In-depth knowledge about the global patterns and dynamics of land surface net water flux (NWF) is essential for quantification of depletion and recharge of groundwater resources. Net water flux cannot be directly measured, and its estimates as a residual of individual surface flux components often suffer from mass conservation errors due to accumulated systematic biases of individual fluxes. Here, for the first time, we provide direct estimates of global NWF based on near-surface satellite soil moisture retrievals from the Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites. We apply a recently developed analytical model derived via inversion of the linearized Richards’ equation. The model is parsi-monious, yet yields unbiased estimates of long-term cumulative NWF that is generally well correlated with the terrestrial water storage anomaly from the Gravity Recovery and Climate Experiment (GRACE) satellite. In addition, in conjunction with precipitation and evapotranspiration retrievals, the resultant NWF estimates provide a new means for retrieving global infiltration and runoff from satellite observations. However, the efficacy of the proposed approach over densely vegetated regions is questionable, due to the uncertainty of the satellite soil moisture retrievals and the lack of explicit parameterization of transpiration by deeply rooted plants in the proposed model. Future research is needed to advance this modeling paradigm to explicitly account for plant transpiration.
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Acknowledgments. MS, AE, and LG acknowledge support from the National Aeronautics and Space Administration (NASA) Terrestrial Hydrology Program (THP, 80NSSC18K152) through Dr. J. Entin and the New (Early Career) Investigator Program (NIP, 80NSSC18K0742) through Dr. A. Leidner and Dr. T. Lee. SBJ and MT acknowledge funding from National Science Foundation (NSF) Grants 1521469 and 1521164. MT and EB acknowledge support from the University of Arizona (UA) College of Agriculture and Life Sciences (CALS) Innovation Venture Investment Project (iVIP). JBF and AJP contributed to this work from the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA and acknowledge support by the NASA SUSMAP program.