LAND DEGRADATION & REHABILITATION, VOL 5,41-55 (1994)

SOIL EROSION AND REDISTRIBUTION ON CULTIVATED AND UNCULTIVATED LAND NEAR LAS BARDENAS IN THE

CENTRAL EBRO RIVER BASIN, SPAIN

T. A. QUINE,* A. NAVAS,?. D. E. WALLING* AND J. MACHINT *Department of Geography, University of Exeter, Amory Building, Rennes Drive, Exeter EX4 4RJ, UK

f Estacion Experimental de Aula Dei, Consejo Superior de Investigaciones Cient$cas, Apartado 202, 50080 Zaragoza, Spain

ABSTRACT The semiarid regions of Spain, including the central part of the Ebro River basin, are under threat due to desertification. Severe erosion, as a result of poor land management, has led to degradation of the soil resource, and there is a clear need for quantitative erosion rate data to evaluate the problem. This study aimed to examine the potential for using caesium-137 to identify the patterns and rates of soil erosion and redistribution within this semiarid environment. Samples for the determination of caesium-137 were collected from uncultivated slopes and cultivated valley floor sites near the head and outlet of a small representative basin in the Las Bardenas area. The measured patterns of caesium-137 mobilization, redistribution and export provide a semiquantitative indication of the variation in erosion within the study site. Calibration of the caesium-137 measurements, taking account of the differing behaviour of radiocaesium on cultivated and uncultivated land, allows estimation of the actual rates of erosion and deposition involved. The results show (1) the erosion rates on the cultivated land (1.6-2.5 kg m-* yr-') are typically more than five times those seen on the uncultivated land (0-2-0-4 kg m-* yr-'), and (2) erosion on the uncultivated land is significantly less severe at the head of the basin than at the outlet. Study of the vegetation cover suggests that lower growing shrubs and grasses may be more effective in reducing erosion in this environment than trees.

KEY WORDS Caesium-137 Erosion rates and patterns Agricultural land Vegetation influence Semiarid Spain

INTRODUCTION

Desertification poses a threat to the semiarid regions of Spain, including the central part of the Ebro River basin (Figure 1). A history of poor land management in this area, including both agricultural practices and deforestation, has given rise to a serious soil erosion problem which is exacerbated by the low rates of soil formation associated with the semiarid climate (average annual rainfall 350-450 mm). There is, therefore, a need to quantify erosion rates to assess both the magnitude and the spatial patterns of soil loss in these fragile agrosystems. In the light of this clear need, an attempt has been made to use caesium-137 to assemble quantitative erosion rate data for this area.

Fall-out caesium-137 has been widely used for documenting erosion and sedimentation in various landscapes and environments around the world (Loughran, et al., 1990; Ritchie and McHenry, 1990; De Roo, 1991; Martz and De Jong, 1991; Walling and Quine, 1992) and the basis of the technique is now well established (Walling and Quine, 1991). The caesium-137 technique has proved to be a reliable method for investigating rates and patterns of erosion and has clear advantages over many classical measuring techniques (Loughran, 1989; Walling and Quine, 1990a). Although to date the approach has been applied mainly in humid temperate areas, there is a growing body of evidence to suggest that it is also applicable in other environments (Menzel, et al., 1987; Quine, et al . , 1992; Quine, et al., 1993a; Zhang, et al . , 1994). Furthermore, initial studies in the Ebro basin have also confirmed this potential (Navas and Walling, 1992).

This paper reports the caesium-137 levels found in six transects located at the head and the outlet of a

CCC 0898-581 2/94/01 0041-1 5 @ 1994 by John Wiley & Sons, Ltd.

Received 20 December 1993 Accepted 2 February 1994

42 T. A. QUINE ET A L .

Figure 1 . Land use and location of the study area

small drainage basin in Las Bardenas (central Ebro River basin), which includes both natural vegetated slopes and cultivated land in the valley bottom. These caesium-137 data have been used to examine the mobilization, redistribution and delivery of radiocaesium, and to derive quantitative estimates of erosion rates and sediment budgets for the area. These quantitative data may provide the basis for evaluating the erosion problem in the area and for developing effective land management and soil conservation strategies.

STUDY AREA

The study basin, which has an area of 0.52 km2, is oriented south-to-north and is located at the foot of the Tertiary plateau of Loma Negra in Las Bardenas (Figure 1). It lies at an altitude of 360-400 m above sea level and is underlain by Miocene sandstones, marls and clays. The mean annual rainfall is 450 mm and strong winds and high temperatures during the summer contribute to its semiarid character.

The valley floor (Figure 2) is characterized by slopes of ca. 5" and is cultivated perpendicular to the line of maximum slope. The main crops are cereals, chiefly barley, which are grown in alternate years. In marked contrast, the valley sides exhibit steep convex-concave slopes (ca. 15') and a natural vegetation of grass, shrubs and small trees (Juniperus thuriferu), which provides an incomplete cover to the soil surface. Skeletal and fragile soils are the most abundant in the basin. These highly erodible soils are classified as calcic cambisols in the valley bottom, whereas calcic regosols are found on the surrounding slopes.

The potential for erosion in the study area is increased further by the erosivity of the precipitation. Short, intense rainstorms frequently generate overland flow and associated rilling on the valley slopes, which in turn mobilize substantial amounts of sediment which is eventually exported from the basin through the central gully cut into the valley floor. Extension of this gully over the flatter valley floor has led to a drastic reduction in the area available for cultivation over recent decades. The study basin is typical of the landscape in the region and therefore provides representative information about the processes of erosion operating in the region and of the rates involved.

SAMPLE COLLECTION AND ANALYSIS

Samples of soil for the determination of caesium-137 were collected from both the head and outlet of the

SOIL EROSION AND REDISTRIBUTION IN SEMIARID AREAS OF SPAIN 43

Figure 2. Study basin from the south

asin on its east-facing side. At each location, three downslope transects 10m apart were established, and imples were collected at 10-12m intervals along each transect. At the head of the basin, transects UC nd UD each had 14 sampling points, whereas transect UL had 15. At the outlet of the basin, transects D, LC and LL had, respectively, 24,23 and 22 sampling points (Figure 1). Along all transects, the upper :n samples were collected from uncultivated land supporting a natural vegetation cover of shrubs and iixed grass, and the remaining samples were collected from cultivated land. Soil samples were collected using an 8cm diameter hand-operated core sampler. Over the convex

ortion of the slope, samples were taken at depth increments of 5 cm down to 30 cm, whereas over the mcave element, samples were taken at 10cm depth increments down to 60cm. These sectioned cores How examination of the changing depth distributions of caesium-137 along the slope transects. Measurement of caesium-137 was undertaken by gamma spectrometry at the Department of Geography,

Jniversity of Exeter, using hyperpure coaxial Ge detectors coupled to a multichannel analyser. All imples were air-dried and lightly ground before being passed through a 2mm sieve. Measurement of iesium-137 was undertaken on a subsample of the finer fraction (<2mm) of each sample, which was )aded into a Marinelli beaker. Counting times (662 keV) were 30000-6OOOOs, providing an analytical recision of 1k6 per cent. The caesium-137 content of the samples can be expressed per unit mass as the ctivity (mBq g-') or per unit area as the inventory (mBq cm-'). The samples were collected in 1989, but I1 caesium-137 measurements have been standardized for caesium-137 decay to 1992.

CAESIUM-137 DATA

,ocal reference inventory Identification of soil loss and gain using the caesium-137 technique is generally based on comparison of

ie magnitude of the caesium-137 inventories over the study site with an equivalent estimate of the total aesium-137 fall-out to the site. The latter value is typically derived by measurement of the caesium-137 iventories at undisturbed, uneroded, level areas supporting permanent vegetation. These sites are

44 T. A. QUINE ET AL.

described as reference sites, and the associated caesium-137 inventories are termed reference in- ventories.

Identification of reference sites which fulfil the above requirements is often difficult, particularly in areas such as Las Bardenas where virtually all of the relatively level areas are cultivated and where uncultivated areas tend to have discontinuous vegetation cover. However, two small areas of suitable uncultivated land were identified on the plateau area at the top of the transects and core samples collected from these areas were used to estimate the reference inventory for the study area (190 k 9 mBqcm-’).

Variation in depth distributions of caesium-137 Figure 3 shows a typical range of caesium-137 depth distributions and their topographic positions along

a representative transect. Profiles (a) to (d) lie within the uncultivated part of the slope and (e) to (g) in the cultivated area. This distinction in land use is reflected in the general shape of the depth distributions. Profiles (a) to (d) have maximum caesium-137 activities in the upper 5-10cm. Below this level, activities decline sharply with depth. These characteristics are consistent with efficient adsorption of the fall-out radiocaesium by the surface soil horizons and minimal downward displacement. In contrast, profiles (e) to (g) are typical of cultivated areas and show relatively uniform caesium-137 activities throughout the plough layer as a result of mixing of the soil-associated caesium-137 by tillage.

Within the two land use zones, further variations in the depth distributions can be related to the long- term pattern of soil erosion and redistribution. Profile (a), near the crest of the slope, is characterized by a gradual decline in caesium-137 activity with depth and an inventory close to the reference level. Both of these features are consistent with a stable location. Profile (b) shows a slightly lower activity of caesium- 137 in the uppermost depth increment and a more rapid decline in activity down the soil profile. These characteristics and the inventory, which is 25 per cent lower than the reference level, indicate that this location has been subject to erosion. Profile (c) shows even clearer evidence of erosion, having much lower caesium-137 activity at the surface and a low total inventory. Profile (d) is similar to profile (a) below a depth of lOcm, but it is characterized by much higher caesium-137 activities above this depth. The total inventory for profile (d) is also high, being 73 per cent greater than the reference inventory. These properties are consistent with the gradual accumulation of caesium-137 in association with deposited sediment at this location.

Equally clear distinctions may be made between the profiles within the cultivated zone. Profiles (f) and (8) both show relatively uniform caesium-137 activity within the plough layer, but minimal caesium-137 below the maximum plough depth (30 cm). Furthermore, both profiles have caesium-137 inventories which are significantly lower than the reference level. These characteristics are consistent with both sites being subject to caesium-137 loss and therefore erosion. Profile (e) has a similarly uniform caesium-137 activity throughout the plough layer, but the elevated levels of caesium-137 extend well below the maximum plough depth. The profile is also characterized by a caesium-137 inventory which exceeds the reference level. These features indicate that this site has received additional caesium-137 in association with deposited sediment. This deposition has led to gradual surface accretion and burial of radiocaesium- bearing soil below the plough depth. The thickness of the layer of elevated caesium-137 activity which lies below the maximum plough depth may be broadly equated with the total depth of sediment deposition at the location since the initiation of significant levels of caesium-137 fall-out (ca. 1960). Profile (e) therefore suggests that ca. lOcm of sediment deposition has taken place over the last 30 years. The location of profile (e) adjacent to the upper boundary of the cultivated area indicates that the most likely source of the deposited sediment is the uncultivated area. This has important implications for the interpretation of caesium-137 redistribution and the assessment of erosion rates, which will be considered in later sections.

The variation in caesium-137 depth distributions and inventories shown by the typical profiles in Figure 3 demonstrates the broad pattern of caesium-137 redistribution on the slope. However, examination of the full data set reveals both small-scale variations in inventories within the land use zones and clear distinctions between the head and the outlet of the basin. The following section, therefore, addresses the additional information about caesium-137 redistribution provided by the full data sets.

SOIL EROSION AND REDISTRIBUTION IN SEMIARID AREAS OF SPAIN 45

/

46 T. A. QUlNE ET A L .

SPATIAL DISTRIBUTION OF CAESIUM-137 PERCENTAGE RESIDUALS ALONG THE TRANSECTS

The extent of caesium-137 loss or gain may be indicated by the percentage residual (P)

P = [ ( I - R)/R)]*100%

where I = point inventory (mBq cm-’) and R = reference inventory (mBq cm-2). Negative residuals indicate the loss of soil-associated caesium-137 and vice versa. The distributions of

caesium-137 residuals along the two sampled slopes are shown in Figure 4. The distributions were derived by interpolating point residuals using the UNIRAS package.

The pattern of caesium-137 redistribution at the head of the basin (Figure 4a) has three important features. Firstly, there is evidence of deposition (indicated by positive residuals) at the top of both the uncultivated and cultivated zones. This emphasizes the openness of the system. The limited area of deposition at the top of the uncultivated zone indicates that a small amount of sediment has been imported from the adjacent cultivated plateau. In contrast, the extensive area of positive residuals at the upper margin of the cultivated zone points to significant sediment transfer from the uncultivated area upslope. Secondly, there is evidence of widespread erosion over the uncultivated area. Net loss of caesium-137 has occurred over 63 per cent of the uncultivated zone and 27 per cent of the area exhibits negative caesium-137 residuals in excess of 25 per cent. Thirdly, high levels of caesium-137 loss occur over the lower part of the cultivated zone. The transition from net gain to net loss of caesium-137 occurs ca. 20 m below the boundary with the uncultivated area and below this point there is a rapid increase in the magnitude of caesium-137 loss. At the base of the slope, adjacent to the gully which runs along the valley bottom, loss of caesium-137 exceeds 75 per cent.

The pattern of radiocaesium redistribution in the sampled area at the outlet of the basin is dominated by negative residuals. Net loss of caesium-137 has occurred over 83 per cent of the uncultivated zone, and over 87 per cent of the cultivated zone. Furthermore, the areas subject to higher levels of net loss are much more extensive than at the head of the basin. Negative caesium-137 residuals in excess of 25 per cent occur over 63 per cent of the uncultivated zone and 77 per cent of the cultivated zone. These data suggest that the transfer of mobilized (sediment-associated) caesium-137 out of the system is significantly more efficient on the slopes near the outlet of the basin. The following section will attempt to summarize the caesium- 137 redistribution over the slopes and address the question of caesium-137 export.

CAESIUM-137 MOBILIZATION, REDISTRIBUTION AND EXPORT

To summarize the pattern of caesium-137 redistribution in each of the sampled areas, the individual transect data have been combined. For each point along the slope the data from the three transects have been combined to provide an estimate of net caesium-137 loss or gain for the slope segment represented by the samples (Figure 5). These data highlight the contrast between the slopes at the head and at the outlet of the basin. At the outlet of the basin, all slope segments are characterized by net caesium-137 loss, in marked contrast with the pattern at the head of the basin, which shows very clearly a zone of net caesium-137 gain at the break of slope.

The contrast between the slopes at the head and outlet of the basin is seen even more clearly when the segment data are combined to examine the patterns of caesium-137 mobilization, redistribution and export along the slopes (Figure 6). At the outlet of the basin, the severity of erosion is demonstrated by both the high degree of caesium-137 mobilization and the limited amount of redeposition. Over the uncultivated area, 33 per cent of the total caesium-137 input has been mobilized and less than a tenth of this has been redeposited, leading to net export of 29 per cent of the total radiocaesium receipt. This pattern is even more marked over the cultivated zone, where 45 per cent of the total caesium-137 input has been mobilized and 44 per cent exported. In contrast, at the head of the basin, erosion over the upper part of the uncultivated area has resulted in mobilization of only 15 per cent of the inventory and more than half of this has been redeposited within the uncultivated area. As a result of this extensive

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48 T. A. OUlNE ET A L .

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redeposition, only 6 per cent of the total input of caesium-137 to the uncultivated slope has been exported. Furthermore, the presence of a zone of positive residuals over the upper part of the cultivated zone may account for the caesium-137 which was exported from the uncultivated area upslope. This area of deposition tends to mask the severity of erosion over the cultivated zone so that only 17 per cent of the caesium-137 fallout onto the zone has been exported. However, this export is a result of severe erosion over the lower 20 m of the zone (Figure 5 ) , in which the level of caesium-137 mobilization exceeds 50 per cent, a figure comparable with that for the cultivated zone near the outlet of the basin.

SOIL EROSION AND REDISTRIBUTION IN SEMIARID AREAS OF SPAIN 49

58 kBa rn-’

38% Total inventory is exported

;ure 6. Summary diagrams of caesium-137 mobilization, redistribution and export along the slopes at the head (a) and outlet (b) of the basin

QUANTIFICATION OF EROSION AND DEPOSITION

though the summary data for caesium-137 mobilization, redistribution and export provide a semi- antitative indication of the severity of erosion in the study basin, their value is limited for several 3sons. Firstly, equivalent rates of caesium-137 loss from cultivated and uncultivated profiles will reflect pificantly different rates of erosion, because of the contrasts in the depth distributions of caesium-137, d therefore surface activity, under the two different land uses. Secondly, it is important to establish timates of the actual rates of erosion involved to evalute the potential impact of erosion on the idscape and the implications for management practices. To estimate erosion rates from caesium-137 measurements it is necessary to establish a reliable lationship between the magnitude of the caesium-137 residual and the total depth of soil loss. If long- rm erosion plot data are available for the site of interest it is possible to establish an empirical lationship between the magnitude of caesium-137 loss and the soil erosion rate. However, such data are rely available and the investigations undertaken by the University of Exeter have therefore commonly iployed a theoretical model or accounting procedure, which simulates the effect of all processes of soil d caesium-137 redistribution, as a basis for deriving calibration relationships (cf. Quine, 1989; Walling d Quine, 1990b). The model used to simulate the impact of erosion on caesium-137 inventories of ltivated profiles takes account of the temporal patterns of caesium-137 fall-out, soil mixing by tillage,

50 T. A. QUINE ET AL.

soil loss by both water erosion and tillage displacement, the plough depth, the fraction of rainfall occurring during the erosion phase, the area subject to erosion and particle size selectivity. The model used for uncultivated sites excludes the impact of tillage on soil mixing and displacement and includes displacement of the radiocaesium by diffusive processes, but in other respects is similar to that for cultivated sites. In both cases the model is used to predict caesium-137 inventories of soil profiles subject to varying rates of erosion and aggradation and the resultant predictions are used to produce site-specific calibration relationships (Figure 7a). The relationships can then be used to estimate the erosion rates represented by the caesium-137 residuals recorded for individual sampling points.

In addition to simulating the impact of erosion on the caesium-137 inventory of the eroded profile, both models simulate the caesium-137 activity of the water-eroded sediment mobilized from the surface of the profile each year. These data are used in conjunction with assumptions about the particle size selectivity of deposition processes to estimate the caesium-137 activity of deposited sediment. These data may then be used to simulate the increase in caesium-137 inventory which would occur for a soil profile subject to sediment deposition. However, the caesium-137 activity of the eroded and deposited sediment will vary according to the erosion rate in the source area. It is, therefore, necessary to take account of the erosion rates and caesium-137 activity of the eroded sediment, associated with the entire source area. This may be accomplished by simulation of caesium-137 redistribution over the entire slope (Quine, et al . , 1993b). However, a simpler approach to the estimation of deposition rates was employed in this study. The erosion rate represented by each negative caesium-137 residual was calculated using the calibration relationships described above (Figure 7a). These data were then used to establish the total mass of soil eroded from each slope segment (i) over the period since the initiation of fall-out (Emi) . The total loss of caesium-137 from each segment was also calculated (Eci) . These data were then used to calculate cumulative loss of caesium-137 and soil down the slope transects, so that it was possible to calculate the time-averaged caesium-137 activity of the transported sediment at each point on the slope. This value was used to estimate the time-averaged caesium-137 activity of the deposited sediment associated with positive caesium-137 residuals. The effects of deposition on the total caesium-137 and sediment transport were included in the calculation, so that at any point on the slope the caesium-137 activity of the transported sediment may be described as follows:

where T,, = caesium-137 activity of sediment transported through segment n (Bq kg-I); Ecj = total negative caesium-137 residual for segment i (Bq m-'); Emi = total soil loss from segment i (kg m-I); Dci = total positive caesium-137 residual for segment i (Bq m'); and Dmj = total soil gain for segment i (kg m-I).

The downslope variation seen in the resulting estimates of the time-averaged caesium-137 activity of transported sediment (Figure 7b) emphasizes the importance of undertaking this calculation, particularly when investigating an open system with different land uses, such as Las Bardenas.

ESTIMATED EROSION RATE DATA

The point estimates of erosion and deposition rate obtained using the procedures outlined in the previous section have been interpolated using UNIRAS to derive the spatial patterns of soil redistribution shown in Figure 8. As may be expected, the distributions show higher erosion rates on the cultivated land than on the uncultivated land, and the increased severity of erosion near the outlet of the basin. The cultivated zone at the outlet of the basin is subject to erosion rates in excess of 0.2 kg m-' yr-' over more than 85 per cent of the area and in excess of 2 kg m-' yr-' over 44 per cent of the area. At the head of the basin the pattern is complicated by deposition over the upper part of the cultivated zone. Nevertheless, 17 per cent of the cultivated zone is subject to erosion rates in excess of 2 kg m-' yr-'. In contrast, no areas within the uncultivated zones are characterized by erosion rates in excess of 2 kg m-' yr-'. The increased severity of

SOIL EROSION AND REDISTRIBUTION IN SEMIARID AREAS OF SPAIN 51

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igure 7. Estimation of soil erosion and deposition from caesium-137 measurements for both the uncultivated and cultivated zones: (a) site-specific calibration relationships and (b) estimated caesium-137 activity in water-eroded sediment

rosion at the outlet of the basin is seen most clearly for the uncultivated zones (comparison of the ultivated zones is problematic because of the widely differing slope lengths). Over the uncultivated zone t the outlet of the basin erosion rates exceed 0.2 kg m-* yr-' over more than 70 per cent of the area and -5 kg m-2 yr-' over 40 per cent of the area. The equivalent areas at the head of the basin are 33 per cent nd 10 per cent, respectively, indicating that erosion at the head of the basin is not only less extensive, but Is0 of lower magnitude than near the basin outlet.

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SOIL EROSION AND REDISTRIBUTION IN SEMIARID AREAS OF SPAIN 53

CONTRASTS IN VEGETATION COVER ON THE UNCULTIVATED LAND BETWEEN THE HEAD AND THE OUTLET OF THE BASIN

The marked contrasts observed in the erosion rates over superficially similar uncultivated sampling areas located at the head and near the outlet of the basin highlight the need for an improved understanding of the controls on erosion in this environment. A preliminary assessment of the potential role of vegetation in explaining the observed contrasts in erosion rates was therefore made. A vegetation survey was undertaken, along the transect lines, recording the species present and the percentage ground cover and the data assembled are illustrated in Figure 9. In both areas almost half of the uncultivated area surveyed was devoid of vegetation or stone cover. This observation is clearly consistent with the elevated erosion rates observed at both sites. When the vegetation cover is examined there is no absolute contrast between the two sites, but there is a clear difference in composition. At the head of the basin, where the lower erosion rates were observed, the vegetated area is dominated by low growing shrubs and grasses (46 per cent of the total, 87 per cent of the vegetated area). In contrast, near the outlet of the basin tree cover represents a significant proportion of the area (24 per cent of the total, 48 per cent of the vegetated area). Although any conclusion based on these data must be tentative, it appears that in this semiarid environ- ment trees which offer little ground surface cover may be less effective in reducing erosion than lower growing surface vegetation. This is clearly an area of investigation which requires further attention.

SEDIMENT BUDGETS AND SOIL EXPORT

Figure 10 summarizes the soil erosion, deposition and delivery data for the study areas at the head and near the outlet of the basin. In addition to illustrating the contrasts between erosion rates over the cultivated and uncultivated areas which have been discussed above, the results emphasize the importance of the cultivated areas with regard to sediment export. Although the uncultivated areas are subject to

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Grasses

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Figure 9. Vegetation cover of the uncultivated slopes near the head (a and b) and mouth (c and d) of the study basin

54 T. A. QUlNE ET A L

A

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Figure 10. Sediment budgets for the slopes near the head (a) and outlet (b) of the study basin

erosion rates which exceed the likely rate of soil formation (ca. 0.1 kg m-* yr-I), they are relatively insignificant in terms of sediment export. It has been suggested that at the head of the basin the majority of the sediment exported from the uncultivated zone is redeposited within the cultivated zone. In any case, the total amount of sediment exported from the uncultivated zone is only ca. 13 per cent of the total for the entire slope, despite the limited length of the cultivated zone. At the outlet of the basin, there is little evidence of sediment redeposition, but the sediment exported from the uncultivated zone neverthe- less represents <lo per cent of the total sediment export from the whole slope. These data suggest that any attempt to reduce the off-site impacts of erosion at this location should focus on erosion control on the cultivated land.

CONCLUSIONS

This study of soil erosion, deposition and delivery has demonstrated the potential of the caesium-137 technique to provide detailed erosion rate data and important insights into the major sediment sources. However, the results also demonstrate the importance of the calibration methodology employed, because interpretations of erosion severity and sediment delivery based solely on the caesium-137 budgets would overestimate the importance of the uncultivated areas.

SOIL EROSION AND REDISTRIBUTION IN SEMIARID AREAS OF SPAIN 55

ACKNOWLEDGEMENTS

Thanks are due to the British Council for funding the fieldwork associated with this study and to MAFF for authorizing a licence to import soil samples.

REFERENCES

De Roo, A. P. J. 1991. ‘The use of 13’Cs as a tracer in an erosion study in South Limburg (The Netherlands) and the influence of

Loughran, R. J. 1989. ‘The measurement of soil erosion’, Progress in Physical Geography, 13, 216-233. Loughran, R. J., Campbell, B. L., Elliott, G. L. and Shelly, D. J. 1990. ‘Determination of the rate of sheet erosion on grazing land

using caesium-137’, Applied Geography, 10, 125-133. Martz, L. W. and De Jong, E. 1991. ‘Using cesium-137 and landform classification to develop a net soil erosion budget for a small

Canadian Prairie watershed‘, Catena, 18, 289-308. Menzel, R. G., Jung, P., Ryu, K. and Um, K. 1987. ‘Estimating soil erosion losses in Korea with fallout cesium-137, Journal of

Applied Radiation and Isotopes, 38,451-454. Navas, A. and Walling, D . E. 1992. Using caesium-137 to assess sediment movement in a semiarid upland environment in Spain,

pp. 129-138 in Erosion, Debris Flows and Environment in Mountain Regions (Proceedings of the Chengdu Symposium, July 1992), IAHS Publication No. 209. D. E. Walling, T. R. Davies and B. Hasholt (eds.).

Quine, T. A. 1989. ‘Use of a simple model to estimate rates of soil erosion from caesium-137 data’, Journal of Water Resources, 8 ,

Quine, T. A., Walling, D. E., Zhang, X. and Wang, Y. 1992. Investigation of soil erosion on terraced fields near Yanting, Sichuan Province, China, using caesium-137, pp. 155-168 in Erosion, Deb& Flows and Evironment in Mountain Regions (Proceedings of rhe Chengdu Symposium, July 1992), IAHS Publication No. 209. D. E. Walling, T. R. Davies and B. Hasholt (eds.).

Quine, T. A,, Walling, D. E. and Mandiringana, 0. T. 1993a. An investigation of the influence of edaphic, topographic and land- use controls on soil erosion on agricultural land in the Borrowdale and Chinamora areas, Zimbabwe, based on caesium-137 measurements, pp. 185-196 in R. A. Hadley and T. Mizuyama (eds.) Sediment Problems: strategies for monitoring, prediction and control (Proceedings of the Yokohama Symposium, July 1993), IAHS Publication No. 217.

Quine, T. A., Walling, D. E. and Zhang, X. 1993b. The role of tillage in soil redistribution within terraced fields on the Loess Plateau, China: an investigation using caesium-137, pp. 149-155 in K. Banasik and A. Zbikowski (eds.) Runoff and Sediment Yield Modelling (Proceedings of the International Symposium held at Warsaw Agricultural University, September 1993). Warsaw Agricultural University Press, Warsaw.

Ritchie, J. C. and McHenry, J. R. 1990. ‘Application of radioactive fallout cesium-137 for measuring soil erosion and sediment accumulation rates and patterns: a review’, Journal of Environmental Quality, 19, 215-233.

Walling, D. E. and Quine, T. A. 199Oa. Use of caesium-137 to investigate patterns and rates of soil erosion on arable fields, pp. 33- 53 in J. Boardman, I. D. L. Foster and J. A. Dearing (eds.) Soil Erosion on Agricultural Land, Wiley, Chichester.

Walling, D. E. and Quine, T. A. 1990b. ‘Calibration of caesium-137 measurements to provide quantitative erosion rate data’, Land Degradation & Rehabilitation, 2 , 161-175.

Walling, D. E. and Quine, T. A. 1991. ‘The use of caesium-137 measurements to investigate soil erosion on arable fields in the UK: potential applications and limitations’, Journal of Soil Science, 42, 147-165.

Walling, D. E. and Quine, T. A. 1992. The use of caesium-137 measurements in soil erosion surveys, pp. 143-152 in Erosion and Sediment Transport Monitoring Programmes in River Basins (Proceedings of the Oslo Symposium, August 1992), IA HS Publica- tion No. 210.

Zhang, X., Quine, T. A., Walling, D. E. and Li, Z. 1994. ‘Application of the caesium-137 technique in a study of soil erosion on gully slopes in a yuan area of the Loess Plateau near Xifeng, Gansu Province, China’, Geografiska Annaler, 76 (A series), 103- 120.

Chernobyl fallout’, Hydrological Processes, 5 , 215-227.

54-81.