Application of unit response approach for spatial prioritization of runoff and sediment sources

Evaluation and control of suspended sediment load as one of the main water quality concerns is important in large basins, especially in basins with planned dam construction. Success in sediment control measures depends, in particular, on proper identification of sediment sources. Since observations alone are normally not adequate for source prioritization/contribution in relation to the downstream sediment loads, hydrological simulation tools may be applied for spatial prioritization of runoff and sediment yield in basins. In this paper, subbasin prioritization was conducted for the Gharasu basin, located in southwestern Iran. Soil and Water Assessment Tool (SWAT) model was calibrated and validated against observed stream flow and sediment data measured at the outlet of the basin. Then, the Unit Response Approach (URA) was applied to prioritize subbasins in terms of their share to the runoff and sediment yield of the main basin outlet. In this technique, two indices were assigned to each contributing unit to determine the degree of changes in the outlet response, i.e. runoff and sediment, caused by the removal of that unit. Within the URA, not only the runoff and sediment load is considered at subbasin scale, but also the effect of runoff and sediment routing throughout the basin up to the main outlet are incorporated. Results showed that subbasins 16 and 3 have the highest runoff and sediment production at their own subbasin outlet, respectively. However, according to the URA-based simulated runoff and sediment yield indices, subbasins 2 and 4 contribute the largest share in the main outlet runoff and sediment yield, respectively. Thus planning for sediment control should primarily focus on subbasin 4 while subsequent plans may be devoted to the subbasin with the next highest sediment contribution.