The quantitative prediction of environmental impacts of land-use changes in watersheds could serve as basis for developing sound watershed management schemes, especially for Philippine watersheds with agroforestry systems. ArcSWAT is a river basin scale model developed to quantify the impact of land management practices on water, sediment, and agricultural chemical yields in large, complex watersheds with varying soils, land use, and management conditions over long period of time. In this study, the model was parameterized in selected Manupali River Sub-watersheds with an aggregate 2,240 ha to simulate the hydrologic effects of land use. Initial results showed that conversion of forest to agricultural lands causes serious erosion and sediment yield in the area.