“…While purely data-driven deep learning techniques, partly led by authors of this whitepaper, have proven to be extremely powerful in hydrologic applications (Shen, 2018;Shen et al, 2018) , especially in modeling soil moisture (Fang et al, 2017(Fang et al, , 2019Fang & Shen, 2020) , streamflow (floods) , snow (Meyal et al, 2020) , and water quality indicators like water temperature (Rahmani et al, 2020) and dissolved oxygen (Zhi et al, 2020) , they are constrained by data availability and cannot make predictions in variables that are not directly observed at large scales, e.g., groundwater flow and evapotranspiration (there are global satellite-based estimates, but they are not direct and contain substantial modeled elements; there are also in-situ data at hundreds of sites, but they are far from covering the heterogeneity of the world). As mentioned earlier, how can we utilize multifaceted observations to inform parts of the water cycle that is not observed?…”