Development of a national-scale VIC hydrological model to project future changes of the water resources of the Philippines

We applied a version of the macro-scale hydrological model VIC, into which we incorporated a 2D lateral groundwater flow model, to simulate the hydrology of the Philippines. We used global data sets to parameterise the model, which uses a ~1km grid to represent each of the country’s islands; global meteorological driving data were downscaled to this resolution. The model was calibrated over the historical period (1990-2019) against available observed river flow time-series by adjusting soil, aquifer, and riverbed hydraulic properties; Nash-Sutcliffe Efficiency scores of up to 0.53 were obtained. We applied projections of future climate for the 2050s and 2070s derived from global climate simulations undertaken by the UK Meteorological Office’s Hadley Centre – the UKCP18 projections – considering two greenhouse gas concentration pathways: RCP2.6 and RCP8.5. Projected future reductions in precipitation translate into decreases in surface runoff, groundwater recharge, and river baseflow, on average, but the simulations highlight regional differences in groundwater and surface water availability over both the historical and future periods.