Analysis of climate change impacts on dependable flow in Lasang River watershed using the SWAT model

Climate change influences streamflow availability and dependability in watersheds due to its effect on the hydrologic cycle. This study assessed the quantitative impacts of climate change on dependable flow in the Lasang River watershed in southern Philippines using the Soil and Water Assessment Tool (SWAT) model. Three climate change scenarios including the baseline conditions and moderate and extreme conditions were formulated based on CMIP6 climate projections in the Philippines. The SWAT model was adequately calibrated and validated (NSE of 0.54 to 0.56) and was subjected to sensitivity and uncertainty analyses to ensure accurate representation of the watershed's hydrological behavior. Results of model simulation and flow duration analysis showed that moderate increases in precipitation and temperature due to climate change have negligible effects on dependable flow (1.1% increase), while extreme climate change conditions would result in relatively greater impacts on the watershed's dependable flow at a 14% increase. Results suggest that the river system may remain practically normal under a moderate climate change scenario, while greater water availability and dependability could be expected from the watershed under an extreme climate change scenario, particularly during the dry season for irrigation and other purposes. However, increased streamflow may still cause seasonal variability and potential dry-season water availability constraints under future climate conditions. Results obtained in this study could be used for proper irrigation system planning, design, and management and at the same time could serve as a basis for policy formulation geared towards sustainable water resources management under changing climatic conditions in the Lasang River watershed.