Evapotranspiration frequently increases during droughts

During droughts, low water availabilities limit soil evaporation and induce stomatal closure to prevent transpiration, leading to reductions in evapotranspiration (ET). At the same time, drought-associated meteorological conditions such as high temperature elevate atmospheric evaporative demand, acting to increase ET. However, the overall effect of drought on the sign of ET anomalies remains unknown, as are the determinants of this response. Positive anomalies during drought (ET+), in particular, are of concern because they quickly deplete water resources, may cause flash droughts, and exacerbate ecosystem stress. Because remotely sensed ET datasets implicitly assume a stomatal response to drought, they cannot provide direct observational constraints of the prevalence of ET+. Eddy covariance tower records are often too short and sparse to adequately sample drought conditions. To avoid these shortcomings, we used a water balance approach to derive a new estimate of ET+ occurrence during droughts by combining total terrestrial water storage (TWS) observations from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) with Global Precipitation Climatology Project precipitation data. The robustness of this approach is demonstrated across 104 hydrological basins. With this new water balance-based estimate, we showed that ET+ during droughts are globally widespread. On average, ET+ occurs in ~45% of drought periods, and it is more likely to occur during milder droughts (with relatively lower P reductions and ample available TWS). CMIP6 Earth system models (ESMs) underestimate the observed ET+ probability by nearly half. This underestimation is particularly large in relatively dry locations with an aridity index (P/PET) below ~1.5 and can be attributed in part to an overly strong ET response to decreases in soil moisture in these regions. Furthermore, ESM’s lack of accounting for variability in plant water stress response traits within plant functional types exacerbates their underestimation of ET+. This demonstrates for the first time that local adaptation of plant water stress response traits reduces the impact of droughts on ET. These process representations should be improved to reduce model uncertainties in predicting drought impacts on the energy-water-carbon nexus.