Assessment of distributed hydrological drought based on hydrological unit map using SWSI drought index in South Korea

This study is to assess the hydrological drought in South Korea in a semi-distributed form using an equation modifying the SWSI (Surface Water Supply Index). Based on the nationwide gauging stations of 61 precipitation, 53 streamflow, 7 dam inflow and 31 groundwater level, South Korea was divided into 32 (sub-regions) and 112 (accounting units) watersheds. The evaluation result was compared with the historical drought records of 2001 which is the driest year in history. The results showed that the monthly-based SWSI value of the 112 watersheds division gives a more reliable representation than that of the 32 watersheds division. In addition, the weekly-based SWSI evaluation accurately showed the continual drought condition while the monthly-based evaluation failed to catch the drought within a month.