Abstract:
Soil erosion assessment for watershed management is one of the major concerns because in reality it is not possible
to conserve all areas under the threat of erosion because of the financial constraints. Geographic Information
Systems are becoming a popular tool when seeking solutions to issues of these kinds, which are spread over large
spatial extents. However it is of utmost importance to ensure a reason~ble assessment of soil erosion because this is
the core to any such decision making. Use of USLE for erosion assessment using GIS poses concerns since some
parameters are polygon specific. This could be overcome easily by the use of grid-based data layers and selecting
appropriate size of grids for a particular data st!t. The most common problem is the computation of the slope length
and slope class parameters of a particular grid of a DEM. The present study describes a different methodology to
compute these factors using a digital elevation model of the study area. A watershed from Sri Lanka covering an
area of 23,000ha was selected for modelling. Field survey results recognised the need to calibrate the model for
planning purposes. Model calibration used a weighting scheme. A comparative analysis of results for the same
project area from the grid-based model and a model that did not use grid-based data indicated an almost same value
of mean annual erosion level for the watershed. This paper also carries a discussion of the key issues that need to
be considered when modelling in different environments. Suitably developing a model in a grid data environment
provides better flexibility to compute erosion levels for different spatial scales. Grid based method also enables the
meaningful use of pixel based remotely sensed land cover information for modelling soil erosion
