A GRACE Derived Groundwater Deficit as an Indicator for Characterization of Hydrological Drought and Understanding Climatic Impact on Groundwater Resources in South Korea

Although South Korea is under the monsoonal influence, the shortage of summer monsoon and relatively dry spring and winter cause droughts on the Korean Peninsula. Since the late 1960s, the consecutive severe droughts in South Korea have caused extensive groundwater development for agriculture and economic growth. Therefore, identifying and estimating drought impact on the country's groundwater resources are crucial for establishing policies and regulations on groundwater exploitation and recharge. Drought is a complex, slow-moving disaster that is challenging to track, characterize, and estimate severity. However, precipitation deficits spread over time through the hydrologic cycle's components; surface runoff, soil moisture, streamflow, and groundwater. Therefore, the Standardized Precipitation Index (SPI) is a commonly used drought index, but it cannot explain its impact on groundwater resources. Thus, this study aims to provide a quantitative method for calculating the frequency and severity of hydrological droughts based on groundwater deficit derived from NASA's Gravity Recovery and Climate Experiment (GRACE) mission data. The estimated annual groundwater deficit in mass over the country varies from -0.07 km3 to 0.04 km3, with a trend in the mass of -1.60×10-3 km3/year from 2002 to 2021. Thirteen hydrological drought events have been identified based on the deficit, and seven of those coincided with recorded historical droughts. The highest total drought severities recorded in 2015 and 2019 are -34.27 km3 and -55.38 km3, respectively. The monthly deficits clearly state the quantity of water necessary to restore normal water storage conditions. So, maximum and minimum days to recover the hydrological droughts are estimated based on the strength of the storage-based approaches. The groundwater storage deficit approach compares favourably with the meteorological drought databases. It accurately marks the beginning, end, and duration of hydrological droughts and their instantaneous severity and peak magnitude.