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Abstract:
Since April 2002, the Gravity Recovery and Climate Experiment (GRACE) satellite mission has been churning
out water storage anomaly data, which has been shown to be a unique descriptor of large-scale hydrological
extreme events. Nonetheless, efforts to assess the comprehensive information from GRACE on total water storage
variations for near-real time flood or drought monitoring have been limited so far, primarily due to its coarse
temporal (weekly to monthly) and spatial (> 150.000 km2) resolution and the latency of standard products of
about 2 months,.
Pending the status of the aging GRACE satellite mission, the Horizon 2020 funded EGSIEM (European
Gravity Service for Improved Emergency Management) project is scheduled to launch a 6 month duration nearreal
time test run of GRACE gravity field data from April 2017 onward, which will provide daily gridded data with
a latency of 5 days. This fast availability allows the monitoring of total water storage variations related to hydrological
extreme events, as they occur, as opposed to a ’confirmation after occurrence’, which is the current situation.
This contribution proposes a global GRACE-derived gridded wetness indicator, expressed as a gravity anomaly
in dimensionless units of standard deviation. Results of a retrospective evaluation (April 2002-December 2015)
of the proposed index against databases of hydrological extremes will be presented. It is shown that signals for
large extreme floods related to heavy/monsoonal rainfall are picked up really well in the Southern Hemisphere
and lower Northern Hemisphere (Africa, S-America, Australia, S-Asia), while extreme floods in the Northern
Hemisphere (Russia) related to snow melt are often not. The latter is possibly related to a lack of mass movement
over longer distances, e.g. when melt water is not drained due to river ice blocking.
