Probabilistic Drought Forecasts and Uncertainty Analysis Using the Modified Surface Water Supply Index in the Korean Peninsula

Recently more accurate drought forecasts and a drought monitoring system have been needed in order to mitigate increases in drought damages. For this purpose, it is necessary to develop, optimize, or improve the appropriate drought index according to the actual situations in Korea. The drought information based on the accurate drought index should be used to manage and make policies to secure water resources. This study proposes techniques for modifying the Surface Water Supply Index (Modified SWSI, MSWSI), which is the hydrological drought index used in Korea, and forecasting monthly droughts using the MSWSIs. First, for modifying SWSIs, this study (1) investigated all officially collected hydro-meteorological factors in the watershed to determine the appropriate factors, and (2) applied the appropriate probability distributions for each factor. Second, for the monthly drought forecasts, this study carried out the ensemble techniques, which included various combinations using historical observations. This procedure was applied to the Nakdong River basin, which consists of 22 sub-basins in the southern region of Korea, for the 2006 and 2014 drought events. The result confirmed that the MSWSI is more effective than the SWSI. Probabilistic drought forecasts using MSWSI also showed excellent results for all events. Last, this study quantified the uncertainties of MSWSIs using maximum entropy. The results revealed that the uncertainties were caused by the selection of MSWSI input components and the probability distributions, and that the uncertainties of the probabilistic forecasts using MSWSIs were reduced in most of the watersheds.