Weighing changes in the water storage of landscapes

Das supraleitende Gravimeter im geodätischen Observatorium Wettzell im Bayrischen Wald (Foto: S. Schröder, GFZ).

20.07.2017: System Earth is highly complex. It consists of multiple interconnected cycles. Measuring the balance of energy and mass fluxes in the environment is therefore a highly demanding task in sciences. Researchers from the GFZ section Hydrology, together with colleagues from the Senate of the City of Berlin, the German Federal Agency for Cartography and Geodesy, and the University of Potsdam now showed that they are able to “weigh” the balance of water.

Within the water cycle, the knowledge on the amount of water that is stored above and below the Earth surface is highly important. It determines how much water is available within the active part of the cycle and therefore takes part in exchange processes between the atmosphere, the land masses, and the ocean. Water is an important global resource, especially in dry areas. The knowledge on how much water is for example stored underground is key to a sustainable water management.

Water in the landscape appears in several storage compartments, such as plant biomass, soil, groundwater, snow cover or surface water bodies. In theory these need to be monitored individually to calculate changes in the total amount of water stored. For several years, the satellite mission GRACE, where the GFZ is involved in, has been monitoring changes in the water balance. However, the data base is only of low temporal and spatial resolution. The team of first author Andreas Güntner chose a quite unusual approach and now published their results in Hydrology and Earth System Sciences.

„Weighing“ gravity

For more than a year, the team had a highly precise gravimeter installed in the geodetic observatory Wettzell in the Bavarian Forest. This instrument induces an electromagnetic field within a superconducting coil. Within this electromagnetic field, a levitating metallic sphere serves as a test mass. Every movement of the test mass is precisely registered. In this way, the gravimeter detects changes in the Earth’s gravitiational attraction that are caused by mass changes due to water storage changes above or below the Earth surface. The gravimeter therewith works like a scale.

Mass changes below and above the instrument are recorded by the measurements. This also allows for monitoring of evapotranspiration: water transpired by plants or evaporating directly from the Earth’s surface reduce the amount of water stored. So far, measurements of actual evapotranspiration was challenging. It is, however, of great importance for estimating how plants react on environmental changes like rising temperatures or water stress, for instance.

The study shows for the first time that a highly sensitive instrument like a superconducting gravimeter can not only be used under laboratory conditions but also under strongly fluctuating weather and environmental conditions in the field. Andreas Güntner says: “This kind of comprehensive monitoring of changes in water balance in the landscape is unprecedented. Our study shows how gravimeter can be used as a hydrological instruments in the field”. (ak)

Original study: Güntner, A., Reich, M., Mikolaj, M., Creutzfeldt, B., Schroeder, S., and Wziontek, H., 2017. Landscape-scale water balance monitoring with an iGrav superconducting gravimeter in a field enclosure. Hydrology and Earth System Sciences 21, 3167-3182. DOI: 10.5194/hess-21-3167-2017

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