Impacts of Land Use Change on Streamflow, Sediment Yield, and Nitrate Loading in a Small Philippine Catchment with Lake Subbasins

Abstract

This study assessed the impacts of land use changes on streamflow, sediment yield, and nitrate loading in the Macopa River Basin and 2 connected lake subbasins, Sampaloc and Bunot, in San Pablo City, Laguna, Philippines. A Soil and Water Assessment Tool (SWAT+) model was developed using topographic, soil, climate, and land use data from 1997 to 2022, incorporating land use layers for 2003, 2010, 2015, and 2020. Calibration for streamflow (2006 to 2007) showed good agreement with data (NSE = 0.73, R² = 0.80), while validation (2017 to 2018) yielded lower accuracy, reflecting limitations in available observed data. Hindcasting results indicated minimal long-term change in streamflow despite rapid urban expansion. Sediment yield increased by nearly 80% over the simulation period, particularly in areas with reduced vegetation cover. Simulated nitrate levels were generally stable but showed sharp increases during periods of high fertilizer application. Spatial analysis indicated that erosion was most pronounced in urbanized and high-slope zones, while nitrate loading patterns remained consistent across land use scenarios, with the highest accumulation downstream. These findings emphasize the importance of preserving vegetative cover in upper catchment areas and integrating land use change considerations into watershed planning.