Global groundwater balance

Groundwater is one of the most important freshwater resources for mankind and for ecosystems. Groundwater is formed by the percolation of water from the Earth's surface, e.g. from precipitation or from rivers and lakes, into deeper layers of soil or rock. We speak of groundwater when the cracks and fissures in a layer of rock are completely saturated with water and the lateral transport of water primarily follows gravity. These saturated layers are called aquifers. In the global water cycle, groundwater is one of the components that move rather slowly. It stays in the aquifers for periods ranging from a few months to many thousands of years.

Groundwater is found practically everywhere in the world. In regions where few other sources of fresh water are available, for example due to low precipitation amounts, humans particularly depend on it. Due to high use by agriculture and industry, the resource groundwater is increasingly threatened. In many areas of the world, more groundwater has been extracted than can be replenished naturally for several years. The results are falling groundwater levels, as for example is the case in northwestern India. As a result, wells used by the population could dry up. In coastal areas, saline and thus undrinkable seawater intrudes into the emptied aquifers.

To counter such developments, it is important to monitor changes in groundwater storage. This can be done most reliably by measuring groundwater levels in wells. However, in most regions of the world there are very few measuring wells or measurements are not performed frequently enough. Moreover, measurements in a well only give information about the situation at that exact location. Often one does not know exactly how large or how thick an aquifer is in which the well is drilled. Therefore, it is difficult to conclude from one measuring point to the conditions in the whole aquifer.

GRACE/ GRACE-FO also allows to closely monitor groundwater. By observing total water storage GRACE/-FO captures storage changes in the terrestrial components of the hydrologic cycle over time. These components include glaciers, snow coverage, surface water in rivers and lakes, water in unsaturated soil zones and groundwater. By other satellite measurements and with the help of computer models, we are able to make rather exact statements about the changes in storage in the above-mentioned components - with the exception of groundwater. Scientists are now taking advantage of this fact.

Here's a little thought experiment: We know that total water storage consists mainly of the five storage components mentioned above. We can already measure four of these components. So if we now subtract the storage changes in these four components from the storage changes in the total water reservoir one quantity should remain: groundwater. This approach is being pursued in the project "A Global GravityGravity is also known as the force of gravity or mass attraction. Gravity is the force that two or more bodies exert on each other due to their mass. The best-known gravitational force is the Earth's gravitational pull. It causes bodies on earth to f...-based Groundwater Product - G3P" by GFZ together with several European partners. In G3P a prototype for a service has been developed that for the first time offers a largely observation-based data product for groundwater with global coverage. First assessment leaves the team optimistic that GRACE/GRACE-FO and G3P will be able to close the gap in the observation of the water cycle in the future.

Text: Dr. Julian Haas, GFZ

• Data for the groundwater storage on G3P-Website
• G3P on GravIS