Historical drought assessment over the contiguous United States using the generalized complementary principle of evapotranspiration

As drought involves diverse natural and human-made processes in hydrologic cycles, it is necessary to track a process-specific variable for efficient drought risk management. In this study, we employed a state-of-the-art formulation of the generalized complementary relationship (GCR) to estimate terrestrial evapotranspiration (ET). The GCR ET estimates were used to assess historical droughts over the contiguous United States (CONUS) in relation to long-term natural fluctuations. Results showed that the GCR provided strong performance in reproducing water balance ET estimates and correlations to the El Nino-Southern Oscillation, when compared to observation-driven estimates and land surface models. For 1982-2011, the GCR yielded better performance than land surface models even with estimated radiation inputs and no wind speed variations. The Standardized Evapotranspiration Deficit Index (SEDI) based on the GCR provided a consistent drought assessment with precipitation-based drought identification and remotely sensed vegetation conditions. The SEDI seems to capture the heat wave-driven flash drought together with water deficit-driven droughts. In the CONUS, the La Nina-like conditions were likely to trigger extensive vegetation droughts with extreme severity. This study found the three long-term environmental factors that affect the general trend of ET deficits over the CONUS, which are the Atlantic Multidecadal Oscillation, the mean air temperature, and the Pacific Decadal Oscillation. Especially, the Atlantic Oscillation seems to be the most significant predictor that could explain the long-term variation of vegetation droughts over the CONUS. This study highlights that the GCR-based SEDI could be useful for monitoring and assessment of drought risks at a continental scale.