Abstract

The dynamic nature of erosion and associated processes and their dependence on climatic, pedological, land cover, land use, and management factors result in spatial and temporal variability. Computing and mapping this variability will produce information which is essential for designing dams, reservoirs, channels, soil conservation management plans, and the evaluation of on-site and off-site damages by soil erosion, land use projects, and transport of pollutants. This thesis presents the Modified Universal Soil Loss Equation (MUSLE) as a promising tool for the spatial modeling of soil erosion by water integrated with the SPatial ANalysis System (SPANS)-GIS. The information that resulted from this integration was used for land-use optimization to minimize sediment yield and maximize watershed farm income by a multi-objective linear programming model. These models were applied to Syahrood, one of the sub-basins of Damavand watershed in Iran, where soil erosion by water is one of the major land-related problems. Runoff erosivity, soil erodibility, slope length, slope steepness, cover management, and erosion control practice factors were computed and included in the digitized and computed Thiessen polygon, land component, slope and land-use maps of the watershed. The sediment yield of each land use was computed by overlaying these maps with the appropriate models in the SPANS-GIS. The optimization process allocated dryland-farming areas to rangelands if no changes are made to the current supporting practice system. The expected annual sediment yield from the entire sub-basin was reduced by 2420 t/y (or by 5%) and the annual net farm income was increased by 3.99 billion Iranian Rial/y (or by 134%). Results demonstrated that interfacing MUSLE with a GIS is an effective method for the prediction of soil erosion in large watersheds with limited data sets. Overlay operations enable the land manager to obtain higher quality results in a shorter period of time compared to manual calculations. A GIS simplifies the extracting of necessary factors from the databases. However the SPANS-GIS 6.0 was weak in preparing a slope map from the digital elevation database for such mountainous areas. The results indicate that application of land-use optimization methods to reduce sediment yields has great potential in the study area and in other watersheds. The methodology developed in this study can provide a useful tool for watershed managers to reduce sediment yield (soil conservation) while increasing the income of the local inhabitants.