Total water storage variability from GRACE mission and hydrological models for a 50,000 km2 temperate watershed: the Garonne River basin (France)

Study Region: Garonne Basin, France. Study Focus: This study analyses water mass variations for the whole Garonne basin (50,000 km(2) drainage area). To do so, Total Water Storage Anomalies (TWSA) from seven global solutions based on the Gravity Recovery And Climate Experiment (GRACE) satellite mission measurements ((similar to)300 km spatial resolution) are inter-compared with TWSA from two hydrological models, SAFRAN-ISBA-MODCOU (SIM) and Soil and Water Assessment Tool (SWAT), between January 2003 and December 2010. New Hydrological Insights for the Region: Despite the small size of the Garonne basin compared to GRACE spatial resolution, good agreement between GRACE solutions and hydrological model TWSA has been found (maximum correlation coefficient (similar to)0.9 and Nash-Sutcliffe Efficiency, NSE, (similar to)0.7). These datasets showed that TWSA in the Garonne basin is mainly due to water stored in the first dozen meters of soil and in the shallow aquifer. To a smaller extent, snow also influences Garonne TWSA. Open surface water TWSA is quite small and TWSA from deep aquifer is negligible. The most important drought period occurred in 2011/2012, due to low precipitation during the two hydrological years and ETR close to previous years. Important precipitation in 2013/2014 helps to refill the water stocks. This study also showed that GRACE and models mismatches should be due to GRACE poor spatial resolution, but also to its monthly time resolution (rarely shown in previous studies).