The consequences of the prolonged drought in Europe were clear in the summer of 2022. Riverbeds were dry, stagnant waters evaporated, leading to numerous impacts on both nature and people. Not only did many aquatic species lose their habitat, but dry soil caused problems for agriculture, and the energy shortage in Europe worsened. Nuclear power plants in France were unable to generate enough electricity due to a lack of cooling water, and hydroelectric power plants also failed to function without sufficient water.
How does groundwater is measured from space?
How do geodesists at TU Graz utilize satellite data to determine groundwater levels? The core of the G3P project lies in two satellites named Tom and Jerry, orbiting Earth in a polar orbit at an altitude of approximately 490 km. The crucial factor is the 200 km gap between the satellites, which must be constantly and accurately measured to ensure that the rear satellite does not catch up with the front.
The satellites' relative speed is impacted by their proximity to large masses, such as mountains. When flying over a mountain, the front satellite temporarily moves faster due to the increased mass, while the rear satellite accelerates when it reaches the mountain. This change in distance is what the geodesists measure with micrometer precision to calculate Earth's gravitational field. To put it into perspective, a hair is around 50 micrometers thick.
The Grace Follow-on satellites Tom and Jerry measure the mass changes on earth. Credit: NASA – JPL-Caltech
Monthly gravity map of the Earth
TU Graz can provide a monthly gravity map of the Earth, thanks to the two satellites Tom and Jerry, which orbit the Earth at a speed of 30,000 km/h, making 15 complete orbits a day. This results in complete coverage of the Earth's surface every month. According to Torsten Mayer-Gürr, "The processing and computational effort required is substantial, as we have a distance measurement every five seconds, amounting to about half a million measurements per month. From these measurements, we then determine gravity field maps."
However, the gravity map alone doesn't provide information on groundwater levels. This is because the satellites detect all changes in mass, without differentiating between bodies of water such as seas, lakes, or groundwater. To arrive at an accurate assessment of groundwater, cooperation with other partners in the EU's G3P project is needed. Torsten Mayer-Gürr and his team at TU Graz provide the total mass, which is then adjusted by subtracting the mass changes in rivers and lakes, soil moisture, snow, and ice. The result is the groundwater levels.
The other masses involved in the EU G3P project have their own experts contributing data. These experts are based in institutions in Austria (such as the Graz University of Technology, Vienna University of Technology, and Earth Observation Data Center EODC), Germany (GeoForschungsZentrum GFZ in Potsdam), Switzerland (University of Bern and University of Zurich), France (Collection Localisation Satellites CLS, Laboratoire d’Etudes en Géophysique et Océanographie Spatiales LEGOS, and Magellium), Spain (FutureWater), Finland (Finnish Meteorological Institute), and the Netherlands (International Groundwater Resources Assessment Centre IGRAC).
Europe’s looming water crisis
The outcome of this collaboration reveals that the water situation in Europe has become very critical. Torsten Mayer-Gürr did not anticipate such a massive impact. "A few years back, I never thought that water would be a concern in Europe, especially in countries like Germany or Austria. We are facing water shortage issues now, which is concerning," he says. According to him, documenting the ongoing drought with data and continuous satellite monitoring is the first step to address this problem.
The European Space Agency (ESA) and NASA will continue this study through the MAGIC (Mass-change And Geoscience International Constellation) project, with TU Graz participating in the data analysis again.
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