Future projection of drought risk in southern Taiwan under changing climate

Water shortage problems caused by droughts are exacerbated in the future due to more frequent and severe droughts induced by climate change. This study aims to project future drought risk in southern Taiwan (Tainan and Kaohsiung) under changing climate in terms of the relationship between water uses and water-supply sources for current and future statuses. A multiplicative formula links drought hazard (bivariate probabilities of drought duration and severity), drought exposure (water demand), and drought vulnerability (ratio of reliable water supply to water demand) is proposed in this study to evaluate drought risks in southern Taiwan for various situations and time periods. Evaluating future drought risks in southern Taiwan using the risk-index approach and the most recent CMIP6 (Coupled Model Intercomparison Project Phase 6) scenarios is the novelty of this study. The results indicate that drought risk index (DRI) of 0.395 in Kaohsiung is far greater than 0.155 in Tainan for current status (2019) because of higher demand and less reliable water supplies in Kaohsiung. Rapidly growing demands in the future result in increased DRIs of 0.364 and 0.482 in Tainan and Kaohsiung, respectively, for future target year of 2036 without newly built water supply facilities. Planned water supply facilities efficiently reduce DRIs to 0.214 and 0.369 in Tainan and Kaohsiung, respectively, for 2036. However, impacts of climate change offset effects of planned facilities and raise DRIs to 0.264 and 0.471 under SSP2-4.5 in Tainan and Kaohsiung, respectively. The high emission scenario SSP5-8.5 further increase DRIs in Tainan and Kaohsiung to 0.278 and 0.488, respectively, in 2036. Inevitably increased water shortage risks are noted in Tainan and Kaohsiung due to rapidly extending industrial water needs in the future without planned water facilities. Such increased water shortage risks are efficiently reduced by planned facilities in both regions. However, climate change impacts deteriorate water shortage risks in the future even under the condition of finished planned facilities. The proposed approach not only provides an efficient framework to calculate water shortage risks for various regions and time periods, but also evaluates effects of drought mitigation measures and impacts of climate change.