Future rainfall characteristics under AR6 climate change scenarios, case study of pasak river basin, Thailand

Thailand is located in Southeast Asia where it is indicated as highly vulnerable to climate change. This country will likely be one of the countries that have experienced negative impacts of climate change. To reduce the impact of climate change on drought and flood, a water management plan under climate change needs to be prepared. The changes of future rainfalls under climate change emission scenarios play an important role and need to be investigated. This study aims to describe projected rainfall characteristics under AR6 climate change scenarios. Rainfalls generated from 10 GCMs were downscaled to a basin scale at the Pasak River Basin using the quantile mapping method. After the downscaling process simulated precipitation significantly improved in both magnitude and volume compared to the observed data using the Nash-Sutcliff coefficient and volume error methods. Slight differences between downscaled and observed precipitation were found with acceptable results using the coefficient of determination. Variabilities of future precipitation under four scenarios (SSP1-2.6, SSP24.5, SSP3-7.0 and SSP5-8.5) of each model were revealed. Annual average of projected rainfalls was slightly different between the models, but large variation compared to others could be found in INM-CM5. The standardized anomaly index was then calculated for historical and two future periods: the near-term (2015-2050) and the long-term (2051-2100), and the frequencies of wet and dry incidents were presented. Very-extremely wet and dry years often occurred in 2051-2100 under SSP2-4.5 and SSP1-2.6 respectively. Finally, the increments in annual rainfalls were obviously seen in SSP2-4.5, SSP3-7.0 and SSP5-8.5 for the near-term and in all scenarios of the long-term. Slight decreasing trend was only found in the near-term under SSP1-2.6.