A global analysis of water storage variations from remotely sensed soil moisture and daily satellite gravimetry

Information on water storage changes in the soil can be obtained on a global scale from different types of satellite observations. While active or passive microwave remote sensing is limited to investigating the upper few centimeters of the soil, satellite gravimetry can detect changes in the full column of terrestrial water storage (TWS), but cannot distinguish between storage variations occurring in different soil depths. Jointly analyzing both data types promises interesting insights into the underlying hydrological dynamics and may enable a better process understanding of water storage change in the subsurface.

In this study, we investigate the global relationship of (1) several satellite soil moisture (SM) products and (2) non-standard daily TWS data from the GRACE and GRACE-FO satellite gravimetry missions on different time scales. The six SM products analyzed in this study differ in their data source, processing level, and soil depth for which the SM information is provided. Original level-3 surface SM data sets of SMAP and SMOS are compared to post-processed level-4 data products (both surface and root zone SM) and a multi-satellite product provided by the ESA CCI. A tailored temporal and spatial masking has been applied to focus on time spans with favorable signal-to-noise ratio and to exclude periods with snow cover or frozen soil.

We sample all TWS and SM data sets to a common 1 degree spatial resolution, decompose each signal into seasonal to sub-monthly frequencies and carry out the comparison with respect to spatial patterns and temporal variability. We find increasingly large correlations between the TWS and SM for deeper integration depths (root zone vs. surface layer) and for post-processed level-4 data products. Even for highpass-filtered (sub-monthly) variations, significant correlations can be found of up to 0.6 in regions with large high-frequency variability. A time-shift analysis shows differences in the temporal dynamics of soil moisture versus TWS storage variations, indicating different water storage dynamics in the different depth layers. Precipitation data have been added to the analysis to enhance the interpretation of the comparison of soil moisture and total water storage variations.