 7 and 19 mm/year in "El Barranco" and "La Charca" plot respectively, and close to 6 and 17 mm/year respectively, using erosion pins.

1

Introduction

Erosion susceptibility in climates with strong seasonal contrasts, such as ISSN0341-8162 @1992 by CATENAVERLAG, W-3302 Cremlingen-Destedt, Germany 0341 8162/92/5011851/US$2.00+0.25

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mediterranean climates, is generally considered to be very high, especially in badland areas. Measured erosion rates in mediterranean badlands vary widely, ranging between 0.45 mm/year (Yair et al. 1982) and 20-30 mm/year (Alexander 1982). This wide range is the result of climatic, lithologic and relief characteristics at the measurement sites as well as the variety spatial and temporal scales over which the measurements were taken. Apart from these factors, erosion rates also depend on the measurement techniques used (Yair et al. 1980). For example, erosion values recorded by means of erosion pins are usually higher than sediment yield data obtained by Gerlach troughs. The sparse measurements of erosion rates in semiarid mediterranean Spain have been carried out mainly in the Southeast by erosion pins (Scoging 1982), Gerlach troughs (Francis 1986) and water collectors (Romero-Diaz et al. 1988). In the Ebro Basin, erosion research has focused on processes (mainly piping, rilling and gullying) using microform measurements and detailed maps (Guti~rrez et al. 1988). Only estimates from the Universal Soil Loss Equation (U.S.L.E.) have been obtained and shown on a map 1:400.000 scale (Lopez-Cadenas et al. 1987). Erosion values from the U.S.L.E. method range between no soil loss and more than G E O MO RPHO I, O (IY

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LEGEND

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Infilled valley Fig. 1: Geomorphological map of the study area (10 km to SW of Huesca). 200 Tm/Ha/year. In order to obtain actual measurements of erosion rates in badland areas and to evaluate the role of the different processes, two experimental sites were located in the central-northern part of the Ebro Basin. Lithology, topography, types of processes and thresholds have all been taken into account in the plot selection.

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2

Environment of the study area

The experimental sites are located near "Castillo de Orris", south of the Pyrenean "Sierras Exteriores", and 10 km south of Huesca (fig. 1). The climate is continental mediterranean with an annual average temperature at Almud~var (6 km to the SW) of 13°C, ranging between 23°C for the July mean and 3°C

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for January. The mean annual precipitation in Lupifi+n (10 km to the NW) is 650 mm, most of which is storm rainfall. Rainfall occurs, primarily, in spring and autumn, but heavier storms may occur between May and October. Larger storms are generally associated with rainfalls of about 30 mm. The regional geology consists of Miocene continental shales with some thin sandstone and limestone layers. The uppermost part of this formation is primarily comprised of limestones that dip gently southwards. These upper limestones form a cuesta with stepped slopes on its front, partially covered by debris. Within these stepped slopes, a network of valleys has developed, most of which are infilled by Holocene deposits. Valleys in the more gently sloping parts have extensive and coalescing alluvial fans at their termini. The erosion study has been carried out at two plots. One is situated in Holocene valley-fill sediments ("El Barranco") and the other is on a slope of exposed Tertiary clay ("La Charca") (fig. 1).

2.1

"El Barranco" plot

The "El Baranco" site is situated within a valley infilled with Holocene sediment and that is devoid of vegetation (photo 1). This area has low relief, and gentle slopes (between 4 and 6°), but there are locally steeper gradients. The Holocene deposits are greater 3.7 m thick and are comprised of fine detritus. X-ray diffraction tests performed in similar Holocene deposits indicate a mineralogy of calcite, quartz, illite, and chlorite with traces of feldspars, dolomite, smectite and interstratified clay minerals. In order to analyse temporal changes

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in surface morphology and to measure erosion rates, a 225 m 2 plot has been mapped, using a 1.5 m mesh. The surface morphology is made up of a dendritic network of rills and some collapsed pipes (fig. 2). The network is divided into eleven microcatchments, between 1 and 60 m 2 in surface area, with a meandering major channel that terminates within a collapsed pipe. The erosion processes are related to the chemical characteristics and physicochemical properties of the materials. At the surface, there is low electrical conductivity (1 mmhos/cm), low sodium absorption ratio (SAR about 1) and a dispersion index about 0.7. In these materials, the erosion processes are splash, overland flow and rilling. Piping processes are more important at deeper levels because of the higher electrical conductivity and SAR values. The growth of the subsurface pipes produces a decrease in mechanical resistance in their roofs triggering collapse. This process is indicated on the surface by pseudo-dolines which reach up to 3 m in diameter. In the "El Barranco" experimental site, chemical and physico-chemical variations with depth may give rise to an increase in erosion in the incised areas. More dispersive levels occuring below 1 m depth may result in increased erosion rates once these levels are exposed.

2.2

"La Charca" plot

A hundred meters to the east of "El Barranco" plot and on a slope of exposed Tertiary clay is the " L a Charca" site. This plot is situated on a 25--30° slope, orientated NW and is devoid of vegetation (photo 2). The site is underlain by 9 m thick Miocene shales except for the top of the slope where there is a 30 cm

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Photo 1" The experimental station "El Barranco" on Holocene valley-fill sediments.

Photo 2: The experimental site "La Charca" on a slope of exposed Tertiary clay.

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thick limestone layer. Chemical analyses have been made for nine separate horizons within the underlying materials. As a whole, these analyses indicate alkaline pH values between 8 and 10, low percentages of organic matter (< l), and 20 to 47% calcium carbonate. Electrical conductivity of water extract from saturated pastes are less than 10 mmhos/cm and varies between 0.7 and 4 mmhos/cm. With the exception of one sample, the concentration of cations in the saturated extract indicates a large excess of soluble sodium (between 18.2 and 104.3 meq. per litre) over calcium and magnesium (both between 2 and 8 meq. per litre). These concentrations give rise to SAR values between 13 and 42, which corespond to 15 and 38% of exchangeable sodium percentage. Dispersion index values obtained from weight ratios between the disaggregated fractions (with and without dispersants) are near unity for most of the samples. X-ray diffraction tests of Miocene clay from near " L a Charca" plot reflect a mineralogy of quartz, calcite, illite, chlorite with traces of smectite, feldspars and interstratified clay minerals. As at "El Barranco", we have made a micromorphology map of an area of about 100 m 2, using a 1.5 m grid. The "La Charca" site is dissected by a parallel network of rills. In some locations, the rills are discontinuous because of pipes (fig. 3). At locations where rill cut off is unimportant, deep narrow braiding rills have developed. A common feature in "La Charca" plot are inlets which collect the water from a small network of rills, reemerging on the surface through outlets. Despite the low amount of swelling clays, swelling and cracking does occur because of the high sodium percentages

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and a moderate salt content. A large amount of overland flow is produced because of low infiltration capacities resulting from presence of the dispersive clays. Pipes have developed in the lower part of the slope where pH is near 10, salt concentration is low to medium, SAR values are above 40 and dispersion index is close to 1. Rills have developed on materials with different chemical and physico-chemical characteristics though these factors may cause variations in rill network density and dew:lopment.

3

Methodology

The aim of this paper is to present measurements of erosion rates and the record of landform variation in relation to rainfall intensity. In "Castillo de Orris" rainfall data are available from the permanent weather station, with rain gauges located in Lupifi~n (10 km to the NW). For the first six months of 1988 (fig. 4), 420 mm of rainfall was recorded with no daily total exceeding 30 mm. In summer, rainstorms normally have an irregular distribution and sometimes a high intensity. During the summer of 1988, there were no rainfall events. In autumn 1988 rainfall was infrequent and of low intensity except on October when there were four consecutive rain-days of greater than 10 m m precipitation (fig. 4). During the first three months of 1989, several low intensity rainfalls (less than 13 mm/day) occured, especially during the 7 last days of February. In order to determine erosion rates related to this rainfall sequence, comparative study methods and direct measurement techniques were used. The comparative measurements utilized micromorphology mapping arid photographic analysis. A map at a scale of 1:33 was

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Fig. 3: Location of erosion pins and microtopographic profiles of "La Charca'" experimental station.

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Fig. 4: Daily rainfall data Jkom the raingauge at LupifiOn. produced using a 1.5 m grid pattern for each plot (figs. 2 and 3) and photographs of each square were taken at intervals. Direct measurements of surface change were carried out by means of erosion pins and a microtopographic profile gauge. The pins were emplaced as follows: they are long enough to stay fixed in the ground; we did not use permanent washers; a mark on the erosion pin was set flush to the soil; pins were established in short rows (cluster points) and at special points where maximum or minimum erosion was expected; and were set perpendicular to the slope. Erosion pins were constructed of 60 cm long and 4 m m diameter steel rods. There were installed at 273 points on the "El Barranco" plot and at 199 points on the " L a Charca" plot. Pins were situated in the intersections of the grid pattern (1.5 m between pins) in addition to others along rills and in interrill areas (photo 3).

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The pins were installed in November 1987 and measurements were made in July 1988 and February 1989. The erosion pin record was analysed by computer which generated isopleth maps (5 m m intervals). During the first period, disturbance caused by pin placement and settling may have increased the apparent erosion rates. In some cases, we have observed that the presence of the erosion pin may disturb the soil in two ways: overland flow and water flow in rills can be disturbed by pins and cause local areas of scour. Conversely, stones and vegetation debris can become trapped on the upslope side of the rod. These obstacles can produce an accumulation on the upslope side and a hollow downslope. Surface changes using profilometers can be monitored without interfering with the surface form or processes (Campbell 1981). The microtopographic profile gauge used (Benito et al. 1988)

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Photo 3: Erosion pins at a rill capture site. The pins situated on rills of the captured microcatchment (left) show ground lowering (up to 26 mm) while in the other part o/" the microcatchment (right) sedimentation was recorded.

Photo 4: Microtopographic pro.file gauge standing on erosion pins.

(.All NA

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is based on Curtis & Cole (1972) and Mosley (1975) profilometers. The device (photo 4) has a 110 cm wide and 90 cm high aluminium frame that acts as a background to a black panel which has white horizontal lines 2 cm apart painted on its front face. Along its lower horizontal edge, a 104 cm long hollow aluminium bar with holes drilled 2 cm apart was fixed. Through these holes, 51 rods 4 mm in diameter may be slide up and down in response to microtopographic variations, The profilometer was maintained in a horizontal position by adjustable vertical tubes mounted on fixed erosion pins. The results were recorded photographically. Data analysis was carried out by computer giving (x,y) values for each rod. A mathematic function was obtained by means of a cubic spline interpolation that results from fitting of series of cubic functions, Sj, in each subinterval [xj, xj+l] for j=0, 1 .... n-1. This cubic spline interpolation allows integration between several measurements on one profile, obtaining ground variation for each profile in mm. The first profile measurement was made in December 1987 and records were also obtained in July 1988 and February 1989.

4

Results

In mediterranean environments with strong seasonal contrasts, a long record of erosion measurement is required to a reliable estimate for average annual soil loss. For this reason the first year's monitoring has to be considered as only approximate. Erosion pins and profilometer methods do however allow soil loss distribution and relationships between processes and erosion rates to be established.

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4.1

"El Barranco" plot

At the "El Barranco" plot, 273 erosion pins were positioned in November 1987 and resurveyed in July 1988 and February 1989. As a whole, erosion pins show an average ground lowering of 5 mm for the first period (with 514 mm rainfall), which assuming 1.6 gr/cm 3 bulk density is equivalent to a soil loss of about 80 Tm/Ha. In this period, most of the erosion occured in the central-south area at rills and at rill junctions (fig. 5). In these areas, ground lowering exceeds 5 m m and is as great as 40 mm within rills. In fig. 5, maximum erosion rates can be observed following the drainage network. In the period between July 1988 and February 1989 (with 102 mm rainfall), erosion pins show a 2 mm average ground lowering (about 32 T m / H a soil loss). The isopleth map (fig. 5) shows a similar distribution to the preceding period, most of the erosion occured at the major rills and especially at rill junctions (with up to 20 m m recorded). As a whole, between November 1987 and February 1989, a 7 m m average ground lowering has been recorded which is equivalent to a soil loss of 112 Tm/Ha. Higher erosion rates occur in the south part of the plot where the rill network is well developed (fig. 5). At the "El Barranco", 33 microtopographic profiles were measured in December 1987 and resurveyed in July 1988 and March 1989. During the first period, the data analysis using a cubic spline interpolation indicates an average ground lowering of 8 mm. During this period, a 126 T m / H a sediment yield was recorded as a result of 420 m m rainfall. Between July 1988 and March 1989, with 174 mm rainfall, the average ground lowering lbr these profiles was 2 m m (equivalent to

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Fig. 6: Selected microtopographic profiles from "El Barranco" plot, prt~file A: Profiles made in December 1987 (continuous line) and in July 1988 (broken line). B: Profiles made in July 1988 (continuous line) and in March 1989 (broken line).

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24 T m / H a of sediment yield). Summing the results for the period from December 1987 to March 1989 results in an average ground lowering of 10 mm/year and soil loss of 150 T m / H a produced as result of 594 mm of rainfall. Profiles of rill sections and interrill areas indicate variations in microtopographic evolution depending on the position in the catchment and on the season of measurement. In general, the majority of ground lowering occurs in and near rills whereas ground lowering on the interrill areas is less. In some cases, erosion and sedimentation rates can be increased because of rill capture (photo 3), pipe collapse, or removal by erosion of the surface crust. In the "El Barranco" plot, part of microcatchment 2 has been captured by microcatchment 1. Profile 18, situated downstream of the capture point, shows erosional and depositional variations in response to this capture (fig. 6). Profiles made in December 1987 and July 1988, before capture occurred, indicate high erosion rates on rill and interrill areas. Average erosional lowering for this profile was 14 mm with a maximum of 34 mm at the main rill. The erosion pins supporting the profilometer show 0 and 5 mm ground lowering while the nearest pin in the bottom of the rill (30 cm above) records 26 m m of ground lowering (photo 3). The profilometer measurements recorded after the capture (between July 1988 and March 1989) reflect that some eroding areas became sedimentation areas, locally up to +2 mm in ground surface gain (fig. 6). This sedimentation is also recorded by erosion pins situated in the rill (+3 ram) while ground lowering (3 and 2 ram) was recorded by the erosion pins located in interrill areas.

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Erosion measurements and their distribution may be related to the temporal and spatial variability in the geomorphic processes, Ground retreat recorded in the July 1988 measurements, is the result of rilling and overland flow processes occuring during spring rainfalls. However, profile measurements made in March reflect desiccation softening and weathering of the surface by frost in winter. The low intensity rainfall in winter cannot flush away the sediment produced by frost which fills channel bottoms and mantles some interrills areas.

4.2

"La Charca" plot

As at the "El Barranco" experimental site, erosion pins were installed in November 1987 and measurements were made in July 1988 and in February 1989. Erosion rates for the first period (fig. 51 indicate an average ground lowering of 15 mm associated with 514 m m of rainfall, which assuming clay bulk density of 2 gr/cm 3 is equivalent to 300 Tm/Ha. Most of the plot is enclosed by the 12 mm contour (fig. 5). However, the upper part of the plot had less than 4 m m of ground lowering. In the second period between July 1988 and February 1989, with a rainfall of 102 mm, the erosion pins recorded an average of 5 mm ground lowering. For this period, the upper part of the plot experienced the least amount of erosion, while the maximum erosion rate was recorded by pins positioned in the rills and where headcut retreat of rills has taken place. As a whole, between November 1987 and February 1989, with 616 mm rainfall, 20 mm of ground lowering has been recorded by erosion pins (fig. 5). In "La Charca'" plot where overland flow is the main erosional process, there were similar denudation

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rates on rill and interrill areas. At the "La Charca" experimental station, 22 microtopographic profile measurements of rill and interrill areas were carried out. The data analysis by cubic spline interpolation indicates that during the December 1987 July 1988 period, there was an average ground lowering equivalent to 15 mm. This is equivalent to about 290 T m / H a soil loss and was associated with 420 mm of rainfall. Between July 1988 and March 1989, microtopograpic measurements indicate an average ground lowering equivalent to 10 mm associated with 174 mm rainfall. Summing the results from December 1987 to March 1989 yields a ground lowering of 25 m m and a soil loss of 584 T m / H a due to 594 m m of rainfall. The analysis of measurement profiles indicates that during the first period of measurement, the slope retreat was parallel to the ground surface while during the second measurement period, the main changes were in the rills. These features are exemplified by profile 19 (fig. 7) where ground lowering in the period December 1987 and July 1988 was 18 ram. The profile retreat is regular and only small differences can be observed between the rill and interrill areas. The profile measured in March 1989 indicates an average of 14 mm (fig. 7) of ground lowering. Erosion rates were highest for the interrill areas. The rills experienced less erosion, in some locations aggraded.

5

Discussion and conclusions

In climates with strong interannual and seasonal contrasts a fifteen month monitoring period can provide only approximate erosion rates. This is especially true for the first three months because possi-

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ble disturbance caused by pin settlement may increase apparent erosion rates. However, this disturbance appears to be small because erosion rates measured both with erosion pins and microtopographic profile gauge were similar. In comparing these data, potential measurement errors of 1 m m for the erosion pins and up to 2 mm for the profilometer need to be considered. In the first six months of 1988, erosion amounts recorded by erosion pins were 5 and 15 mm at the "El Barranco" and the "La Charca" plots, respectively, while measurements made by the profilometer indicated 8 and 15 mm (tab. 1). During the second eight months at the "'El Baranco'" plot, erosion pins and the profilometer indicate a lowering of 2 and 1.5 mm respectively. At the "La Charca" plot, erosion values recorded by means of pins and the profilometer were 5 and I0 mm, respectively (tab. 1). Over the entire fifteen month measuring period, erosion measurements of 7 mm by erosion pins and 9 mm using the microtopographic profile gauge were recorded at "El Barranco", and 20 and 24 m m respectively were recorded at "La Charca". Note that both methods lead to similar results (tab. 1); with the slight differences possibly due to measurement errors and to the location of most of profilometer measurements across rills where ground lowering is usually higher. As significant as erosion rates, is spatial and temporal variability in the geomorphic processes. In summer, the strong temperature contrasts produce desiccation softening and weathering in the surface cover giving the sediment yield. However, because of infrequent rainfall, this sediment may not be transported far, filling rills and smoothing the surface. Similar effects could be pro-

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Plot A

Period

Ground lowering mm

Erosion rates Tin/Ha

Rainfall mm

Barranco Barranco Barranco Charca Charca Charca

11/87 7/88 7/88 2/89 11/87 2/89 11/87-7/88 7/88 2/89 11/87 2/89

5 2 7 15 5 20

80 32 112 300 100 400

514 102 616 514 102 616

Barranco Barranco Barranco Charca Charca Charca

12/87-7/88 7/88 3/89 12/87 3/89 12/87 7/88 7/88 3/89 12/87 3/89

8 2 10 15 10 25

126 24 150 290 195 485

420 174 594 420 174 594

Tab. l: Erosion rates using erosion pins ( A ) and the microtopographic profile gauge ( B ) in "El Barranco" and "La Charca" experimental sites.

duced by frost heaving in the winter. In spring and autumn, the sediments stored in rills and on interrill areas are flushed away by runoff and the rills are incised. This variability observed by photography, is also recorded by the microtopographic profile gauge in the July and March surveys. During the first period (December July), the "La Charca" plot profiles show a parallel ground lowering on rill and interrills areas. However, for the "El Barranco" plot, ground lowering was higher in the major rills than in the interrill areas (figs. 6 and 7). At both sites, this record may reflect that the sediment produced by desiccation softening and weathering of the surface cover has been flushed away by runoff during spring rainfalls. At "El Barranco", runoff is concentrated in rills causing incision. The second record carried out in March 1989, after winter, but before the spring rainfall, indicates an irregular pattern of erosion and in some locations, sedimentation in rills and on rill slopes (figs. 6 and 7). This contrast is not so clearly recorded

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by the erosion pin measurements. On the "El Barranco" plot during the first measurement period, erosion pin measurements indicate that the majority of the erosion occurred around and along rills and at rill junctions. During the second period, some of those erosional areas became areas of sedimentation (fig. 5). In isopleth maps made for the "La Charca" plot, contours have a perpendicular pattern in relation to the slope (fig. 5). Hence, erosion rates appear to depend on slope angle, distance from the upper part of the plot, and to lithologic variability, with only small differences between rill and interrill areas. For the measurements made in winter (February 1989), the main differences are higher erosion rates at rill headcuts, resulting from small-scale mass movement processes. The erosion measurements record high erosion rates from interrill areas. At "El Barranco" where 90% of" the plot comprises interrill areas (fig. 2), both measurement methods indicate that 75% of the total denudation is due to interrill

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processes. At " L a C h a r c a " (fig. 3), interrill areas are 80% of total surface and produce 60--75% of the total erosion. Percentage of sediment production in interrill areas is higher because the area covered by interrill zones is significantly higher. In terms of ground surface lowering, seasonal changes of lowering between rill and interrill areas can be observed in both experimental plots. However, for the total period measured, similar rates of ground lowering were observed on rill and interrill areas at the "La Charca" plot without significant changes in morphology. These observations indicate that a dynamic equilibrium occurs between the main components of the badland system (Schumm & Lichty 1965), suggesting that over the total period of observation and in small areas, a state close to "steady state" may be maintained. Dynamic equilibrium may be interrupted when extrinsic and intrinsic changes cause the system to cross a threshold. At the "El Barranco" plot, increased of erosion rates may be expected when denudation reaches the more highly erodible levels, with high SAR values and dispersion index close to 1.

The authors are very grateful to Dr. A.M. Harvey (University of Liverpool) and Dr. J.E. O'Connor (University of Arizona) for reviews of the manuscript and useful suggestions, and to Luis Randez of the "Departamento de Matemfitica Aplicada" (Universidad de Zaragoza) for help in the mathematical analysis of the profilometer data. This work has been partly supported by the project CA1CYT, PB 85-0392.

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BENITO, G., GUTIERREZ, M. & S A N C H O , C. (1988): Perfiladores de microtopografias para el control de secciones transversales de canales. In: M. Sala & F. Gallart (Eds.), Metodos y Tecnicas para la Medicion en C a m p o de Procesos Geomorfologicos. 54~57. Monografia SEG nr, I, Zaragoza.

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Address of authors: Dr. G. Benito Dr. M. Gutitrrez Dr. C. Sancho Departamento de Geologia Facultad de Ciencias Universidad de Zaragoza 50009 Zaragoza Spain

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