Spatial patterns of soil water balance on intensively cultivated hillslopes in a semi-arid environment: the impact of rock fragments and soil thickness

During past decades, a diverse system of subsistence agriculture in south-e ast Spain (annual rainfall of less than 300 mm) has been overturned in favo ur of large-scale plantations of almond trees without consideration for top ography and related spatial patterns in soil hydrological properties. The o bjective of this paper is to investigate the spatial pattern in soil physic al properties induced by this cultivation system, and to highlight its impa cts on the water balance. Soil properties were recorded along hillslopes wi th shallow soils developed on slates and greywackes in the upper part of th e Guadalentin drainage basin (Murcia region). Frequent tillage of these alm ond plantations covering entire hillslopes has resulted in denudation by ti llage erosion on the topographic convexities, as well as transport of rock fragments and fine earth along the slopes. These processes have created a s ystematic spatial pattern of soil thickness and rock fragment content: shal low and stony soils on the topographic convexities and deep soils with a ro ck fragment mulch in the concavities at the foot of the slopes. At the same time, a negative relationship between rock fragment content and fine earth bulk density was observed. The impact of this spatial pattern in soil prop erties on the water balance was evaluated using the PATTERN one-dimensional hydrological and plant growth model, The model simulates the water balance of soil profiles covering the observed variation in soil thickness, stonin ess and bulk density. The model results indicate that the highest rates of infiltration, evaporation and drainage, as well as the lowest rates of over land flow are restricted to shallow soils on the hilltops. In contrast, the deeper soils in the valley bottoms produce a more stable moisture regime t han shallower soils, which tend to saturate and dry out quickly. These mode l results are in agreement with the spatial patterns of almond productivity : an asymptotic increase with soil thickness. Copyright (C) 2000 John Wiley & Sons, Ltd.