Hydrological modelling of the Benguet Province, Philippines

This report describes the development and application of a hydrological model to the Benguet Province, Philippines. This is based on a downscaled version of the Philippine Hydrological Model. The model simulates the natural hydrological cycle; any surface water and groundwater abstractions or dams are not considered. The model is driven by two sets of climate data: for the historical period (1979 to 2024) by ERA5 climatology and for 1980 to 2089 by UKCP18 climate change projections.
The historical model is compared with observations at gauging stations on the rivers Agno, Bauang, Bued and Galano. There are discrepancies between the simulated and the observed flows; however, there are uncertainties in the observations and the climate data, and surface-water abstractions and dams affect the river flows. For example, the hydrological model does not consider any abstractions from groundwater and surface-water sources or dams, which both impact the flows in the Agno river basin. Comparing the simulated flows with catchment size, we find that there is a linear relationship between catchment size and simulated naturalised river flow, which supports the inference that gauge flows are both uncertain and affected by abstractions.
The future climate change projections suggest a change in the flow regimes of all simulated rivers. In general, higher flows are suggested to increase and lower flows to decrease. The magnitude of this change varies from catchment to catchment and is predominantly driven by the projected change in precipitation.
Considering a representative greenhouse gas concentration pathway (RCP) of 8.5, a scenario in which estimated global average temperatures rise to 4.3°C above the preindustrial level by 2100, the high flows (5 per cent exceedance) across the assessed locations increase on average by 18 per cent (15 to 24 per cent), whereas the low flows (95 per cent of exceedance) decrease on average by 27 per cent (-48 to 5 per cent). There are some differences between the subcatchments and ensemble members for the climate change projections. More detail is shown in Figure 1 for four river flow gauging stations. The boxplots show the projected change in river flow for different flow percentiles and illustrate the uncertainty between the different members of the ensemble of climate projections.