Groundwater Storage Anomalies
Groundwater storage variations with global coverage and monthly resolution displayed here are the results of the project
Global Gravity-based Groundwater Product (G3P)
.
G3P has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement nº 870353. The purpose of G3P was to develop a prototype for a new Copernicus service. Accordingly, data presented here should be treated as such.
As terrestrial water storage (TWS) variations observed by GRACE/GRACE-FO represent the sum of the variations of several storage compartments including groundwater, a cross-cutting combination of TWS variations with other individual water storage variations based on the existing portfolio of the
Copernicus services
is applied. Accordingly, groundwater storage anomalies (relative to a long-term mean) are derived from the anomalies of the other storage compartments as follows:
groundwater storage = terrestrial water storage – soil moisture - glacier mass - snow water equivalent - surface water storage
The individual quantities given in this equation as well as their uncertainties can be visualized by clicking on the respective name of a particular quantity next to the spatial plot, or by using the corresponding pulldown menu in the spatial plot. It is important to note that:
- all storage products represent anomalies relative to the long-term mean over the period 2002/04 through 2020/12 (omitting GRACE/GRACE-FO data gaps, i.e., months where no GRACE/GRACE-FO observations are available);
- all the different storage anomalies are provided in terms of mm of equivalent water height;
- the GRACE/GRACE-FO-based TWS anomalies are derived from the
COST-G combined gravity field solutions
;
- to match the spatial resolution of the necessarily filtered GRACE/GRACE-FO-based TWS anomalies, a Gaussian smoothing with 250 km filter radius is applied to the anomalies of all the other individual storage compartments;
- for soil moisture, glacier mass and snow water equivalent, storage products are also provided during GRACE/GRACE-FO data gaps (this is currently not possible for surface water storage due to technical reasons, but planned for coming updates).
The current version of the G3P prototype is v1.12 and covers the time period 2002/04 through 2023/09. Gridded products as displayed in the spatial plot can be
downloaded in NetCDF format
.
Time series of averaged anomalies for certain regions in CSV format can be directly downloaded from this site by clicking on the download button above the time series plot. Use the corresponding pulldown menu in the spatial plot to select between the following types of predefined regions: aquifers, river basins or climatically similar regions (regions with similar precipitation properties). Click on the spatial plot to select a particular region. The geometries of the predefined regions are available
here for the aquifers
,
here for the river basins
and
here for the climatically similar regions
.
G3P v1.12 has replaced version v1.11 which is still available
here
.
Note: the data set on snow water equivalent contains a faulty entry for June 2005. This entry also impacts groundwater storage in that month. We recommend to exclude this month entirely from any analyses. Beyond this, the entry had no effect on processing, anomaly calculation or any other variable in any other month.
A list of key references for the individual water storage compartments can be found at the bottom of this page.
Groundwater storage
Groundwater storage anomalies calculated by subtracting the aggregated and filtered (Gaussian smoothing with 250 km radius) storage variations for soil moisture, glaciers, snow, and surface water from the GRACE/GRACE-FO-based terrestrial water storage variations.
Groundwater storage uncertainty
Uncertainty estimates of the groundwater storage anomalies, calculated by error propagation
of the individual uncertainties of terrestrial water storage, soil moisture, glaciers, snow,
and surface water.
Soil moisture
Filtered soil moisture (SM) anomalies accounting for the water content in the first 2 meters of the soil column based on the Copernicus Climate Change Service (C3S) Surface Soil Moisture data; C3S data are first gap-filled using the DCT-PLS algorithm, and then propagated via the exponential filter method to approximate SM beyond the surface layer, up to a maximum depth of 2 meters.
Soil moisture uncertainty
Uncertainty estimates of the filtered soil moisture (SM) anomalies computed by employing the law standard of uncertainty propagation adapted to the exponential filter method (i.e. accounting for its parameter and model structural uncertainties).
Glacier mass
Filtered glacier mass anomalies; to obtain anomalies corresponding to a regular grid, an area
transformation considering the glaciated area of each grid cell is applied; to reconvert to
gridded glacier mass loss in units of Gt, the water height per grid cell needs to be multiplied
by the total area of the grid cell; note that due to filtering, glacier mass is underestimated
by about a 40% mainly caused by signal leakage into the ocean in coastal areas.
Glacier mass uncertainty
Time-variable component of the uncertainty estimates for the filtered glacier mass anomalies;
glacier mass change uncertainties are based on the inherited uncertainties from the glaciological and the geodetic input datasets and their variability, at a 95% confidence interval (1.96 std); uncertainty propagation considers the glaciated area of a grid cell and the number of independent observations; finally, to convert from specific mass balance uncertainties to water mass loss (Gt) and water height uncertainties (mm), an additional source of error related to the glacier area and area change rates are considered.
Snow water equivalent
Filtered snow water equivalent (SWE) anomalies; SWE is a measure for the amount of liquid water
stored in a snowpack (corresponding to the resulting water column of a snowpack when completely
melted), defined as the product of snow depth of the snow layer and its density.
Snow water equivalent uncertainty
Uncertainty estimates of the filtered snow water equivalent anomalies per grid point.
Surface water storage
Filtered surface water storage anomalies, based on the Copernicus Global Flood Awareness System (GloFAS) version 4.0; the underlying model to GloFAS is the global hydrological model Lisflood; the combined surface water storage comprises model output and state variables for river water storage, lake water storage and water storage in man-made reservoirs; due to limitations in spatial and temporal coverage, the observation-based surface water products further developed within G3P are not included in this prototype version; please refer to the public G3P project reports for further information.
Surface water storage uncertainty
Uncertainty estimates of the filtered surface water storage anomalies, based on an ensemble mean between Lisflood and the WaterGAP Global Hydrological Model (WGHM); the following steps are performed:
- Calculate delta between monthly time series of Lisflood and WGHM surface
water storage anomalies for the period of 2002/04 through 2016/12,
- divide the time series of deltas by 2,
- subtract this new delta time series from the Lisflood anomaly time series
and convert to absolute values to generate the uncertainty time series for
the above period, and
- the climatology of the uncertainty values from 2002/04 to 2016/12 is
calculated and applied to the period from 2017/01 to 2020/12 to complete
the time series.
Terrestrial water storage
GRACE/GRACE-FO-based water mass anomalies representing the sum of all water storage
compartments including soil moisture, glaciers, snow, surface water, and deep groundwater;
not corrected for spatial leakage.
Terrestrial water storage uncertainty
Uncertainty estimates of the terrestrial water storage anomalies based on a spatial covariance model (Boergens et al., 2022) and global time-variable uncertainty estimates, given as standard deviation per grid point.
