A method for formulating a new composite drought hazard index for assessment of multiple drought hazards under projected climate and land use changes in agricultural areas: a case of Wang River Basin, Thailand

This study introduces a new method for formulating a new composite drought hazard index that takes into account multiple hazard factors that influence droughts. The same concept can also be applied to other types of hazards such as floods and others. The objective of this study is to combine these hazard factors into a composite hazard index and compare it with single hazard factors in order to understand the overall impact of these hazards. To illustrate its practical application, a case study on droughts in the Wang River Basin in Thailand is presented. Field investigations on topography, soil type, climate, land use, and crops grown indicate that meteorological and hydrological factors have the greatest impact on droughts’ effects on people. The meteorological hazard factor is represented by the standardized precipitation evapotranspiration index, while the hydrological hazard factor is represented by the standardized runoff index. These factors are then calculated and incorporated into the newly developed formula to derive the composite drought hazard index. The study considers different drought timescales of 1-, 3-, 6-, and 24-months to assess the ability of rice, field crops, orchards, and teak trees to tolerate water scarcity. Furthermore, the study examines the impact of climate change and land use change on droughts during the historical baseline period from 2001 to 2020 and the projected future period from 2021 to 2100. Two climate change scenarios, RCP4.5 and RCP8.5, along with land use scenarios, are considered when calculating historical and future drought hazards. According to the UNEP’s Gap Report in 2016, the scenario RCP8.5 is recommended as the most conservative approach.