This study employs the GIS-based Revised Universal Soil Loss Equation (RUSLE)-Sediment Delivery Ratio (SDR) model to assess the spatial variability of soil erosion (SE) and sediment yield (SY) in the 450 km2 Bongabong Watershed in eastern Mindoro Island. As inputs to the model, important datasets like Digital Elevation Model (DEM), 30-year rainfall records, land use and soil maps, and published literature are acquired for watershed soil loss analysis. To estimate SE, the RUSLE model considers five factors in raster data formats, namely: rainfall and runoff erosivity (R factor), soil erodibility (K factor), topography (LS factor), cover and management (C factor), and support practice (P factor). The application of GIS in obtaining and computing these factors has greatly improved the accuracy of results and reduced the time of RUSLE execution. For predicting SY, the RUSLE-SDR model is used, in which SDR is obtained as a function of basin area based on the equation developed by Julien P. (2002). The calculated sediment yield allows the estimation of the annual sediment replenishment rate (sustainable sediment yield) of Bongabong catchment area that has been incessantly exposed to massive sediment extraction. The results of the study reveal that both the upper mountainous region and alluvial plains require implementation of sediment management measures suitable for the type of river sub-basins within the watershed. All sediment management options involving removal and disturbance of river deposits must be strictly evidence-based and must be performed in the least environmentally damaging manner.
INTRODUCTION
Soil Loss
Soil erosion is a physical phenomenon wherein soil particles are detached from the soil mass and washed downslope by surface runoff (Kaffas & Hrissanthou, 2019). It is a widespread degradation process that naturally occurs on sloping land aggravated by surface water runoff or by wind in areas with scarce vegetation (Montanarella, 2016). Soil erosion has negative impacts in the field of agriculture due to the decreased soil productivity and arable land size, which thus leads to economic problems triggered by food scarcity in agricultural-reliant areas (Bhandari, Joshi, Regmi, & Awasthi, 2021). Soil erosion also has adverse environmental impacts in terms of water quality and pollution due to stream sedimentation (Lal, 1998), as well as flooding (Poesen & Hooke, 1997).
Soil erosion is considered one of the Philippines' significant environmental problems and poses a threat to the country’s agricultural sustainability (IBON Foundation, 2006). The geomorphologic and meteorological characteristics of the country increases the susceptibility to soil erosion, given its steep topography and high precipitation. Furthermore, the country also relies heavily on the agriculture with about 40% of the labor force employed in the agricultural sector. Thus, soil erosion and land degradation have serious ramifications for the country’s economic status and social well-being.