Drought-prone regions host many of the world's least developed countries, which are experiencing increasing drought risk as a result of population pressures and increasing drought frequencies (UNDRR, 2015). This is evident in Somalia, where estimated fatalities from the 2011 drought exceeded 250,000 and a further drought in 2017 required humanitarian assistance for 6 million people (FAO, 2018(FAO, , 2019. Drought early warning systems (DEWSs) aim to monitor and forecast drought risk, enabling early interventions that mitigate the most severe socioeconomic impacts. To formulate this risk, DEWSs require evidence of drought intensity and community exposure collected by monitoring systems (Brown, 2014; UNDRR, 2015).
Current Drought Monitoring Methods for Early WarningA range of regional-scale, satellite-based indicators are used to monitor physical drought development. The Standardized Precipitation Index (SPI) is a popular indicator for monitoring meteorological drought (McKee et al., 1993;Van Loon, 2015). The SPI indicates rainfall anomaly compared to the long-term average for that chosen time period (Sheffield et al., 2014). For agricultural drought (Van Loon, 2015), satellite-based indicators are used such as soil moisture and the Normalized Difference Vegetation Index (NDVI; Liang et al., 1994;NASA Worldview, 2022;Tarpley et al., 1984). These indicate available soil moisture for vegetation growth, which in turn indicates crop failure in areas where agriculture is nonirrigated (Brown, 2014; FEWS NET, 2021; FSNAU, 2021;Sheffield et al., 2014). However, in most arid zones during late dry seasons, water availability for human consumption depends on groundwater supply. As such, these surface indicators are not ideal for assessing hydrological droughts-the most intense form of drought that causes water shortages (