Theia SAGSA Water Storage Anomalies

GRACE and GRACE-FO spatial missions
The GRACE (Gravity Recovery And Climate Experiment; Tapley et al., 2004) and GRACE-FO (GRACE-Follow On; Landerer et al., 2020) missions enable the measurement of spatial and temporal variations in the Earth's gravity field with a spatial resolution of a few hundred kilometers and a temporal resolution ranging from weeks to months. To derive variations in continental water stocks, a few post-processing steps must be applied. The latter aim at reducing the instrumental noise, at estimating the very long wavelength components (degree 1 and 2) of the gravitational field to which satellites are not very sensitive, and at removing signals related to the deformation of the solid Earth (glacial isostatic adjustment and co-seismic deformations). The uncertainties associated with the processing and post-processing of GRACE and GRACE-FO data are estimated using an ensemble approach developed by Blazquez et al. (2018).

Data description
The products distributed in this collection are assessments of monthly Terrestrial Water Storage nomalies (TWSA), estimated from GRACE and GRACE-FO measurements. TWSA includes all water stocks from the Earth surface to deep aquifers: surface water (rivers, lakes, floodplains), water retained by vegetation, water under snow form, soil moisture, and groundwater. TWSA are expressed as equivalent water height (EWH) anomalies in millimeters for each position expressed in water cubic kilometers and are calculated relative to the temporal average of the series over the period January 2005 - December 2014.

TWSA are derived from SAGSA Level 3 ensemble products, available on GEODES and documented at the following link: GEODES SAGSA Project. The gridded ensemble products, expressed in meters of equivalent water height (m EWH), are multiplied by each ellipsoidal surface to obtain water volumes in cubic kilometers. Final hydrogravimetry products are generated by applying a landmask to the data. This land mask, that also includes drainage basins’ masks, is distributed through the variable “land_mask” in the hydrogravimetry products of this collection as well as in the ensemble products available on GEODES. In addition, a variable “surface” that provides the surface of all land pixels is included in the datasets to allow the user to convert the equivalent water heights into equivalent water volumes. For more details, a describes the Level 3 TWSA products of this collection. A netcdf file containing masks of hydrological basins and the ratio, for each grid cell and each Global Runoff Data Centre (GRDC) basin, between the area overlapped by the basin in the cell and the total cell area is available: . This time-independent dataset (that applies to all products in the collection) could be useful to hydrologists. The Level 3 gravimetric dataset is updated twice a year to complete the time series with the most recent products and to incorporate software updates that refine and improve the processing of Level 2 GRACE and GRACE-FO data. Version 2.2 of “Theia SAGSA GRACE Terrestrial Water Storage Anomalies” collection is the most recent version of this dataset and is recommended for use. The versions’ history of the “Theia SAGSA GRACE Terrestrial Water Storage Anomalies” collection is presented in the table below: