Evaluation of Development and Land Use Change Impacts on Stream Flow and Sediment Yield in Margalla Hills

Abstract

Water resources are affected by development activities and land use change which is undeniable and most significant. Land use change can alter the hydrology of a watershed. The present study has been carried out for Rawal and Simly Dams ‘watershed. This study analyzed the historical land use changes which had taken place in catchment area for the period 1975-2012 (38 years) for Rawal Dam and for period of 1983-2012 (30 years) for Simly Dam. The main objectives of this study were to identify the land use change pattern over time, to obtain land use information from satellite imagery classification using GIS and Remote Sensing and to analyze its effects on rainfall-runoff and runoff-sediment relationships for the catchment areas of Rawal and Simly Dam. In this study the catchment area of the Rawal and Simly Dam was delineated on topographic sheets and using Planimeter the catchment area of Rawal Dam was computed as270 km2 and the catchment area of Simly Dam was computed as150km2. Using digital elevation model the catchment area of Rawal Dam was computed as 272.8km2 and Catchment area of Simly Dam as 153.5km2.To check the effect of land use changes on rainfall-runoff and runoffsedimentation relationship for Rawal and Simly Dam Double Mass Curve analysis was performed. Double Mass Curves with trend curves of annual rainfall-runoff, Monsoon month’s rainfall-runoff were drawn for both Rawal and Simly Dam site. It can be concluded that slope of the trend curve is changing for both Rawal and Simly Dam, which shows that land use changes have affected the rainfall-runoff and runoff-sedimentation relationship. To quantify land use change in catchment, the satellite imageries of 1992, 2000 and 2010 were used. The unsupervised classification was performed and 6 land use classes water body, forest, vegetation & agriculture, rangeland, built-up area and bare land were made. Land use analysis of 1992-2010 for Rawal catchment has shown 2.4 km2decrease in water bodies’ area, 13.6km2 decrease in forest area , 37.4km2 decrease in vegetation and agriculture, 28.3km2 increase in range land, 16.6km2 increase in built up area and8.5km2increase in bare land. Similarly land use analysis of Simly Dam from 1992-2010 has shown 0.6 km2 decrease in water bodies area ,18.7 km2 decrease in forest, 4.1km2 decrease in vegetation and agriculture,6.8 km2increase in rangeland,6.5km2 increase in built-up area and10.1km2 increase in bare land. It is recommended that more meteorological stations should be installed in both catchments to monitor meteorological conditions, sediment data should be collected on yearly basis. GIS and remote sensing with Double Mass Curve Analysis should be used for watershed management.