Water Quality Modeling Using Soil and Water Assessment Tool: A Case Study of Rawal Watershed

Abstract

The water quality of Rawal watershed has been highly deteriorated because of increased concentration of organic nitrogen and phosphorous coming from non-point sources causing eutrophication. The main objective of this study was to use a watershed scale model SWAT (Soil & Water Assessment Tool) as a modeling tool for predicting the impact of the Land Use change on water quality of Rawal Lake. Specific objectives were to (a)develop pollution source inventory mapping(b) quantify the pollutant load (organic nitrogen and phosphorus) with respect to Land Use change that causes eutrophication and (c) model the alternative Best Management Practices (BMP’s) and evaluate their effectiveness. In the pre field phase the collection and preprocessing of data (DEM & Landsat images (30m resolution)) was done. Land Use (2001 & 2010) classification was done using Landsat imagery in ArcGIS. For the pollution source inventory, mapping the study area was surveyed. In the post field phase the model was calibrated (2002-2006) by using the observed and simulated surface runoff data and then validated (2007-2010) .The, model’s accuracy was further verified by using R2 and Nash Sutcliffe Efficiency (NSE). SWAT simulations resulted in 392.40 % and 391.72 % increase in the quantity of organic nitrogen and phosphorus with respect to Land Use change (2001 & 2010). By the application of BMP’s such as   filter strips, the average reduction in the concentration of N could be 75.155% while that of P could be 96.93%, similarly by the application of terracing the average reduction in the concentration of N could be 52.73% while that of P could be 94.18% in the selected sub basins. By the application of contouring the average reduction of 35.64% for N and 92.09% for P could be achieved. This study would provide decision maker a source of information on most feasible and cost effective BMP’s.