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The impact of the farming practice of remodelling hillslope topography on badland morphology and soil erosion processes
Catena 40 Ž2000. 229–250 www.elsevier.comrlocatercatena
The impact of the farming practice of remodelling hillslope topography on badland morphology and soil erosion processes Michele ` L. Clarke a
a,)
, Helen M. Rendell
b
Centre for EnÕironmental Management, School of Geography, UniÕersity of Nottingham, UniÕersity Park, Nottingham NG7 2RD, UK b Geography Laboratory, Arts C, UniÕersity of Sussex, Falmer, Brighton BN1 9QN, UK Received 13 January 1998; received in revised form 11 August 1998; accepted 23 September 1998
Abstract Badland landforms, created in Plio–Pleistocene clay landscapes of the Basilicata region of southern Italy, form marginal features in a semi-arid landscape dominated by the widespread cultivation of subsidised cereals. These badland features are high relative relief forms exhibiting a high drainage density and steep slopes, with slope angles typically in excess of 358. Economic incentives to increase agricultural productivity have resulted in the practice of remodelling these marginal areas using heavy earth-moving equipment. Remodelling the badland features creates longer slopes at lower angles, which can be cultivated using conventional farm machinery. These changes in hillslope morphology have altered the degree to which soil erosion processes operating in these areas are spatially coupled. In the badland areas, erosion and deposition are strongly localised with minimal sediment delivery to ephemeral or perennial channel systems. The economically-driven change in land use from visually striking badland areas to newly remodelled fields for agricultural use results in an increase in the coupling of drainage networks and a net increase in soil erosion. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Badlands; Calanchi; Biancane; Soil erosion; Land use change; Common agricultural policy
1. Introduction Visually striking badland landscapes are found in fine-grained clastic sedimentary bedrock regions of central and southern Italy, principally in the provinces of Marche )
Corresponding author. E-mail: [email protected]
0341-8162r00r$20.00 q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 9 . 0 0 0 4 7 - 8
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ŽCalzecchi-Onesti, 1957., Tuscany ŽGuasparri, 1993. and Basilicata ŽRendell, 1982, 1986.. These badlands consist of deeply-dissected, unvegetated or poorly-vegetated landforms of high relative relief and high drainage density. In the region of Basilicata Žformerly known as Lucania. it has been estimated that badlands occupy 32,125 ha in the Bradano, Basento, Cavone and Agri catchments ŽRadina, 1964.. The badland areas are located in the middle and lower reaches of these catchments on Plio–Pleistocene marine clays, which were deposited as part of the Fossa Bradanica ŽBradanica Trough. fill complex ŽSabato and Tropeano, 1994.. The climate of Basilicata ranges from high mountains areas in the western part of the region with mean annual rainfall in excess of 2000 mm and mean annual temperature of 108C to relatively low-lying areas bordering the Ionian Sea with mean annual rainfall of less than 700 mm and mean annual temperature in excess of 158C ŽCataudella, 1987.. The climate of the Salandrella–Cavone area of Basilicata is Mediterranean semi-arid with more than 65% of rainfall occurring during the autumn and winter months. Summers are characterised by long periods without rain, with temperatures in excess of 408C and relative humidities of less than 40% ŽServizio Meteorologico Aeronautica Militare.. Within the main river valleys, agricultural productivity on river terraces is enhanced by irrigation allowing the cultivation of citrus fruit orchards and crops such as tomatoes, melons and strawberries ŽRendell, 1986.. On higher slopes and in the majority of the river catchments, current land use is dominated by the European Union ŽEU. subsidised cultivation of durum wheat Žfor pasta., which can be grown on the clay terrain, without artificial irrigation, yielding about 2.5 t hay1 ŽBianchi et al., 1993.. Most winter cereal cultivation occurs on slopes shallower than 208 but the use of caterpillar-tracked machinery has enabled steeper slopes to be cultivated and thus badland features have been increasingly confined to marginal scarp slopes. Changing land management strategies and increased mechanisation has resulted in the deep ploughing of slopes to increase water retention capacity ŽLandi, 1989.. This improvement in mechanisation and an increase in crop specialisation has resulted in economical changes which, in order to improve the cost–benefit ratio to the farmer, has lead to a general increase in field area, removal of terraces and a ‘smoothing of hillslopes’ ŽZanchi, 1989.. These economic incentives have meant that there is a tradition of using bulldozers ŽCalzecchi-Onesti, 1954, 1957. and even explosives ŽPavari, 1911. to remodel and reclaim areas of badland landforms in Italy for agricultural productivity. In Tuscany, Guasparri Ž1993. has shown that substantial areas of former badlands have been obliterated since the 1960s and in Basilicata, major remodelling of badland slopes has occurred since the 1970s ŽRendell, 1986. and is still continuing today Žsee Fig. 7.. Badlands are conventionally considered to be areas of extreme soil erosion. This paper describes the impact on soil erosion of remodelling these badlands area for increased agricultural productivity. Land degradation by severe soil erosion has been linked to desertification in this and other parts of the Mediterranean Basin ŽThornes and Brandt, 1996. and it has been suggested that there is an increasing risk of further degradation in response to both physical and socioeconomic factors ŽPerez-Trejo, 1994.. The Salandrella–Cavone study area is therefore an ideal location in which to examine the interaction of these factors in relation to land use change.
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2. Study area The study area lies between the Basento and Agri rivers, predominantly in the Salandrella–Cavone river catchment ŽFig. 1.. The area is underlain by up to 500 m ŽENI-AGIP, 1972. of Plio–Pleistocene clays which are capped in places by locally variable, uncemented to partially-cemented, Upper Pleistocene sands and conglomerates. Uplift and differential incision has left pedestal-type outcrops of the conglomerate which lie up to 250 m above the river valleys ŽRendell, 1975. and support hilltop settlements such as Pisticci, Ferrandina and Pomarico. At Pisticci, the clays are incised to a depth of 200 m relative to the current valley of the Cavone river. Clay mineralogy shows a dominance of sodium montmorillonite, although kaolinite and illite are also present ŽRendell, 1975.. The clays are inorganic silty-clays of medium plasticity and disperse rapidly when wetted ŽRendell, 1982, 1986. making them prone to both surface and subsurface erosion. The grain size of the clays is typically 55–65% by weight finer than 8 f Ž3.9 mm. and 90–95% finer than 4f Ž62.5 mm. ŽRendell, 1975.. Given the dispersive nature of the clays, the high relative relief and the seasonality of the climate, it has been suggested that this area is prone to severe soil erosion ŽAlexander, 1982; Cataudella, 1987..
3. Land use change impacts on badland landforms In order to assess the impact of landscape remodelling on soil erosion it is necessary to compare the initial and final topographic forms involved in hillslope modification. This is extremely difficult because Ž1. the majority of the landscape remodelling occurred prior to 1986 Žbased on analysis of aerial photographs. and Ž2. regional legislation in 1994 has made remodelling of landforms of geomorphological or geological interest illegal in order to preserve the landscape heritage of these characteristic badland features ŽLegge Regionale, 1994, no. 28.. The imposition of fines is not a sufficient deterrent to prevent remodelling particularly when compared with the economic benefits to the farmer of increasing land productivity, and thus, during 1996–1997 we were fortunate to obtain slope geometry data from an area both before and after remodelling. In all other cases we have chosen to characterise the morphology of badland-type sites ŽSerra Pizzuta, Serra del Purgatorio, Mesola della Zazzera. and take measurements of badland areas adjoining remodelled slopes Žsee Fig. 9 for an example. to compare these data with those for parts of the same slopes that are known to have been remodelled based on field observations and analysis of aerial photographs.
4. Characteristics of badland landforms Two distinct types of badland landform have been recognised in Italy ŽAlexander, 1982; Sdao et al., 1984; Pinna and Vittorini, 1989.: calanchi are steep gullies with knife-edge ridges and biancane are small, conical or dome-shaped forms. In order to
232 M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Fig. 1. Map of the study area showing location of sites described in the text.
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characterise the typical morphology of these landforms in this area of Basilicata, biancane and calanchi were studied at three sites within the Salandrella–Cavone catchment. Serra Pizzuta and Serra del Purgatorio lie to the west and south of the hilltop town of Pisticci, whilst Mesola della Zazzera is located to the northwest of Craco and south of Ferrandina ŽFig. 1.. Measurements of slope length and angles at the sites were undertaken either by Abney level and tapes or surveyed using a Wild T60 theodolite and staff. 4.1. Biancane At the Serra Pizzuta and Serra del Purgatorio sites, biancane forms were surveyed. Slope angles were measured at 0.5-m intervals along north–south and east–west transects. A total of 26 individual biancane forms were studied, 20 from the Serra Pizzuta study area, where biancane forms are developing as a scarp calanchi slope retreats, and six from Serra del Purgatorio, where the forms are dissociated from scarp retreat. In both areas the biancane are developed on a relatively low angle basal pediment of 5–108. The morphological data for the biancane populations from both sites ŽTables 1 and 2. show a dominance of steep slope angles in the range 30–388. Slope angles appear not to differ as a function of aspect ŽTable 1.. Mean slope lengths ŽTable 2. varied from 2.7 m to 5.4 m. Examples of biancane profiles are given in Fig. 2. As the forms get smaller they become more symmetrical, with the same steep slope angles maintained on the smaller forms. The bases of biancane are marked by a sharp break of slope ŽFig. 3. and a low-angle pediment covered by redeposited fine-grained sediment. The steepest slope elements on the biancane, those with slopes in excess of 538, show evidence of gravitational creep. Biancane surfaces are covered by a typical ‘popcorn’ covering of weathered bedrock, traversed by a network of desiccation cracks, which form at a
Table 1 Biancane slope angles measured in 0.5-m-length slope segments Slope aspect N
S
E
W
Serra Pizzuta Maximum Minimum Mean"standard deviation Number of slope segments
638 78 33.4"13.38 125
688 18 35.7"13.18 198
708 18 35.5"14.28 184
628 28 34.3"13.58 133
Serra del Purgatorio Maximum Minimum Mean"standard deviation Number of slope segments
558 138 37.5"8.78 46
688 18 33.2"12.48 33
568 48 31.1"14.78 36
748 38 37.9"13.88 45
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Planform Žm.
Serra Pizzuta Serra del Purgatorio
Height Žm.
Length of slope Žm.
N–S
E–W
N
S
E
W
N
S
E
W
7.52"4.60 5.21"1.56
6.03"3.05 5.27"1.51
1.72"0.80 2.32"0.80
2.91"1.79 1.44"0.83
2.32"1.56 1.48"0.73
1.82"1.03 2.24"1.23
3.70"2.57 3.75"1.33
5.36"3.22 2.67"0.94
4.21"2.74 2.88"1.22
3.35"1.54 3.71"1.55
The mean and standard deviation of measurements from individual biancane are given.
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Table 2 Dimensions and slope lengths of biancane from twenty forms at Serra Pizzuta and six forms at Serra del Purgatorio
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Fig. 2. Profiles of two biancane Ža. and Žb. from Serra Pizzuta.
variety of scales. The depth of the weathered layer is typically in the range 10–20 mm, but may reach as much as 40 mm thick, developing in response to seasonal wetting and
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Fig. 3. Break in slope at base of a biancane at Serra Pizzuta showing the low angle of the basal pediment.
drying. Biancane surfaces may be covered with small rainsplash pillars which develop beneath marine fossils of bivalves as they weather out of the clay. Estimates of the rate of surface lowering of the forms were obtained from a series of erosion pins placed along north–south and east west transects on three of the Serra Pizzuta biancane ŽTable 3.. These show a mean erosion rate of 15–23 mm yry1 for Serra Pizzuta during 1993–1994 which are similar in magnitude to the 19–27 mm yry1 Table 3 Erosion pin data showing the mean and standard deviation of surface lowering Žmm. found on different aspect slopes of biancane at Serra Pizzuta in the period between April 1993 and April 1994 Form
Number of pins
N
S
E
W
Biancana 1 Biancana 2 Biancana 3
66 32 71
19"5 23"6 15"4
22"4 15"9 20"3
21"2 – 20"5
16"7 20"6 20"5
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Table 4 Annual rainfall Žmm. for the period 1980–1988, derived from daily rainfall records at stations in Tuscany and Basilicata Tuscany Siena
Basilicata Taverna d’Arbia Monte Oliveto
Maximum 1048.4 1240.1 Minimum 558.2 487.0 Mean"standard 785.73"157.06 804.97"220.26 deviation
Pomarico
Pisticci
974.2 626.9 899.9 495.6 265.10 319.0 697.26"162.83 413.14"137.40 512.21"181.55
Tuscany data were obtained from the Istituto Idrografico di Pisa and Basilicata data from the Instituto Idrografico di Catanzaro.
found at Stazione Craco ŽFig. 1. in the Salandrella–Cavone catchment during 1978–1980 ŽAlexander, 1982.. The Serra Pizzuta data show no difference in lowering rates as a function of aspect, or with distance from the biancane summit. This implies that biancane erosion at this site is dominated by a process which operates uniformly over the entire form, i.e., rainsplash. Erosion is detachment-limited which is controlled by the thickness of the weathered layer and rate of movement of the wetting front. Mean rates of wetting advance of around 7 mm in 90 min have been found for desiccated unweathered clays from this area ŽRendell, 1975.. Many of the biancane show micro-rilling features with dimensions of around 1–5 mm following topographic low points and randomly traversing bedrock joints, showing that there is clearly no lithological control on the pattern of flow networks. These rills may reflect the collapse of micropiping in the upper 10–15 mm of the weathered crust. The thin fractured and weathered crust cannot sustain significant piping, although micropipes of dimensions - 5 mm may connect crack networks. Field evidence during rainstorm events show that cracks in the weathered surface seal after 7 mm of persistent rain, resulting in overland flow. Micro-piping has been proposed as the dominant process controlling biancane formation in Tuscany ŽTorri et al., 1994; Torri and Bryan, 1997.. This is not the case in Basilicata. The differences in dominant process may be related to differences in climate. The site studied by Torri et al. in southeastern Tuscany receives between 480 and 1240 mm of annual rainfall Žbased on 1980–1988 data; see Table 4., averaging 690 to 805
Table 5 Measurements of calanchi slope angles Calanchi location Slope feature
Serra Pizzuta Major chutes
Minor chutes
Interfluves
Mesola della Zazzera Major chutes
Minimum angle Maximum angle Mean"standard deviation Number of segments
218 548 36.5"6.18 57
288 558 41.5"6.28 32
168 518 37.7"6.78 108
198 65.58 43.02"9.628 109
Angles measured in 0.5-m-length segments upslope.
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Fig. 4. Profiles of calanchi at ŽA. Serra Pizzuta and ŽB. Mesola della Zazzera.
mm yry1 Žin agreement with the values of Torri et al., 1994. and resulting in a superficial weathered layer thickness of 50–150 mm ŽTorri et al., 1994., which is at least five times the depth of the weathered layer found in the more arid area of
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239
Basilicata. Annual rainfall in this region of Basilicata varied from 413 to 512 mm between 1980 and 1988 Žthe comparable period to the Tuscany data.. Depth of the weathered layer will control piping efficiency, and ultimately affect the overall form of the biancane ŽTorri and Bryan, 1997.. Comparison of Basilicata biancane forms with those described by Calzolari and Ungaro Ž1998. in Tuscany appears to confirm that there is a genuine difference in form types. For example, the maintenance of form with increasingly smaller biancane found at Serra Pizzuta and Serra del Purgatorio sites in Basilicata contradicts the findings of Torri and Bryan Ž1997. who argue that in Tuscany, the dominance of micropiping results in a penultimate collapsed souffle-like form. ´ Biancana form types are defined and described for Tuscany by Calzolari and Ungaro Ž1998.. 4.2. Calanchi In Basilicata, calanchi forms are seen on erosional terrace scarps of relict fluvial systems and incised bowl-shaped gully head areas close to hill crests. In this study we have concentrated on those features associated with the more prevalent erosional scarps. Calanchi were studied at two of these sites on south-facing escarpments at Serra Pizzuta and Mesola della Zazzera. Profiles were obtained in 0.5-m segments ŽTable 5; Figs. 4 and 5.. At both sites gully thalwegs, or chutes, were separated by knife-edge ridges. The Serra Pizzuta profiles indicate that chutes and ridges are parallel to each other ŽFig. 2. suggesting that all features on the slope are retreating at the same rate allowing maintenance of high slope angles Ž36.58 to 41.58.. At the Mesola della Zazzera site, the
Fig. 5. The calanchi front at Mesola della Zazzera.
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measured chutes were slightly steeper Ž438. than those at Serra Pizzuta. Calanchi slope lengths are in the estimated range of 25–60 m Žbeing too steep to measure by hand for the entire length. at overall slope angles greater than 358. In addition to the geomorphic processes operating on biancane forms Žwetting and drying, rainsplash, micro-piping and micro-rilling., the calanchi in these study areas show both visual ŽFig. 6. and morphological evidence of acting as effective debris flow chutes for weathered material accumulated in the headward regions of their discrete drainage basins. At both study sites the break of slope at the base of the calanchi chutes occurred at an accumulation zone forming debris flow cones Žsee Fig. 5.. 5. Slope remodelling The remodelling of badland landforms in Italy has involved: Ži. the modification of calanchi forms by smoothing out the sharp break of slope at the top Žhead. of the calanchi and removal of the knife-edge ridges; Žii. the isolation of calanchi forms, leaving small areas on otherwise remodelled slopes; Žiii. the obliteration of calanchi forms, creating a completely new slope; and Živ. the obliteration of biancane to produce a smoothed slope surface, created by cutting off the tops of the hummocks. With the availability of increasingly powerful earth-moving equipment, the tendency appears to have been to favour options Žii. to Živ. ŽCalzecchi-Onesti, 1957.. In Basilicata, remodelling carried out within the last 20 years has involved the implementation of options Žiii. and Živ. resulting in the complete obliteration of badland forms.
Fig. 6. 3Ža. Active erosion on calanchi during a rainstorm, Basento Valley southeast of Tricarico. Žb. Evidence for debris flows after a rainstorm at Serra Pizzuta.
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Fig. 6 Žcontinued..
Examples of the kinds of morphological changes involved in remodelling both biancane and calanchi in the Salandrella–Cavone river catchment are shown in Table 6. Two examples of remodelling biancane and three of calanchi have been studied and are described below. Observation of these new slopes suggests that they appear to be prone to rilling and gully development. 5.1. Remodelled biancane At Manca Tortoniero, which lies approximately 2 km to the west of the Pisticci–Craco road, two biancane swarms, located on opposite slopes of a drainage divide, were observed being actively remodelled in August 1996 ŽFig. 7a.. The site was revisited in August 1997 ŽFig. 7b. and measurements were taken of the change in slope geometry. Both newly remodelled fields Žone facing ESE and the other WNW. showed evidence of
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Location
Initial badland landform
Slope aspect
Initial slope angles
Initial slope length Žm.
Remodelled slope angles
Remodelled slope length Žm.
Change in angle
Change in slope length Žm.
Manca Tortoniero Manca Tortoniero Pantone della Fica San Bernardino La Piana
Biancane swarm Biancane swarm Calanchi Calanchi Calanchi
ESE WNW WSW W SW
378 378 408 418 468
-5 -5 25 22 63
168 178 178 128 168
113 99 90 34 90
y218 y208 y238 y298 y308
q108 q94 q65 q12 q27
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Table 6 The change in slope morphology and characteristic created by hillslope transformations achieved by remodelling in Basilicata
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Fig. 7. Ža. Biancane on an ESE facing slope at Manca Tortoniero being actively remodelled, August 1996. Žb. The same location in August 1997. The abandoned town of Craco is visible on the skyline to the top left of the photograph.
active soil erosion during the subsequent winter of 1996–1997, exhibiting large rills at topographic low points in the slope. The rill formed in the ESE facing slope ŽFig. 8a.
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Fig. 8. Large rills formed in the topographic lowpoints of the newly remodelled fields at Manca Tortoniero Ža. on the ESE slope—note the gully system at the field base Žb. on the WNW slope.
was 101.2 m long and ranged in depth from 0.16 to 0.32 m and width from 0.18 to 0.49 m wide. The rill in the WNW facing slope ŽFig. 8b. was 69.3 m long and ranged in depth from 0.06 m to 0.28 m and width from 0.09 to 0.32 m wide. Calculations of rill
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geometry indicate that erosion of slope sediment and rill formation during the winter of 1996–1997 resulted in removal of at least 8.6 m3 and 5.7 m3 of sediment from the ESE and WNW fields, respectively. Inspection of slope surfaces in August 1997 revealed the presence of networks of pronounced desiccation cracks up to 0.03 m wide and 0.5 m deep and which may leave the slopes preferentially prone to subsurface erosion by piping. The large rills on both slopes are interconnected with major gully systems located at the field base Žheadcuts currently 2.5 m deep feeding into channels in excess of 15 m deep. which in turn drain into the Salandrella River. Thus, remodelling these slopes has resulted in the formation of a connecting link with the ephemeral drainage network catchment of the Salandrella–Cavone system. Soil eroded from the large rills formed in these remodelled fields in winter 1996–1997, can therefore potentially be transported into the perennial fluvial system of the Salandrella–Cavone and out into the Ionian Sea. Future monitoring of these fields is planned with particular interest in the potential of headward retreat of the gully system upslope fed by the connecting large rills and potential pipes fed from large surface cracks. Headward extension of the gully systems would lead to an irreversible loss in field size, once the erosional forms are too large to be ploughed in. 5.2. Remodelled calanchi Three remodelled calanchi sites were studied ŽFig. 1.. All of these sites were remodelled between 1986 and 1996, based on aerial photograph coverage and field surveillance.
Fig. 9. Remodelled slope at La Piana on the Torrente il Gruso.
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5.2.1. Pantone della Fica The calanchi front at Pantone della Fica faces 2408 ŽWSW. with a 408 scarp which is 25 m long. Part of this front has been remodelled resulting in a lower angle slope of 178 and an increase in slope length to 90 m, and has involved the obliteration of a dirt road which ran along the base of the old calanchi front. Formation of a lower slope angle has resulted in the creation of a new dirt road used by local farm traffic, orthogonal to the old road and running upslope and over the top of the remodelled calanchi, providing a compacted surface for overland flow runoff during storm events. Whilst probably being the most direct route, the creation of a road through the calanchi badlands would be impossible prior to remodelling due to the extreme steepness of the 408 slope. As at other locations in this area where dirt roads traverse slopes, the new road provides an ideal focus for future gully development, resulting from the creation of a local drainage system and flow convergence on to the road during storm events. Runoff from the road at breaks of slope or below bends results in sediment entrainment and thus, this site, like many others on this clay terrain, has a high future soil loss potential. 5.2.2. San Bernardino The San Bernardino site lies to the northwest of Pantone della Fica and consists of a remodelled slope adjacent to the remnants of the previously extensive calanchi scarp. The slopes face 2708 ŽW. and are located on the east side of a minor road to Ferrandina. Slope angle on the calanchi scarp is 418 and the length of slope is 22 m and this has
Fig. 10. Ža. A rotational landslide in a remodelled field near Craco after harvest, September 1997. Žb. Shallow debris flows in a remodelled slope near Grassano, cultivated with durum wheat, April 1996.
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Fig. 10 Žcontinued..
been remodelled into a 34-m slope of only 128. The remodelled slope had been used for durum wheat cultivation during the 1996 and 1997 growing seasons. 5.2.3. La Piana This site, shown in Fig. 9, lies on the northern bank of the Torrente il Gruso which is a tributary of the Salandrella River, southeast of Ferrandina. Measurements on the adjacent calanchi scarp to the east of the remodelled slope indicate that bare 63-m slopes of 468 have been converted into 90-m slopes of 168 and cultivated with durum wheat. Wheat growth at this site is very patchy implying low overall yields. Poor land management is indicated by the blue-grey nature of the remodelled field resulting from the deep ploughing of bedrock. Nevertheless, an attempt has been made to cultivate wheat on the bedrock without the addition of substantial quantities of organic matter to improve soil fertility.
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6. Slope remodelling impacts on landscape morphology and soil erosion The slope transformations described above involve increasing slope lengths whilst decreasing slope angles by more than 208. These changes have repercussions for the spatial organisation and coupling of erosion processes on the slopes. Prior to remodelling, badland slopes dominated by calanchi or biancane, although visually spectacular, are characterised by very localised processes of erosion and deposition of material. The biancane at Serra Pizzuta may exhibit erosion rates of 20 mm yry1 but the material eroded is deposited adjacent to the erosional landforms. Similarly, the calanchi exhibit very localised deposition of material carried by mass movement or debris flows. In many cases these badlands are either completely or almost completely spatially divorced from local channel systems so that sediment delivery to the channel systems does not occur. Although the calanchi and biancane forms are mostly unvegetated, they are surrounded by areas of scrub vegetation. After remodelling, ploughed slopes remain unvegetated, this is critical during the late summer and early autumn periods and when they are particularly vulnerable to the type of convective rainstorms that characterise the end of summer in central and southern Italy. The slope transformations increase the effective catchment for rainfall by reducing slope angle Ža change from 408 to 158 for the same slope length results in an increase of 26% in catchment area for rain falling vertically. and increasing slope length. Whereas detachment of material by rainsplash is the dominant process on the biancane, on the remodelled slopes rainsplash, ponding and overland flow create process interactions delivering sediment off-slope and potentially off-site. As has been shown, the new slope lengths will facilitate the development of concentrated overland flow with the development of rills and gullies delivering water and sediment downslope and off-field into water courses and on to roads. Another impact of remodelling is an apparent increase in the occurrence of relatively small-scale rotational landslides ŽFig. 10. and shallow debris flows, with typical depths of 0.8 m. These tend to be sufficiently superficial in morphology that, like many rills, they can be ploughed in at the beginning of each cultivation cycle and are therefore recurrent but short-lived landscape features. Deep ploughing techniques used in this area turn over sediment to a depth of 1.4 m.
7. Implications of changing land use policy The practice of remodelling badland hillslopes to increase agricultural productivity from the clay landscape has been exacerbated by the provision of EU subsidies to cultivate durum wheat. The pattern of subsidy changed in June 1992 with the introduction of a staged reduction in intervention price for durum wheat. This change has been coupled with a dual subsidy system whereby for 1995–1996, a hectarage payment of around 643,000 lire hay1 was given with an additional yield-related value of 163,000 lire hay1 Žfor a yield of 3 t hay1 . which amounts to at least 40% of farm income ŽBianchi et al., 1993.. The continuing hectarage-based economic incentive to increase landscape productivity has inadvertently resulted in an increase in soil erosion in these
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areas, where comparatively stable badland landforms have been replaced by lower angle agricultural fields which have a higher potential for soil erosion by off-site removal of sediment. The issue of cereal subsidies has, to some extent, been balanced by the provision of subsidies to farmers to maintain areas of fallow land by the introduction of ‘set-aside’. The decision by the EU in 1997 to discontinue subsidising set-aside will have a negative effect on measures to prevent soil erosion, as it becomes economically more beneficial to cultivate all fallow land. Economic incentives may result in a further increase in the number of illegally remodelled badland slopes. The use of subsidies for specific crops may also increase the likelihood of soil erosion. For example, in the 1997 growing season, there has been an increase in the number of fields in the study area given over to the cultivation of sunflowers. The percentage ground cover of crops like sunflowers is low and thus, the potential for soil erosion during a growing season is greater than for a larger ground cover crop such as durum wheat. Land use policy and its subsequent economic implications are thus important in determining some of the controlling factors on soil erosion.
8. Conclusions In Basilicata, the practice of using heavy earth-moving machinery to remodel calanchi and biancane badland landforms changes slope morphology. Initially steep badland scarps of ) 358 are lowered by 20–308 to form more gentle slopes of 12–178 which can be ploughed and then cultivated for cereals. Slope lengths are also greatly increased resulting in a spatial coupling of soil erosion processes and an extension of existing drainage networks. This change in land use, promoted by economic incentives, has resulted in an increase in potential soil erosion in this area. The effects of ‘soil quarrying’ for infrastructure improvement or land preparation may be considered to be soil degradation processes ŽPoesen and Hooke, 1997. in that soil is physically removed from the landscape or locally repositioned on it. However, it is the geomorphological and associated hydrological changes to the slopes, as a result of these landscape remodelling activities, that have the potential for significantly increasing soil erosion and land degradation.
Acknowledgements We would like to thank Dott. F. Belelli of the Unione Provincale Agricoltori di Siena and Dott. C. de Giovanni of the Istituto di Allevamento delle Piante, Bari for technical data, Geom. G. Alfieri of the Istituto Idrografico di Catanzaro for meteorological data from Basilicata, and the Santangelo family of Tricarico for their continuing hospitality.
References Alexander, D.E., 1982. Difference between ‘calanchi’ and ‘biancane’ badlands in Italy. In: Bryan, R., Yair, A. ŽEds.., Badland Geomorphology and Piping. Geo Books, Norwich, UK, pp. 71–87.
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Bianchi, A., Boggini, G., Di Fonzo, N., Pasquini, M., Pogna, N.E., Ronga, G., Azzoni, A., 1993. La risposta della durogranicoltura italiana alla nuova politica agricola comunitaria. L’Informatore Agrario 34, 27–33. Calzecchi-Onesti, A., 1954. Sistemazioni del Terreno e Fertilita` in Collina. Ramo Editoriale Degli Agricoltori, Rome, 107 pp. Calzecchi-Onesti, A., 1957. Sistemazioni in Collina. Ramo Editoriale Degli Agricoltori, Rome, 350 pp. Calzolari, C., Ungaro, F., 1998. Characterization and quantitative spatial analysis of geomorphic features of a badland Žbiancane. area Žcentral Italy.. Catena 31, 237–256. Cataudella, M. ŽEd.., 1987. Altante della Basilicata: il Territorio per Immagini. Regione Basilicata, 47 pp. ENI-AGIP, 1972. Acque Dolce Sotteranee. Milano. Guasparri, G., 1993. I lineamenti geomorfologici dei terreni argillosi pliocenici. In: La Storia Naturale della Toscana Meridionale. Monte dei Paschi, Siena, pp. 89–106. Landi, R., 1989. Revision of land management systems in Italian hilly areas. In: Schwertmann, U., Rickson, R.J., Auerswald ŽEds.., Soil Erosion Protection Measures in Europe. Catena Soil Technology Series, 1, pp. 175–188. Legge Regionale, 1994. Individuazione, classificazione, istituzione, tutela e gestione delle aree naturali protette in Basilicata. 28 June 1994, No. 28. Pavari, A., 1911. Le crete senesi e lo scasso con la dinamite. L’Italia Agricola, Piacenza. Perez-Trejo, F., 1994. Desertification and land degradation in the European Mediterranean. EUR 14850 EN, Office of the Official Publications of the European Communities, Luxembourg. Pinna, S., Vittorini, S., 1989. Su alcune caratteristiche delle argille plioceniche della Valle Dell’Era ŽToscana. in rapporto alla genesi di calanchi e biancane. Geografia Fisica Dinamica Quaternaria 12, 131–137. Poesen, J.W.A., Hooke, J.M., 1997. Erosion, flooding and channel management in Mediterranean environments of southern Europe. Progress in Physical Geography 21, 157–199. Radina, B., 1964. Contributo alla conoscenza del dissesto geologico del versante Ionico Lucano. Bolletino Societa Naturalistica, Napoli 73, 211–265. Rendell, H.M., 1975. Clay hillslope erosion in a semi-arid environment: the Basento Valley, southern Italy. Unpublished PhD thesis, University College London. Rendell, H.M., 1982. Clay hillslope erosion rates in the Basento Valley, S. Italy. Geografiska Annaler 64A, 141–147. Rendell, H.M., 1986. Soil erosion and land degradation in southern Italy. In: Fantechi, R., Margaris, N.S. ŽEds.., Desertification in Europe. Commission of European Communities, Brussels, pp. 184–193. Sabato, L., Tropeano, M., 1994. Cenni sul sistema catena–avanfossa–avampaese in Italia Meridionale. In: Guida alle escurzioni. Congresso. Soc. Geol. Italia, Bari, 1994. Quaderni Bibliografico della Provincia di Matera, 15, pp. 11–32. Sdao, G., Simone, A., Vittorini, S., 1984. Osservazioni geomorfologiche su calanchi e biancane in Calabria. Geografia Fisica Dinamica Quaternaria 7, 10–16. Thornes, J., Brandt, J., 1996. Mediterranean Desertification and Land Use. Wiley, Chichester. Torri, D., Bryan, R., 1997. Micropiping processes and biancane evolution in southeast Tuscany, Italy. Geomorphology 20, 219–235. Torri, D., Colica, A., Rockwell, D., 1994. Preliminary study of the erosion mechanisms in a biancana badland ŽTuscany, Italy.. Catena 23, 281–294. Zanchi, C., 1989. Drainage as a soil conservation and soil stabilizing practice on hilly slopes. In: Schwertmann, U., Rickson, R.J., Auerswald ŽEds.., Soil Erosion Protection Measures in Europe. Catena Soil Technology Series, 1, pp. 73–82.
The impact of the farming practice of remodelling hillslope topography on badland morphology and soil erosion processes Michele ` L. Clarke a
a,)
, Helen M. Rendell
b
Centre for EnÕironmental Management, School of Geography, UniÕersity of Nottingham, UniÕersity Park, Nottingham NG7 2RD, UK b Geography Laboratory, Arts C, UniÕersity of Sussex, Falmer, Brighton BN1 9QN, UK Received 13 January 1998; received in revised form 11 August 1998; accepted 23 September 1998
Abstract Badland landforms, created in Plio–Pleistocene clay landscapes of the Basilicata region of southern Italy, form marginal features in a semi-arid landscape dominated by the widespread cultivation of subsidised cereals. These badland features are high relative relief forms exhibiting a high drainage density and steep slopes, with slope angles typically in excess of 358. Economic incentives to increase agricultural productivity have resulted in the practice of remodelling these marginal areas using heavy earth-moving equipment. Remodelling the badland features creates longer slopes at lower angles, which can be cultivated using conventional farm machinery. These changes in hillslope morphology have altered the degree to which soil erosion processes operating in these areas are spatially coupled. In the badland areas, erosion and deposition are strongly localised with minimal sediment delivery to ephemeral or perennial channel systems. The economically-driven change in land use from visually striking badland areas to newly remodelled fields for agricultural use results in an increase in the coupling of drainage networks and a net increase in soil erosion. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Badlands; Calanchi; Biancane; Soil erosion; Land use change; Common agricultural policy
1. Introduction Visually striking badland landscapes are found in fine-grained clastic sedimentary bedrock regions of central and southern Italy, principally in the provinces of Marche )
Corresponding author. E-mail: [email protected]
0341-8162r00r$20.00 q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 9 . 0 0 0 4 7 - 8
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ŽCalzecchi-Onesti, 1957., Tuscany ŽGuasparri, 1993. and Basilicata ŽRendell, 1982, 1986.. These badlands consist of deeply-dissected, unvegetated or poorly-vegetated landforms of high relative relief and high drainage density. In the region of Basilicata Žformerly known as Lucania. it has been estimated that badlands occupy 32,125 ha in the Bradano, Basento, Cavone and Agri catchments ŽRadina, 1964.. The badland areas are located in the middle and lower reaches of these catchments on Plio–Pleistocene marine clays, which were deposited as part of the Fossa Bradanica ŽBradanica Trough. fill complex ŽSabato and Tropeano, 1994.. The climate of Basilicata ranges from high mountains areas in the western part of the region with mean annual rainfall in excess of 2000 mm and mean annual temperature of 108C to relatively low-lying areas bordering the Ionian Sea with mean annual rainfall of less than 700 mm and mean annual temperature in excess of 158C ŽCataudella, 1987.. The climate of the Salandrella–Cavone area of Basilicata is Mediterranean semi-arid with more than 65% of rainfall occurring during the autumn and winter months. Summers are characterised by long periods without rain, with temperatures in excess of 408C and relative humidities of less than 40% ŽServizio Meteorologico Aeronautica Militare.. Within the main river valleys, agricultural productivity on river terraces is enhanced by irrigation allowing the cultivation of citrus fruit orchards and crops such as tomatoes, melons and strawberries ŽRendell, 1986.. On higher slopes and in the majority of the river catchments, current land use is dominated by the European Union ŽEU. subsidised cultivation of durum wheat Žfor pasta., which can be grown on the clay terrain, without artificial irrigation, yielding about 2.5 t hay1 ŽBianchi et al., 1993.. Most winter cereal cultivation occurs on slopes shallower than 208 but the use of caterpillar-tracked machinery has enabled steeper slopes to be cultivated and thus badland features have been increasingly confined to marginal scarp slopes. Changing land management strategies and increased mechanisation has resulted in the deep ploughing of slopes to increase water retention capacity ŽLandi, 1989.. This improvement in mechanisation and an increase in crop specialisation has resulted in economical changes which, in order to improve the cost–benefit ratio to the farmer, has lead to a general increase in field area, removal of terraces and a ‘smoothing of hillslopes’ ŽZanchi, 1989.. These economic incentives have meant that there is a tradition of using bulldozers ŽCalzecchi-Onesti, 1954, 1957. and even explosives ŽPavari, 1911. to remodel and reclaim areas of badland landforms in Italy for agricultural productivity. In Tuscany, Guasparri Ž1993. has shown that substantial areas of former badlands have been obliterated since the 1960s and in Basilicata, major remodelling of badland slopes has occurred since the 1970s ŽRendell, 1986. and is still continuing today Žsee Fig. 7.. Badlands are conventionally considered to be areas of extreme soil erosion. This paper describes the impact on soil erosion of remodelling these badlands area for increased agricultural productivity. Land degradation by severe soil erosion has been linked to desertification in this and other parts of the Mediterranean Basin ŽThornes and Brandt, 1996. and it has been suggested that there is an increasing risk of further degradation in response to both physical and socioeconomic factors ŽPerez-Trejo, 1994.. The Salandrella–Cavone study area is therefore an ideal location in which to examine the interaction of these factors in relation to land use change.
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
231
2. Study area The study area lies between the Basento and Agri rivers, predominantly in the Salandrella–Cavone river catchment ŽFig. 1.. The area is underlain by up to 500 m ŽENI-AGIP, 1972. of Plio–Pleistocene clays which are capped in places by locally variable, uncemented to partially-cemented, Upper Pleistocene sands and conglomerates. Uplift and differential incision has left pedestal-type outcrops of the conglomerate which lie up to 250 m above the river valleys ŽRendell, 1975. and support hilltop settlements such as Pisticci, Ferrandina and Pomarico. At Pisticci, the clays are incised to a depth of 200 m relative to the current valley of the Cavone river. Clay mineralogy shows a dominance of sodium montmorillonite, although kaolinite and illite are also present ŽRendell, 1975.. The clays are inorganic silty-clays of medium plasticity and disperse rapidly when wetted ŽRendell, 1982, 1986. making them prone to both surface and subsurface erosion. The grain size of the clays is typically 55–65% by weight finer than 8 f Ž3.9 mm. and 90–95% finer than 4f Ž62.5 mm. ŽRendell, 1975.. Given the dispersive nature of the clays, the high relative relief and the seasonality of the climate, it has been suggested that this area is prone to severe soil erosion ŽAlexander, 1982; Cataudella, 1987..
3. Land use change impacts on badland landforms In order to assess the impact of landscape remodelling on soil erosion it is necessary to compare the initial and final topographic forms involved in hillslope modification. This is extremely difficult because Ž1. the majority of the landscape remodelling occurred prior to 1986 Žbased on analysis of aerial photographs. and Ž2. regional legislation in 1994 has made remodelling of landforms of geomorphological or geological interest illegal in order to preserve the landscape heritage of these characteristic badland features ŽLegge Regionale, 1994, no. 28.. The imposition of fines is not a sufficient deterrent to prevent remodelling particularly when compared with the economic benefits to the farmer of increasing land productivity, and thus, during 1996–1997 we were fortunate to obtain slope geometry data from an area both before and after remodelling. In all other cases we have chosen to characterise the morphology of badland-type sites ŽSerra Pizzuta, Serra del Purgatorio, Mesola della Zazzera. and take measurements of badland areas adjoining remodelled slopes Žsee Fig. 9 for an example. to compare these data with those for parts of the same slopes that are known to have been remodelled based on field observations and analysis of aerial photographs.
4. Characteristics of badland landforms Two distinct types of badland landform have been recognised in Italy ŽAlexander, 1982; Sdao et al., 1984; Pinna and Vittorini, 1989.: calanchi are steep gullies with knife-edge ridges and biancane are small, conical or dome-shaped forms. In order to
232 M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Fig. 1. Map of the study area showing location of sites described in the text.
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
233
characterise the typical morphology of these landforms in this area of Basilicata, biancane and calanchi were studied at three sites within the Salandrella–Cavone catchment. Serra Pizzuta and Serra del Purgatorio lie to the west and south of the hilltop town of Pisticci, whilst Mesola della Zazzera is located to the northwest of Craco and south of Ferrandina ŽFig. 1.. Measurements of slope length and angles at the sites were undertaken either by Abney level and tapes or surveyed using a Wild T60 theodolite and staff. 4.1. Biancane At the Serra Pizzuta and Serra del Purgatorio sites, biancane forms were surveyed. Slope angles were measured at 0.5-m intervals along north–south and east–west transects. A total of 26 individual biancane forms were studied, 20 from the Serra Pizzuta study area, where biancane forms are developing as a scarp calanchi slope retreats, and six from Serra del Purgatorio, where the forms are dissociated from scarp retreat. In both areas the biancane are developed on a relatively low angle basal pediment of 5–108. The morphological data for the biancane populations from both sites ŽTables 1 and 2. show a dominance of steep slope angles in the range 30–388. Slope angles appear not to differ as a function of aspect ŽTable 1.. Mean slope lengths ŽTable 2. varied from 2.7 m to 5.4 m. Examples of biancane profiles are given in Fig. 2. As the forms get smaller they become more symmetrical, with the same steep slope angles maintained on the smaller forms. The bases of biancane are marked by a sharp break of slope ŽFig. 3. and a low-angle pediment covered by redeposited fine-grained sediment. The steepest slope elements on the biancane, those with slopes in excess of 538, show evidence of gravitational creep. Biancane surfaces are covered by a typical ‘popcorn’ covering of weathered bedrock, traversed by a network of desiccation cracks, which form at a
Table 1 Biancane slope angles measured in 0.5-m-length slope segments Slope aspect N
S
E
W
Serra Pizzuta Maximum Minimum Mean"standard deviation Number of slope segments
638 78 33.4"13.38 125
688 18 35.7"13.18 198
708 18 35.5"14.28 184
628 28 34.3"13.58 133
Serra del Purgatorio Maximum Minimum Mean"standard deviation Number of slope segments
558 138 37.5"8.78 46
688 18 33.2"12.48 33
568 48 31.1"14.78 36
748 38 37.9"13.88 45
234
Planform Žm.
Serra Pizzuta Serra del Purgatorio
Height Žm.
Length of slope Žm.
N–S
E–W
N
S
E
W
N
S
E
W
7.52"4.60 5.21"1.56
6.03"3.05 5.27"1.51
1.72"0.80 2.32"0.80
2.91"1.79 1.44"0.83
2.32"1.56 1.48"0.73
1.82"1.03 2.24"1.23
3.70"2.57 3.75"1.33
5.36"3.22 2.67"0.94
4.21"2.74 2.88"1.22
3.35"1.54 3.71"1.55
The mean and standard deviation of measurements from individual biancane are given.
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Table 2 Dimensions and slope lengths of biancane from twenty forms at Serra Pizzuta and six forms at Serra del Purgatorio
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
235
Fig. 2. Profiles of two biancane Ža. and Žb. from Serra Pizzuta.
variety of scales. The depth of the weathered layer is typically in the range 10–20 mm, but may reach as much as 40 mm thick, developing in response to seasonal wetting and
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
236
Fig. 3. Break in slope at base of a biancane at Serra Pizzuta showing the low angle of the basal pediment.
drying. Biancane surfaces may be covered with small rainsplash pillars which develop beneath marine fossils of bivalves as they weather out of the clay. Estimates of the rate of surface lowering of the forms were obtained from a series of erosion pins placed along north–south and east west transects on three of the Serra Pizzuta biancane ŽTable 3.. These show a mean erosion rate of 15–23 mm yry1 for Serra Pizzuta during 1993–1994 which are similar in magnitude to the 19–27 mm yry1 Table 3 Erosion pin data showing the mean and standard deviation of surface lowering Žmm. found on different aspect slopes of biancane at Serra Pizzuta in the period between April 1993 and April 1994 Form
Number of pins
N
S
E
W
Biancana 1 Biancana 2 Biancana 3
66 32 71
19"5 23"6 15"4
22"4 15"9 20"3
21"2 – 20"5
16"7 20"6 20"5
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237
Table 4 Annual rainfall Žmm. for the period 1980–1988, derived from daily rainfall records at stations in Tuscany and Basilicata Tuscany Siena
Basilicata Taverna d’Arbia Monte Oliveto
Maximum 1048.4 1240.1 Minimum 558.2 487.0 Mean"standard 785.73"157.06 804.97"220.26 deviation
Pomarico
Pisticci
974.2 626.9 899.9 495.6 265.10 319.0 697.26"162.83 413.14"137.40 512.21"181.55
Tuscany data were obtained from the Istituto Idrografico di Pisa and Basilicata data from the Instituto Idrografico di Catanzaro.
found at Stazione Craco ŽFig. 1. in the Salandrella–Cavone catchment during 1978–1980 ŽAlexander, 1982.. The Serra Pizzuta data show no difference in lowering rates as a function of aspect, or with distance from the biancane summit. This implies that biancane erosion at this site is dominated by a process which operates uniformly over the entire form, i.e., rainsplash. Erosion is detachment-limited which is controlled by the thickness of the weathered layer and rate of movement of the wetting front. Mean rates of wetting advance of around 7 mm in 90 min have been found for desiccated unweathered clays from this area ŽRendell, 1975.. Many of the biancane show micro-rilling features with dimensions of around 1–5 mm following topographic low points and randomly traversing bedrock joints, showing that there is clearly no lithological control on the pattern of flow networks. These rills may reflect the collapse of micropiping in the upper 10–15 mm of the weathered crust. The thin fractured and weathered crust cannot sustain significant piping, although micropipes of dimensions - 5 mm may connect crack networks. Field evidence during rainstorm events show that cracks in the weathered surface seal after 7 mm of persistent rain, resulting in overland flow. Micro-piping has been proposed as the dominant process controlling biancane formation in Tuscany ŽTorri et al., 1994; Torri and Bryan, 1997.. This is not the case in Basilicata. The differences in dominant process may be related to differences in climate. The site studied by Torri et al. in southeastern Tuscany receives between 480 and 1240 mm of annual rainfall Žbased on 1980–1988 data; see Table 4., averaging 690 to 805
Table 5 Measurements of calanchi slope angles Calanchi location Slope feature
Serra Pizzuta Major chutes
Minor chutes
Interfluves
Mesola della Zazzera Major chutes
Minimum angle Maximum angle Mean"standard deviation Number of segments
218 548 36.5"6.18 57
288 558 41.5"6.28 32
168 518 37.7"6.78 108
198 65.58 43.02"9.628 109
Angles measured in 0.5-m-length segments upslope.
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Fig. 4. Profiles of calanchi at ŽA. Serra Pizzuta and ŽB. Mesola della Zazzera.
mm yry1 Žin agreement with the values of Torri et al., 1994. and resulting in a superficial weathered layer thickness of 50–150 mm ŽTorri et al., 1994., which is at least five times the depth of the weathered layer found in the more arid area of
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239
Basilicata. Annual rainfall in this region of Basilicata varied from 413 to 512 mm between 1980 and 1988 Žthe comparable period to the Tuscany data.. Depth of the weathered layer will control piping efficiency, and ultimately affect the overall form of the biancane ŽTorri and Bryan, 1997.. Comparison of Basilicata biancane forms with those described by Calzolari and Ungaro Ž1998. in Tuscany appears to confirm that there is a genuine difference in form types. For example, the maintenance of form with increasingly smaller biancane found at Serra Pizzuta and Serra del Purgatorio sites in Basilicata contradicts the findings of Torri and Bryan Ž1997. who argue that in Tuscany, the dominance of micropiping results in a penultimate collapsed souffle-like form. ´ Biancana form types are defined and described for Tuscany by Calzolari and Ungaro Ž1998.. 4.2. Calanchi In Basilicata, calanchi forms are seen on erosional terrace scarps of relict fluvial systems and incised bowl-shaped gully head areas close to hill crests. In this study we have concentrated on those features associated with the more prevalent erosional scarps. Calanchi were studied at two of these sites on south-facing escarpments at Serra Pizzuta and Mesola della Zazzera. Profiles were obtained in 0.5-m segments ŽTable 5; Figs. 4 and 5.. At both sites gully thalwegs, or chutes, were separated by knife-edge ridges. The Serra Pizzuta profiles indicate that chutes and ridges are parallel to each other ŽFig. 2. suggesting that all features on the slope are retreating at the same rate allowing maintenance of high slope angles Ž36.58 to 41.58.. At the Mesola della Zazzera site, the
Fig. 5. The calanchi front at Mesola della Zazzera.
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M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
measured chutes were slightly steeper Ž438. than those at Serra Pizzuta. Calanchi slope lengths are in the estimated range of 25–60 m Žbeing too steep to measure by hand for the entire length. at overall slope angles greater than 358. In addition to the geomorphic processes operating on biancane forms Žwetting and drying, rainsplash, micro-piping and micro-rilling., the calanchi in these study areas show both visual ŽFig. 6. and morphological evidence of acting as effective debris flow chutes for weathered material accumulated in the headward regions of their discrete drainage basins. At both study sites the break of slope at the base of the calanchi chutes occurred at an accumulation zone forming debris flow cones Žsee Fig. 5.. 5. Slope remodelling The remodelling of badland landforms in Italy has involved: Ži. the modification of calanchi forms by smoothing out the sharp break of slope at the top Žhead. of the calanchi and removal of the knife-edge ridges; Žii. the isolation of calanchi forms, leaving small areas on otherwise remodelled slopes; Žiii. the obliteration of calanchi forms, creating a completely new slope; and Živ. the obliteration of biancane to produce a smoothed slope surface, created by cutting off the tops of the hummocks. With the availability of increasingly powerful earth-moving equipment, the tendency appears to have been to favour options Žii. to Živ. ŽCalzecchi-Onesti, 1957.. In Basilicata, remodelling carried out within the last 20 years has involved the implementation of options Žiii. and Živ. resulting in the complete obliteration of badland forms.
Fig. 6. 3Ža. Active erosion on calanchi during a rainstorm, Basento Valley southeast of Tricarico. Žb. Evidence for debris flows after a rainstorm at Serra Pizzuta.
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
241
Fig. 6 Žcontinued..
Examples of the kinds of morphological changes involved in remodelling both biancane and calanchi in the Salandrella–Cavone river catchment are shown in Table 6. Two examples of remodelling biancane and three of calanchi have been studied and are described below. Observation of these new slopes suggests that they appear to be prone to rilling and gully development. 5.1. Remodelled biancane At Manca Tortoniero, which lies approximately 2 km to the west of the Pisticci–Craco road, two biancane swarms, located on opposite slopes of a drainage divide, were observed being actively remodelled in August 1996 ŽFig. 7a.. The site was revisited in August 1997 ŽFig. 7b. and measurements were taken of the change in slope geometry. Both newly remodelled fields Žone facing ESE and the other WNW. showed evidence of
242
Location
Initial badland landform
Slope aspect
Initial slope angles
Initial slope length Žm.
Remodelled slope angles
Remodelled slope length Žm.
Change in angle
Change in slope length Žm.
Manca Tortoniero Manca Tortoniero Pantone della Fica San Bernardino La Piana
Biancane swarm Biancane swarm Calanchi Calanchi Calanchi
ESE WNW WSW W SW
378 378 408 418 468
-5 -5 25 22 63
168 178 178 128 168
113 99 90 34 90
y218 y208 y238 y298 y308
q108 q94 q65 q12 q27
M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Table 6 The change in slope morphology and characteristic created by hillslope transformations achieved by remodelling in Basilicata
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243
Fig. 7. Ža. Biancane on an ESE facing slope at Manca Tortoniero being actively remodelled, August 1996. Žb. The same location in August 1997. The abandoned town of Craco is visible on the skyline to the top left of the photograph.
active soil erosion during the subsequent winter of 1996–1997, exhibiting large rills at topographic low points in the slope. The rill formed in the ESE facing slope ŽFig. 8a.
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M.L. Clarke, H.M. Rendellr Catena 40 (2000) 229–250
Fig. 8. Large rills formed in the topographic lowpoints of the newly remodelled fields at Manca Tortoniero Ža. on the ESE slope—note the gully system at the field base Žb. on the WNW slope.
was 101.2 m long and ranged in depth from 0.16 to 0.32 m and width from 0.18 to 0.49 m wide. The rill in the WNW facing slope ŽFig. 8b. was 69.3 m long and ranged in depth from 0.06 m to 0.28 m and width from 0.09 to 0.32 m wide. Calculations of rill
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geometry indicate that erosion of slope sediment and rill formation during the winter of 1996–1997 resulted in removal of at least 8.6 m3 and 5.7 m3 of sediment from the ESE and WNW fields, respectively. Inspection of slope surfaces in August 1997 revealed the presence of networks of pronounced desiccation cracks up to 0.03 m wide and 0.5 m deep and which may leave the slopes preferentially prone to subsurface erosion by piping. The large rills on both slopes are interconnected with major gully systems located at the field base Žheadcuts currently 2.5 m deep feeding into channels in excess of 15 m deep. which in turn drain into the Salandrella River. Thus, remodelling these slopes has resulted in the formation of a connecting link with the ephemeral drainage network catchment of the Salandrella–Cavone system. Soil eroded from the large rills formed in these remodelled fields in winter 1996–1997, can therefore potentially be transported into the perennial fluvial system of the Salandrella–Cavone and out into the Ionian Sea. Future monitoring of these fields is planned with particular interest in the potential of headward retreat of the gully system upslope fed by the connecting large rills and potential pipes fed from large surface cracks. Headward extension of the gully systems would lead to an irreversible loss in field size, once the erosional forms are too large to be ploughed in. 5.2. Remodelled calanchi Three remodelled calanchi sites were studied ŽFig. 1.. All of these sites were remodelled between 1986 and 1996, based on aerial photograph coverage and field surveillance.
Fig. 9. Remodelled slope at La Piana on the Torrente il Gruso.
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5.2.1. Pantone della Fica The calanchi front at Pantone della Fica faces 2408 ŽWSW. with a 408 scarp which is 25 m long. Part of this front has been remodelled resulting in a lower angle slope of 178 and an increase in slope length to 90 m, and has involved the obliteration of a dirt road which ran along the base of the old calanchi front. Formation of a lower slope angle has resulted in the creation of a new dirt road used by local farm traffic, orthogonal to the old road and running upslope and over the top of the remodelled calanchi, providing a compacted surface for overland flow runoff during storm events. Whilst probably being the most direct route, the creation of a road through the calanchi badlands would be impossible prior to remodelling due to the extreme steepness of the 408 slope. As at other locations in this area where dirt roads traverse slopes, the new road provides an ideal focus for future gully development, resulting from the creation of a local drainage system and flow convergence on to the road during storm events. Runoff from the road at breaks of slope or below bends results in sediment entrainment and thus, this site, like many others on this clay terrain, has a high future soil loss potential. 5.2.2. San Bernardino The San Bernardino site lies to the northwest of Pantone della Fica and consists of a remodelled slope adjacent to the remnants of the previously extensive calanchi scarp. The slopes face 2708 ŽW. and are located on the east side of a minor road to Ferrandina. Slope angle on the calanchi scarp is 418 and the length of slope is 22 m and this has
Fig. 10. Ža. A rotational landslide in a remodelled field near Craco after harvest, September 1997. Žb. Shallow debris flows in a remodelled slope near Grassano, cultivated with durum wheat, April 1996.
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Fig. 10 Žcontinued..
been remodelled into a 34-m slope of only 128. The remodelled slope had been used for durum wheat cultivation during the 1996 and 1997 growing seasons. 5.2.3. La Piana This site, shown in Fig. 9, lies on the northern bank of the Torrente il Gruso which is a tributary of the Salandrella River, southeast of Ferrandina. Measurements on the adjacent calanchi scarp to the east of the remodelled slope indicate that bare 63-m slopes of 468 have been converted into 90-m slopes of 168 and cultivated with durum wheat. Wheat growth at this site is very patchy implying low overall yields. Poor land management is indicated by the blue-grey nature of the remodelled field resulting from the deep ploughing of bedrock. Nevertheless, an attempt has been made to cultivate wheat on the bedrock without the addition of substantial quantities of organic matter to improve soil fertility.
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6. Slope remodelling impacts on landscape morphology and soil erosion The slope transformations described above involve increasing slope lengths whilst decreasing slope angles by more than 208. These changes have repercussions for the spatial organisation and coupling of erosion processes on the slopes. Prior to remodelling, badland slopes dominated by calanchi or biancane, although visually spectacular, are characterised by very localised processes of erosion and deposition of material. The biancane at Serra Pizzuta may exhibit erosion rates of 20 mm yry1 but the material eroded is deposited adjacent to the erosional landforms. Similarly, the calanchi exhibit very localised deposition of material carried by mass movement or debris flows. In many cases these badlands are either completely or almost completely spatially divorced from local channel systems so that sediment delivery to the channel systems does not occur. Although the calanchi and biancane forms are mostly unvegetated, they are surrounded by areas of scrub vegetation. After remodelling, ploughed slopes remain unvegetated, this is critical during the late summer and early autumn periods and when they are particularly vulnerable to the type of convective rainstorms that characterise the end of summer in central and southern Italy. The slope transformations increase the effective catchment for rainfall by reducing slope angle Ža change from 408 to 158 for the same slope length results in an increase of 26% in catchment area for rain falling vertically. and increasing slope length. Whereas detachment of material by rainsplash is the dominant process on the biancane, on the remodelled slopes rainsplash, ponding and overland flow create process interactions delivering sediment off-slope and potentially off-site. As has been shown, the new slope lengths will facilitate the development of concentrated overland flow with the development of rills and gullies delivering water and sediment downslope and off-field into water courses and on to roads. Another impact of remodelling is an apparent increase in the occurrence of relatively small-scale rotational landslides ŽFig. 10. and shallow debris flows, with typical depths of 0.8 m. These tend to be sufficiently superficial in morphology that, like many rills, they can be ploughed in at the beginning of each cultivation cycle and are therefore recurrent but short-lived landscape features. Deep ploughing techniques used in this area turn over sediment to a depth of 1.4 m.
7. Implications of changing land use policy The practice of remodelling badland hillslopes to increase agricultural productivity from the clay landscape has been exacerbated by the provision of EU subsidies to cultivate durum wheat. The pattern of subsidy changed in June 1992 with the introduction of a staged reduction in intervention price for durum wheat. This change has been coupled with a dual subsidy system whereby for 1995–1996, a hectarage payment of around 643,000 lire hay1 was given with an additional yield-related value of 163,000 lire hay1 Žfor a yield of 3 t hay1 . which amounts to at least 40% of farm income ŽBianchi et al., 1993.. The continuing hectarage-based economic incentive to increase landscape productivity has inadvertently resulted in an increase in soil erosion in these
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areas, where comparatively stable badland landforms have been replaced by lower angle agricultural fields which have a higher potential for soil erosion by off-site removal of sediment. The issue of cereal subsidies has, to some extent, been balanced by the provision of subsidies to farmers to maintain areas of fallow land by the introduction of ‘set-aside’. The decision by the EU in 1997 to discontinue subsidising set-aside will have a negative effect on measures to prevent soil erosion, as it becomes economically more beneficial to cultivate all fallow land. Economic incentives may result in a further increase in the number of illegally remodelled badland slopes. The use of subsidies for specific crops may also increase the likelihood of soil erosion. For example, in the 1997 growing season, there has been an increase in the number of fields in the study area given over to the cultivation of sunflowers. The percentage ground cover of crops like sunflowers is low and thus, the potential for soil erosion during a growing season is greater than for a larger ground cover crop such as durum wheat. Land use policy and its subsequent economic implications are thus important in determining some of the controlling factors on soil erosion.
8. Conclusions In Basilicata, the practice of using heavy earth-moving machinery to remodel calanchi and biancane badland landforms changes slope morphology. Initially steep badland scarps of ) 358 are lowered by 20–308 to form more gentle slopes of 12–178 which can be ploughed and then cultivated for cereals. Slope lengths are also greatly increased resulting in a spatial coupling of soil erosion processes and an extension of existing drainage networks. This change in land use, promoted by economic incentives, has resulted in an increase in potential soil erosion in this area. The effects of ‘soil quarrying’ for infrastructure improvement or land preparation may be considered to be soil degradation processes ŽPoesen and Hooke, 1997. in that soil is physically removed from the landscape or locally repositioned on it. However, it is the geomorphological and associated hydrological changes to the slopes, as a result of these landscape remodelling activities, that have the potential for significantly increasing soil erosion and land degradation.
Acknowledgements We would like to thank Dott. F. Belelli of the Unione Provincale Agricoltori di Siena and Dott. C. de Giovanni of the Istituto di Allevamento delle Piante, Bari for technical data, Geom. G. Alfieri of the Istituto Idrografico di Catanzaro for meteorological data from Basilicata, and the Santangelo family of Tricarico for their continuing hospitality.
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