Storage-based Drought Indices Using Satellite Gravimetry and External Climatology

Abstract:

Extreme events like drought have occurred more frequently and with higher intensity over the last two decades. Droughts in Australia (Millennium drought), California, northern India, and southeastern Brazil are just a few examples which have resulted from climate change and unsustainable water management. Satellite observations have improved our understanding of the temporal and spatial variability of droughts. Since March 2002, the Gravity Recovery and Climate Experiment (GRACE) and its successor GRACE Follow-On (GRACE-FO) have been observing variations in Earth's gravity field yielding unprecedented information about changes in total water storage anomaly (TWSA). The TWS vertically integrates all forms of water on and beneath land surface including surface water, soil moisture, snow, and groundwater storage.

Drought indices characterize drought in terms of their severity, location, duration and timing. Several drought indices have been developed based on the GRACE water storage anomaly relative to a GRACE-based climatology. This approach, though, suffers from the short record of GRACE, about 15 years. In our approach, the limited duration of the GRACE observations necessitates the use of external datasets of TWSA with a more extended period for climatology. Drought characterization comes with its own uncertainties due to the inherent uncertainty in the GRACE data, the various post-processing approaches of GRACE data, and different options for external datasets on the other hand.

This study offers a modified storage-based drought index and compares it with the strengths and weaknesses of existing drought indices. Furthermore, the uncertainty of the final index is quantified. We also assess the sensitivity of major global river basins to the duration of the observations. The study allows for a more informative storage based drought, including uncertainty, thus enabling a more realistic risk assessment.