Spatial patterns of land degradation and their impacts on the water balance of rainfed treecrops: A case study in South East Spain

Rainfed permanent crops such as almonds, olives and vines cover important areas in the drier parts of the Mediterranean. The acreage of almonds has expanded rapidly into marginal soils of the hillslopes of southeast Spain. This expansion staffing in the 1970s was reinforced by the subsidies under the EU Common Agricultural Policy since the late 1980s. Trees are widely spaced and the soil in between is kept bare to reduce competition with weeds for the scarce rainfall. Hence, the large areas of bare soil are vulnerable to water erosion. However, recently it has been demonstrated that soil redistribution by tillage can be an even more important cause of degradation. This paper investigates the systematic variation in soil properties as a result of soil redistribution by tillage of a small catchment (21 ha in Murcia Region, southeast Spain) converted to almond groves in the late 1970s. Furthermore, the impacts of the spatial variation in soil properties on the water balance of the almond cropping system are evaluated. The results of a spatially distributed tillage erosion model, WATEM, are validated by a topographic survey of accumulation and removal of soil along field borders (root mean square difference (RMS) = 6.7 ton ha(-1) y(-1)). On the hillslopes, soil loss by tillage erosion amounts to 26.6 ton ha(-1) y(-1) while sedimentation occurs at a rate of 21.1 ton ha(-1) y(-1). The difference between erosion and sedimentation on the hillslopes results in a net transport of sediment towards the valley bottom, where the sediment is retained behind 17 earthen dams. Overall, these currently store 156.5 tons ha(-1), which is about half their total capacity. The area suffering from erosion is larger (53%) than the area undergoing sedimentation (34%) or the area undergoing a change within the accuracy of the model prediction (13%). Hence, soils in a large part of the catchinent become gradually thinner and stonier. The PATTERN model, developed to describe the hydrology of thin and stony soils, was validated on two columns of bare soil for which wetting and drying runs were monitored. Frequent tillage destroys the roots in the plough layer and forces the widely spaced trees to develop a lateral root system in the broken bedrock below. The model results indicate that evaporation losses (92.8-94.7 mm per year) are similar for all soil profiles regardless of landscape position. The water draining below the plough layer (i.e. 67% of the rainfall) will be used by the tree crop to develop its canopy covering the fields only partially (i.e. the transpiring surface is a fraction of the total area). However, the frequent ploughing causes important soil redistribution, which reduces soil thickness on the one hand and fills up the storage behind dams in the valley bottom on the other hand. (c) 2006 Elsevier B.V. All rights reserved