Using GRACE as an Observational Constraint for Exponential Depth Extrapolation of Satellite Soil Moisture 

Abstract

Changes in terrestrial water storage (TWS) can be observed globally by the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) mission, whereas microwave remote sensing primarily provides information on near-surface soil moisture. However, obtaining reliable information on water storage changes in deeper soil layers remains a challenge. A simple yet effective approach is the use of an exponential filter, which approximates deeper soil moisture by accounting for the delayed response to surface soil water variations. Yet, constraining these deeper storage dynamics at the global scale remains difficult. GRACE-derived TWS offers a global observational constraint on integrated subsurface water storage variations.This study utilises soil moisture products from the European Space Agency Climate Change Initiative (ESA CCI), including both surface and root zone soil moisture, together with daily GRACE-derived TWS to assess the potential of GRACE for constraining exponential depth extrapolation of satellite soil moisture. Focusing on detrended, deseasonalised sub-seasonal signals, we compare the agreement between daily GRACE-derived TWS and exponentially filtered soil moisture for different soil moisture integration depths. This allows us to assess how well the observed and depth-extrapolated soil moisture dynamics represent sub-seasonal TWS variability and to identify regions where storage dynamics beyond the represented soil depth may be relevant. The results demonstrate the potential of GRACE as an observational constraint for depth-extrapolating satellite soil moisture and reveal spatial differences in the relevance of deeper subsurface storage dynamics.