Modeling hydrological response to land use/cover change: case study of Chirah Watershed (Soan River), Pakistan

A ssessment of land use change impacts on watershed hydrologic processes is of great importance to draw the attention of water resource managers and policy makers for better planning and management of the shrinking resources for the survival of surrounding population of the region. Chirah watershed is an important hydrologic unit which generates an average of 0.53 MCM water per day. Soan River originates from here, supplying an average inflow of 0.2 MCM day −1 to Simly Dam. Since a couple of decades, rainfall variability coupled with growing population is threatening Soan River flows. Deforestation and land use/land cover (LULC) changes have affected the natural pathways of water, decreased groundwater recharge, and triggered flash floods. Therefore, the aim of this research was to assess the impact of LULC changes on the hydrologic processes of the watershed using the Soil and Water Assessment Tool (SWAT), a hydrological model to propose management strategies. LULC change assessment was carried out on Landsat-5 TM and OLI_TIRS images through ‘Supervised Classification’ at four-time steps (1990, 1999, 2010, and 2016). Due to limited ground observations, corresponding Google Earth images were used in mapping. The SWAT model was evaluated using the Nash-Sutcliffe efficiency (NSE) and relative volumetric error (RVE) objective functions on monthly time steps. The NSE achieved during calibration and validation, 0.63 and 0.45, respectively, indicates acceptable levels of model performance despite outliers in the hydro-meteorological dataset. Similarly, the RVE was less than ±5% during both periods, indicating that the model simulated water balance reasonably well. Changes in LULC resulted in alteration of all the hydrological parameters. Conversion of scrub thorny forest (14.74%) into built-up areas (9%) resulted in higher imperviousness which triggered runoff to 24% and lowered groundwater recharge by 8% in 27 years indicating that the hydrologic regime is sensitive to LULC changes.