Adaptation of the revised universal soil loss equation to map spatial distribution of soil erosion in tropical watersheds: a GIS/RS-based study of the Upper Mahaweli River Catchment of Sri Lanka

Soil erosion is a serious issue in highland areas of Sri Lanka that are dominated by extensive agricultural activities. Methods capable of rapidly assessing soil erosion are required to determine suitable conservation methods and to monitor their effectiveness to preserve this non-renewable resource. The widely used method of determining soil erosion rate is the use of field measurements and/or empirical modeling. Although the latter method along with the use of GIS/Remote Sensing has become very popular, its main limitations are the lack of model validation and lack of comparison of modeled rates with acceptable soil loss tolerance values. In this study, soil erosion rates of six sub-catchments were calculated with the revised universal soil loss equation (RUSLE) following five different methods in GIS/RS environment and then compared with field-based measurements for verification. The mean modeled soil erosion rates of the six sub-catchments were within 0.12 and 7.70 t ha(-1) y(-1), whereas the field-based rates range from 1.14 to 15.83 t ha(-1) y(-1). Moreover, modeled erosion classes within the sub-catchments were similar to field-based localized erosion rates over different land use types quantified from plot experiments. Hence, we recommend the integration of RUSLE with GIS/RS as an effective tool for soil erosion hazard mapping at the catchment scale. We propose a typical rate of 1 t ha(-1) y(-1) and an upper limit of 2 t ha(-1) y(-1) as the soil loss tolerance level for the study area based on experimentally derived soil production rates. Finally, a soil erosion map of the entire Upper Mahaweli River Catchment was developed using the best method, and then its sub-catchments were ranked according to severity of soil erosion. The methodology adapted here can be used as a guide in land management and soil conservation programs.