Divergent flash drought risks indicated by evaporative stress and soil moisture projections under warming scenarios

Abstract

Offline drought indices have been utilizable for monitoring drought conditions, but their reliability in projecting future drought risks is uncertain due to non-meteorological influences on atmospheric water demand (Ep). This study investigates the impact of surface resistance sensitivity (rs) to elevated CO2 (eCO2) on projections of future flash droughts (FD). We evaluate FD risks across an East Asian region during a historical period (1981-2020) and a future period (2021-2060) using two Evaporative Stress Index (ESI) series. One series employs the conventional Penman-Monteith (PM) equation for Ep, while the other incorporates a generic rs sensitivity to eCO2 derived from advanced Earth System Models (ESMs). We compare the FD risks identified by the two ESI series with assessments based on soil moisture data from atmospheric reanalysis and multiple ESM projections under two emission scenarios linked with the Shared Socioeconomic Pathways. Results showed that the response of rs to eCO2 has had minimal influences on temporal variations of ESI for the past decades, likely due to its low sensitivity and the masking effects of other environmental factors. However, for the future decades, the ESI projected by the conventional PM equation significantly diverged from soil moisture projections, overestimating future FD risks even under a low emission scenario. We found that incorporating the generic rs sensitivity into the PM equation did not simply resolve the disparity in FD frequencies between ESI and soil moisture projections. Many associated factors contributing to stomatal responses to eCO2 complicate the understanding of future flash drought risks. This study suggests that overreliance on the conventional Ep formula, which neglects non-meteorological effects, could decrease the ability of ESI to detect future FD events under eCO2.