**5. Conclusion**

*Geodetic Sciences - Theory, Applications and Recent Developments*

the unknown evapotranspiration to be determined [71].

estimated at ∼624 ± 32 Gt with respect to the mean value.

continents using the GRACE Level-2 products.

**4.3 Detection of extremes events**

interactions [69]. There is also a significant contribution of river discharge in the spatial distribution of seasonal water storage with a dependency of climate [70]. Regional time variations of evapotranspiration rate - expressed in mm/yr. - can be also derived by integrating and solving the water mass balance equation, which relates TWS on land provided by GRACE, precipitation data from the Global Precipitation Climatology Centre, runoff given by a global land surface model and

Gravimetric satellites missions also image the extreme climate events in the whole Earth system. Floods and droughts have been largely studied in different

In the case of the Amazon basin, GRACE has revealed periods of extreme droughts and floods. During the 2005 drought, the TWS in the river and floodplains of the Amazon basin was 70% below its average for the 2003–2007 period [71]. However, in 2009 gravity measurements display an exceptional flood associated to La Niña event [26]. The maximum value of TWS in the entire Amazon basin was

New detection approaches based on GRACE data are developed to identify drought episodes and their severity [72]. Advances in GRACE data treatment

*Differences of weekly-averaged of GRACE solutions derived by the Kalman filter approach as proposed by [73], before and during the Katrina and Rita episodes (top); and comparison with the anomalies of TRMM* 

**68**

**Figure 6.**

*precipitation for the same periods (bottom).*

During its sixteen years of operation, the GRACE mission provided a novel source of information on variations of water mass on lands at unprecedented spatial and temporal resolutions. This mission offered an exceptional dataset for studying large-scale water mass redistributions, and for the very first time, the opportunity to monitor water changes in all the hydrological compartments and from regional to global scales. While spherical harmonics solutions were firstly used, regional and local approaches have already demonstrated the possible access to spatial (better localization of structures by construction) and temporal (through daily updates using Kalman filter strategies, e.g. see [73]) scales that were higher than those that were offered by global SH solutions. Additional bibliographic resources and useful information about GRACE can be found on the GRACE Tellus web site [74]. The GRACE mission ended in 2017, and later on a partnership between NASA and the German Research Centre for Geosciences (GFZ) decided to schedule GRACE Follow-On (GRACE-FO) mission to launch in 2018 in order to ensure the continuity of GRACE-type space gravimetry. By using a similar twin satellite configuration of the low-Earth and nearly polar orbit at 300–500 km altitude, the GRACE-FO mission is following its successful predecessor [75]. Additionally, it carries a demonstrator of laser system to measure the inter-satellite distance and velocity, and hence for an improved precision. It is promising for providing new perspectives in hydrology studies, such as refined long-term mass balance estimates of surface water storage and ice sheets. These new data that offer continuity with the previous GRACE observations will be of first interest for hydrology of global hydrological model calibration to constrain their operations through assimilation techniques.
