Impacts of Land Use-Land Cover Changes on Streamflow and Water Balance of Stung Sangkae Catchment Using SWAT

The Soil and Water Assessment Tool (SWAT) model was used to examine the effects of LULCC on hydrological processes in the catchment. The model was evaluated through uncertainty sensitivity analysis, calibration, and validation. The study found that from 1998 to 2018, the coverages of forest land, wetland, and paddy rice decreased by 5.1%, 1.4%, and 1.2%, respectively. The forest land and wetland were converted into cultivated land with an increase of about 7.6% over the study periods. These changes resulted in significant variations of water balance, including an increase of average annual surface runoff by 10 mm, a 3 mm reduction of the average annual lateral flow, a 6.2 mm reduction of the average annual groundwater, a reduction of the average annual evapotranspiration by 0.97 mm, and an increase of average annual water yield by 0.9 mm. The SWAT model results showed a reasonable agreement with the observed values, with a coefficient of determination (R2) of 0.41, Nash Sutcliff efficiency (NSE) of 0.40, and percent of bias (PBIAS) of 0.34. The validated water discharge and water balance results in 2018 indicated that LULCC have impacted the environmental sustainability, especially the streamflow that caused flooding at the downstream watershed due to decline in forest cover at the watershed. The Stung Sangkae catchment is the third largest of Tonle Sap Basin river system with an area of 6,051 km2. The catchment area is mostly lowlands with elevations of less than 100 m above the mean sea level, and with gentle slopes. There are six main soil types within the catchment area and 13 land uses. Of the 13 land uses, 25% are planted with annual crops such as cassava, corn, sugarcane, etc., and 24% is under rice production. The region receives most of its rainfall during the southwest monsoon from May to November, with an average annual rainfall of 1,318 mm/year. The SWAT model was used to estimate, predict, and assess the effects of soil characteristics, land management, and climate change on water resource, sediment, nutrient cycling, and crop growth in the catchment. The model simulates various hydrologic processes, including surface runoff calculation, evapotranspiration, groundwater flow, lateral flow, and percolation. The required input data of SWAT model in Stung Sangkae catchment were collected from different sources both locally and globally, including a digital elevation model (DEM) with a 30 m resolution.