Assessing the Impacts of Land Use Policy on Soil Erosion Risk: Possibilities and Constraints of Geographical Information Systems

Abstract

Soil erosion is regarded as a major and widespread soil degradation process. The consequences of soil erosion occur both on- and off-site. On-site consequences are particularly important on agricultural land where the redistribution of soil within a field, the loss of soil from a field, the breakdown of soil structure and the decline in organic matter and nutrients result in a reduction of the cultivable soil depth and a decline in soil fertility (Morgan, 1996). Off-site problems result from sedimentation downstream which reduces the capacity of rivers and drainage ditches, enhances the risk of flooding, blocks irrigation canals and shortens the design life of reservoirs (Verstraeten and Poesen, 1999). Sediment is also a pollutant in its own right, and through the chemicals absorbed it can increase the levels of nitrogen and phosphorus in water bodies and result in eutrophication (Steegen et al., subm.). The rate of soil loss is normally expressed in units of mass or volume per unit area per unit time. Young (1969) quotes annual rates of the order of 0.0045 Mg ha-1 for areas of moderate relief and 0.45 Mg ha-1 for steep relief. For comparison, rates from agricultural land are in the range of 5 to 500 Mg ha-1 (Morgan, 1996; Van Rompaey et al., 2000).