Dynamic downscaling and bias correction of rainfall in the Pampanga River Basin, Philippines, for investigating flood risk changes due to global warming

Tomoki Ushiyama, Akira Hasegawa, Mamoru Miyamoto, Yoichi Iwami
Received 2016/09/01, Accepted 2016/10/29, Published 2016/11/30

Tomoki Ushiyama1), Akira Hasegawa1), Mamoru Miyamoto1), Yoichi Iwami1)

1) International Centre for Water Hazard and Risk Management, Public Works Research Institute

The Pampanga River Basin in Philippines suffers from floods due to typhoons or monsoonal rainfall every year. Assessment of changes in flood risk due to global warming is, therefore, an important issue for this flood-vulnerable basin. We studied possible changes in rainfall features under present (1979–2003) and future (2075–2099) climates using dynamic downscaling of MRI-AGCM experiments. The GCM projections were downscaled into a finer resolution of 5 km for hydrological simulation (catchment size of 10,500 km2). The downscaled rainfall overestimated the number of weak rainfall events, which subsequently resulted in overestimation of monthly rainfall. Bias correction was carried out for downscaled rainfall with reference to raingauge rainfall to perform cumulative distribution mapping. The simulation results found that monthly rainfall would change slightly between present and future climate conditions, with extreme rainfall very likely to increase in the future. The annual maximum 48 h rainfall with a 50-year return period may increase from 320 mm under the present climate to 470 mm (MRI-AGCM 3.2S) or 530 mm (MRI-AGCM 3.2H) under the RCP8.5 future climate scenario. This increase in extreme rainfall in the future would have a significant impact on this vulnerable river basin.

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