Tomoki Ushiyama

ANALYSIS OF LONG-TERM RAINFALL-RUNOFF-INUNDATION IN THE CHAO PHRAYA RIVER BASIN

Recent development of a rainfall-runoff-inundation (RRI) model allows simultaneous simulation of river discharges and flood inundations in large river basins. Because of the structural simplicity with fast computation, the model is suitable for flood risk mapping with climate change projections. To meet the objective, however, it is necessary to test the model ability for long term simulations. This study employs land surface model (LSM) output of actual evapotranspiration for forcing the RRI model over the period of 1980-2004 in the Chao Phraya River basin. The first simulation result with parameters calibrated for 2011 floods was not satisfactory. The RRI model could not hold sufficient soil water in dry seasons to meet the evapotranspiration demand by the LSM. As a result, the model overestimated the annual runoff. To solve the issue, the subsurface model in mountainous area was updated to incorporate unsaturated zone with thicker soil layer, which improved significantly the simulation results of monthly runoff at various locations.