The 2011 monsoon season was exceptionally heavy, leading to extensive and long-lasting flooding in the Chao Phraya river basin. Flooding was exacerbated by rapid expansion of urban areas into flood plains and was the costliest natural disaster in the country's history, with direct damages estimated at US$ 45 billion. The present study examines the flood behavior in 2011 and flood impact from changing climate. Two generations of the global climate model (GCM), ensembles CMIP3 and CMIP5, are statistically downscaled through historical 20th century and future projections. The majority of GCMs overestimate the dry spell (in June and July) and underestimate the peak precipitation (in May and September). However, they can simulate the mean precipitation reasonably well. Use of the Multi Model Mean shows continuously increased precipitation from near-future to far-future, while the Multi Model Median shows increased precipitation only for the far-future. These findings in changing precipitation are assessed by flood simulation. With several adaptation measures, flood in the lower Chao Phraya river basin cannot be completely avoided. One of the best practices for a high flood risk community is to raise the house with open space in the first floor. This is promoted as one resilient approach in Thailand.

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