Dear Colleagues,

Earth observation (EO) satellite missions provide invaluable global-to-regional estimates of hydrologic variables from atmosphere to lithosphere. From these EO missions, the Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission (GRACE-FO) record data that are inverted to so-called terrestrial water storage (TWS), i.e., a virtual summation of surface and subsurface water storage. The separation of TWS into its individual water-storage compartments requires careful postprocessing that challenges the use of these data for hydrological applications. Here, specific analysis of auxiliary mission data would improve the knowledge of spatial and temporal dynamics of TWS and its subdomains. Satellite altimetry missions provide measurements of water-level changes in seas, lakes, and rivers. Various satellite missions, including SMOS, SMAP, ASCAT, AMSR2, and Sentinel-1, provide multiyear to multidecadal soil-moisture estimates at various soil depths. Moreover, seasonal changes in vegetation water content related to microwaves (e.g., SMOS vegetation optical depth) or optical observations (e.g., MODIS normalized difference water index) may also influence TWS dynamics. However, a reliable and physically sound relationship between measured signals, and their respective spatial and temporal dynamics with water storage is considerably complex and demands research in this scientific field.

Despite the increasing interest in using remote-sensing techniques for subsurface hydrology, its benefits need to be realized for monitoring vegetation, surface, and root-zone soil water storage, as well as groundwater storage from space. Therefore, the goal of this Special Issue is to demonstrate the contribution of satellite observations, and physical, conceptual, and/or statistical modeling techniques for estimating these water-storage components and their changes from  local (catchment) to global (hydrological-cycle) scales. Contributions introducing the latest developments in terms of new sensors and satellite missions that will be available in the near future, as well as those addressing the integration of remote-sensing products with surface and groundwater process models, including techniques for data assimilation, are particularly invited. Contributions dealing with all components of water storage between atmosphere and bedrock are welcome, and may include in situ measurements and/or nonremotely sensed datasets for parameter estimation at various spatial and temporal scales. Examples of potential focus areas are:

• Advances in combining remote-sensing techniques with hydrological (process) models to provide spatially distributed, high-resolution water-storage variations.
• Multisensor approaches for the estimation of water-storage status and variations to indicate individual contributions, when applicable, of soil water, lakes, vegetation, and groundwater contributions to TWS signals.
• Space- and air-borne or ground-based experiments to study water-storage estimation techniques of compartments between Earth surface and bedrock.
• Case studies on a global or local scale for dedicated water-storage assessment with intercomparisons of in situ observations, remote sensing products, and modeling results.
• Data assimilation, machine learning, and statistical or physical approaches to improve TWS estimation.

Prof. Dr. Ehsan Forootan
Dr. Carsten Montzka
Dr. Thomas Jagdhuber
Prof. Dr. Vagner Ferreira
Guest Editors

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• water storage
• water content
• hydrology
• hydrological modeling
• remote sensing
• GRACE
• GRACE-FO
• microwaves
• radiometer
• altimeter
• SAR
• multisensor
• data fusion
• assimilation
• lakes
• rivers
• soil moisture
• root-zone moisture
• groundwater