Predicting the Effects of Land Use on Runoff and Sediment Yield in Selected Sub-watersheds of the Manupali River Using the ArcSWAT Model

Abstract

The quantitative prediction of environmental impacts of land use changes in watersheds could serve as a basis for developing sound watershed management schemes, especially for Philippine watersheds with agroforestry systems. The Soil and Water Assessment Tool (SWAT) model was parameterized and calibrated using data from two Manupali River subwatersheds with an aggregate area of 200 ha, to simulate the effect of land use change on runoff volumes, sediment yield and streamflows. Model simulation results demonstrated that SWAT can predict runoff volumes and sediment yield with Nash- Sutcliffe Efficiency (NSE) ranging from 0.77 to 0.83 and 0.55 to 0.80, respectively. Simulation of land use change scenarios using the SWAT model indicated that runoff volume and sediment yield increased by 3% to 14% and 200% to 273%, respectively, when 50% of the pasture area and grasslands is converted to cultivated agricultural lands. Consequently, this results in a decrease of baseflow of 2.8% to 3.3%, with the higher value indicating a condition of the watershed without soil conservation intervention. Moreover, an increase of 15% to 32% in runoff volume occurs when the whole subwatershed is converted to agricultural land. This accounts for 39% to 45% of the annual rainfall to be lost as surface runoff. While simulation results are subject to further validation, this study has demonstrated that the Soil and Water Assessment Tool (SWAT) model can be a useful tool for modeling the impact of land use changes in Philippine watersheds.