Downscaled Climate Projections for Rainfall Extremes and Drought in Pampanga River and Pasig-Marikina-Laguna-Lake Basin

The Philippines is one of the most climate-vulnerable countries in the world, with Central Luzon often experiencing extreme rainfall, floods, and droughts that are expected to worsen with ongoing warming. Using regional evidence from CORDEX–SEA (CMIP5) and CMIP6 multi-model ensembles, which show stronger rainfall extremes across Southeast Asia, this study produces basin-scale projections for the Pampanga River Basin and the Pasig-Marikina-Laguna-Lake Basin. High-resolution (5 km) climate projections were created with the Weather Research and Forecasting (WRF) model, dynamically downscaled from MRI-AGCM 3.2H (60 km) and 3.2S (20 km), and bias-corrected using the quantile-mapping method. The study analyzed projected changes in seasonal rainfall, heavy-rainfall indices (≥50 mm/day; 95th–99th percentiles), drought indicators (CDD, CWD), and design rainfall (from annual maxima) under RCP2.6 and RCP8.5 scenarios. The results show a clear intensification of heavy rainfall in both basins, especially during the southwest monsoon and under RCP8.5, with annual-maximum rainfall distributions indicating a consistent rise in extreme values and higher design rainfall for all return periods. Spatial patterns vary by basin: Pampanga has the largest increases in the lower western catchment, while the Pasig-Marikina-Laguna-Lake Basin shows a more uniform increase across the basin. At the same time, more consecutive dry days (CDD) and fewer consecutive wet days (CWD) suggest increased hydroclimatic variability, with wetter wet seasons and drier dry periods. These basin-specific, bias-corrected projections offer physically consistent data for flood and drought hazard assessment, agricultural and economic modeling, and climate-resilient water infrastructure design.