Abstract
The Advanced Microwave Scanning Radiometer for the Earth Observing System and Advanced Microwave Scanning Radiometer 2 sensors (AMSR) have provided multifrequency microwave measurements of the global terrestrial water cycle since 2002. A new AMSR surface wetness index (ASWI) was developed by analyzing the near‐surface atmospheric vapor pressure deficit (VPD), surface volumetric soil moisture (VSM), and land surface fractional open water (FW) time series from an established AMSR Land Parameter Data Record (LPDR). The ASWI allows for multicomponent and independent satellite assessments of near‐surface drought conditions by exploiting the weighted anomalies of VPD, VSM, and FW. Comparisons between ASWI and more traditional drought metrics, including the Palmer moisture anomaly index (PDSI‐Z) and the U.S. Drought Monitor, showed generally consistent classifications of drought severity for three major droughts over the Contiguous United States since 2002. The AWSI showed moderate (0.3 ≤ R ≤ 0.7 for 56% of area) to strong (R > 0.7 for 29% of area) correlations with the PDSI‐Z during the summer months (June–August) from 2002 to 2017. ASWI and PDSI‐Z differences were attributed to AMSR retrieval uncertainties and the different aspects of drought represented by the indices. Comparisons between ASWI and the Gravity Recovery and Climate Experiment drought severity index (GRACE‐DSI) showed strong correspondence (R = 0.61) in regions where possible long‐term total water storage changes occurred. The sole reliance of the ASWI on satellite microwave remote sensing and continuing AMSR2 operations enables effective global monitoring of drought conditions while providing new information on the atmosphere, soil, and surface water components of drought.
Plain Language Summary
Extreme hydrologic events affect the economy, environment, and society. The anomalies of the water cycle components are associated with drought events. Considering the capability of multifrequency microwave remote sensing in observing multiple water cycle components, this study developed a new surface wetness index by analyzing near‐surface atmospheric vapor pressure deficit, volumetric soil moisture, and land surface water inundation time series from an established satellite data record. The new index showed consistent assessment of major drought events relative to traditional drought monitors while providing enhanced information on the water cycle components of drought. A combined analysis of the index with satellite total water storage measurements provides complementary information on the different aspects of droughts, including relatively dynamic surface water changes and slower evolving groundwater conditions. The index enables independent and continuous satellite assessment of global drought conditions.
Key Points
A new surface wetness index was developed from satellite‐observed vapor pressure deficit, soil moisture, and surface water cover
The index is consistent with traditional drought monitors but allows for analyzing the relative importance of water cycle components
The satellite‐based multicomponent index allows for independent monitoring of global drought conditions