A quantitative, GRACE-based framework for regional hydrologic drought characterization

Prolonged hydrologic drought disturbs the natural state of ecosystems, stresses regional water supplies, and can adversely affect agricultural production. Determining the severity of hydrologic drought traditionally lies in evaluations of historical rainfall, stream flow, and soil moisture; yet, a comprehensive measure of the magnitude of a drought's impact on all components of the terrestrial hydrologic system, including surface, soil, and groundwater storage, remains absent from standard drought analyses. NASA's Gravity Recovery and Climate Experiment (GRACE) satellite mission fills a gap by providing monthly measures of terrestrial water storage anomalies based on time-variable gravitational fields. We present a new quantitative, GRACE-based framework for measuring the severity of hydrologic drought. GRACE observations are used for drought characterization by calculating the deviation of monthly-average terrestrial water storage anomalies from the regional climatological reference, where negative deviations represent storage deficits. Each deficit conveys the volume of water that would be required to recover from a drought. We then use modeled terrestrial water storage assimilated with GRACE data as a means of vertically and spatially deconstructing the propagation of drought severity. Assimilation downscales and vertically disaggregates GRACE's total water storage signal permitting the estimation of groundwater. Lastly, we estimate drought recovery time using the finite water storage deficits in the calculation of a likely time for recovery based on statistical percentiles of storage change distributions, for every month through the end of the event. This GRACE-based framework has potential to serve as an addition to future drought application and monitoring tools.