The Indispensable Role of Evapotranspiration in Driving Terrestrial Water Storage Drought Development

Terrestrial Water Storage (TWS) drought is a major hydrological hazard with severe impacts on water resources security, crop yield, natural ecosystem production, and socioeconomic stability. Precipitation has long been assumed as the major driver in the development of TWS drought, but recent work highlights evapotranspiration (ET) as a key additional driver—yet its specific mechanisms and relative importance remain underexplored. In this study, we first derive ET using the terrestrial water budget from observational data (2003–2019) and then propose a diagram to unravel the role of ET anomalies and precipitation minus runoff (PR) anomalies in driving TWS drought intensification and recovery across diverse climate regions. Our results show an asymmetric role of ET in TWS drought dynamics: positive ET anomalies (ET⁺, ET exceeding climatology) frequently drain TWS and intensify TWS drought, while negative ET anomalies (ET^-) preserve TWS and promote TWS recovery. Regional patterns of ET and PR in driving TWS drought development differ markedly. Drought intensification is driven mainly by the combination of ET⁺ and PR^- in arid regions, while ET⁺ often offsets PR⁺ to lead drought intensification in humid regions. Drought recovery is predominantly driven by PR⁺ in hyper-humid and humid regions but is more commonly dominated by ET^- than by PR⁺ in arid and semi-arid regions. These findings provide new perspectives into the complex, indispensable role of ET in TWS drought development, highlighting the need to incorporate ET processes into improved drought monitoring, prediction, and management frameworks.