Hydrology and gravimetry (Assessing groundwater mass balance: Keynote lecture: Hydrogeodesy in Membach and Rochefort)

For the 10 last years, terrestrial and satellite (GRACE) gravity measurements have reached such a precision that they can be of interest to better monitor underground water masses. First, we show that terrestrial measurements provide high-precision information about the time evolution of mass changes in the few kilometres square around the gravimeter. Then, examples of the possibilities and limitations of terrestrial measurements are given in Membach, close to Eupen, and in the Rochefort karst system. In Membach, we show that the evapotranspiration can be directly inferred from continuous gravity measurements: as water evaporates and transpires from terrestrial ecosystems, the mass distribution varies through the system, changing its gravity field at the level of, or smaller than 10-10 g per day. This corresponds to 2.0 mm of water over an area of 50 ha. The strength of this method is its ability to ensure a direct, traceable and continuous monitoring of actual ET for years at the mesoscale (~50 ha) with a precision of a few tenths of mm of water. In Rochefort, gravity measurements at the surface and in the cave allows separating the water contained in the unsaturated zone from the saturated one and therefore monitoring groundwater content changes that occur in the unsaturated zone only.