On the co-evolution of geodetic observations and Earth System Models

International audienceWhile surface freshwater storage components are easily observable components of our landscapes, a major partof continental water resides and flow below ground in groundwater systems (GW) and is broadly inaccessible todirect observations. GW sustain ecosystems with high quality freshwater, support food security, however, they arehardly represented within global land models. The main difficulties lie in the long response time of GW and thecomplexity water flow within heterogeneous geological layers.In this presentation, we show how geodesy, i.e. the study of the Earth’s shape and gravity field, can support theimplementation of GW within models. First, gravity observations from GRACE have revolutionized our vision ofthe water cycle, by its sensitivity to total water storage changes. Another fundamental processes lies in the physicalinteractions between water and rock: fluid pressure gradients is the engine for water flow, but also generate adeformation that is measurable at the surface with current geodetic tools (GPS). New opportunities also emergewith the development of seismic noise monitoring, highly sensitive water pressure changes. All these instrumentsprovide valuable information on shallow and confined GW storage changes and contribution to water cycle, on avariety of spatial scales, from 10 km to 1000s km scales. Considering the development of dense internationalnetworks (GPS, seismic stations), and new satellite missions (InSAR from Sentinel, GRACE-FO), there is a wideopportunity to improve the representation of GW within global models.We support the analysis with recent examples at all scales, and show the link between the current knowledge ofwater flow and geodetic analyses. While a goal of the geodesy is to provide independent measures of the watercycle, it is clear that an iterative analysis is the most effective. It is anticipated that global models will guide theformulation of geodetic water storage variation estimates as, in turn; geodetic estimates will contribute to globalhydrological modeling