Complementary gravimetric monitoring of water storage changes - lysimeters and superconducting gravimeters

Creutzfeldt, B.¹; Güntner A.¹; Merz, Bruno¹; Wziontek, H.²
1 GFZ German Research Centre for Geosciences, Section Hydrology, Potsdam, Germany; 2 Federal Agency for Cartography and Geodesy (BKG), Section National Reference Systems for Gravity, Frankfurt am Main, Germany; benjamin.creutzfeldt@gfz-potsdam.de

Lysimeters provide an accurate measurement of the gravimetric soil moisture change by continuously recording the mass of a soil monolith. Weighable, suction-controlled and monolithic lysimeters can also be considered as very accurate devices to measure precipitation and deep drainage and they are, in fact, the only device to directly measure the actual evapotranspiration. Superconducting gravimeters (SG) are currently the best performing gravimeters to study relative gravity variations in time in terms of precision and temporal resolution. The Geodetic Observatory Wettzell is the only place where both gravimetric monitoring systems - a state-of-the-art lysimeter and a dual sphere SG - operate in parallel at a distance of around 40 m. In this study, we present the first comparison of lysimeter and SG measurements. Lysimeter data are used to estimate WSC at the field scale in combination with complementary hydrological observations and a hydrological 1D model. At the Geodetic Observatory Wettzell (Germany), WSC in the snow pack, top soil, unsaturated saprolite and fractured aquifer are all important terms for the local water budget. We calculate the hydrological gravity response from the estimated WSC, taking into account the mass distribution in space. A comparison of the gravity response and the SG residuals shows that local WSC and the SG residuals are highly correlated on the event and seasonal scale. We find that the gravity residuals are caused to a larger extent by local WSC than previously stated. Lysimeter measurements significantly improve the independent estimation of WSC and thus provide a better way of reducing the local hydrological effect from gravimeter measurements.