About Global Gravity-based Groundwater Product
Groundwater is an essential factor for ecosystems and humanity alike. It ensures ecosystem stability, energy and food security, and promotes human health. Groundwater is the largest component of global liquid freshwater resources in the water cycle, providing about 30% of the total freshwater. Groundwater accounts for 33% of the global water withdrawals by mankind, with more than two billion people depending on groundwater as primary water resource. However, despite its importance, groundwater is often not included in sustainable water management actions and plans. This typically leads to uncontrolled withdrawals exceeding the renewal rates, with the consequence of dramatically declining groundwater tables in the past decades.
Recognizing the importance of groundwater, the Global Climate Observing System (GCOS) defined groundwater as one of the Essential Climate Variables (ECV), i.e., an observation that is required to systematically observe the Earth`s changing climate. While several other ECVs are accessible as operational products in the portfolio of the Copernicus Services (European Union’s Earth Observation Programme), groundwater is not. The reason for this, but also for limited data availability on groundwater in general for many regions worldwide, is in part due to its obscure nature hidden below the Earth’s surface.
Between 2002 and 2017, the Gravity Recovery and Climate Experiment (GRACE) satellite mission has demonstrated the potential and the value of globally observing mass variations on and below the Earth’s surface, based on the time-variable gravity field of the Earth. With the launch of the German-American GRACE-FO (Follow-on) mission in 2018, a meanwhile nearly 17-year time series of global mass changes as an expression of the effects of climate change, human water use on the water cycle and of other geophysical phenomena can be achieved.
While GRACE and GRACE-FO monitor Total Water Storage (TWS) variations, i.e., the sum of all storage compartments of the continental hydrosphere, numerous studies have shown for selected regions and aquifers that the groundwater storage can be assessed by subtracting the other storage compartments from TWS, i.e., the variations of snow, glaciers, surface water bodies and soil moisture as relevant for a particular region of interest. This is typically done by hydrological or land surface models estimating these components. The procedures and the type of models, however, vary from one study to another. Furthermore, the modelled hydrological components contain a high degree of uncertainty which translates into uncertainties of the derived groundwater storage. Observational data from satellites or in-situ measurements have only been used in few studies for the subtraction process so far. There is no consistent global freely accessible data set on groundwater storage variations based on satellite gravity data yet.
The Global Gravity-based Groundwater Product (G3P) project aims at developing a product of groundwater storage variations with global coverage and monthly resolution from 2002 until present by a cross-cutting combination of GRACE and GRACE-FO satellite gravity data with water storage data that are based on the existing portfolio of the Copernicus services. G3P is developed for later operational implementation as Essential Climate Variable (ECV) Groundwater into the Copernicus Climate Change Service, Lot 'Land hydrology & cryosphere'.
This product was developed by:
Helmholtz Zentrum Potsdam Deutsches GeoForschungsZentrum (GFZ) - Coordinator, Universität Bern (UB), Technische Universität Graz (TUG), Ilmatieteen Laitos (FMI), Technische Universität Wien (TUW), Collecte Localisation Satellites SA (CLS), International Groundwater Resources Assessment Centre (IGRAC), Universität Zürich (UZH), Magellium SAS (MAG), Université Paul Sabatier Toulouse III (LEGOS), FutureWater SL (FUWA), Earth Observation Data Centre for Water Resources Monitoring GmbH (EODC)