Changes in hydrological extremes as revealed by GRACE/GRACE FO data

Severe floods and droughts have been increasing in frequency and severity around the world. System detection of changes in hydrological extremes is essential for preparation and planning and requires spatially and temporally consistent observational data. The Gravity Recovery and Climate Experiment (GRACE) and its follow-on (FO) mission have been providing terrestrial water storage (TWS) data since 2002. Representing vertically integrated water storage changes that include soil moisture, groundwater, snow/ice and surface water, GRACE TWS data provide an unparalleled opportunity to study hydrological extremes in a manner that is not possible with any other type of observations. In particular, the ability to detect groundwater storage changes makes this record especially valuable for understanding the depth and extent of changes in hydrological extremes.

Using a spatial-temporal clustering algorithm, we identified more than 1,000 wet and dry events globally and found that, across four major climate classes, hydrological extremes became more severe during the study period (2002-2021), suggesting increases in amplitudes of TWS changes. Concurrent increases in severity of both wet and dry extremes suggest that severity of weather whiplash events, as California recently experienced, has also increased. Significant changes in frequency of occurrence were found in the Dry climate where the number of wet/dry events decreased/increased, indicating a drying trend in that climate. Drying trends were also found in several densely populated regions that may be attributed to increases in water demands and evapotranspiration in responses to global warming. In the Continental climate where temperature has risen faster than global average, the number of dry events increased significantly. Impacts of climate changes were also found in the most intense events, ranked using spatial temporal accumulate TWS anomalies, including the worldwide most intense dry event which occurred in the warmest year on record, 2016, in Brazil. Intensification of the hydrological cycle is most distinctly reflected in the global total intensity of extreme events which increased significantly in correlation with global temperature. These results underscore the importance of using GRACE/FO data for examining and quantifying changes in hydrological extremes, with implications for water management and emergency responses.