Journal of Geodynamics 59–60 (2012) 157–167 Contents lists available at SciVerse ScienceDirect Journal of Geodynamics j ourna l ho me page: http://www.elsevier.com/locate/jog Modeling the water resources of the Black and Mediterranean Sea river basins and their impact on regional mass changes T. aus der Beek a,∗ , L. Menzel a , R. Rietbroek b , L. Fenoglio-Marc c , S. Grayek d , M. Becker c , J. Kusche b , E.V. Stanev d,e a Department of Geography, Heidelberg University, Germany b Institute of Geodesy and Geoinformation, University of Bonn, Germany c Institute of Physical Geodesy, University of Darmstadt, Germany d Institute for Chemistry and Biology of the Sea, University of Oldenburg, Germany e Institute for Coastal Research, GKSS Research Center, Germany a r t i c l e i n f o Article history: Received 12 October 2010 Received in revised form 11 November 2011 Accepted 30 November 2011 Available online 8 December 2011 Keywords: GRACE Mass transport Oceanography Altimetry WaterGAP a b s t r a c t For the first time, a dedicated release of the hydrology and water use model WaterGAP3, has been devel- oped to spatially explicit calculate hydrological fluxes within river basins draining into the Mediterranean and Black Sea. The main differences between the new regional version of the global WaterGAP3 model and the previously applied global version WaterGAP2 can be found in the spatial resolution, snow mod- eling, and water use modeling. Comparison with observations shows that WaterGAP3 features a more realistic representation of modeled river runoff and inflow into both seas. WaterGAP3 generates more inflow to both seas than WaterGAP2. In the WaterGAP3 simulation, contributions to the total runoff into the Black Sea from individual discharge regions show in general a good agreement to climatology derived runoff, but lesser importance of Georgian rivers for the basin’s water. After the successful model validation WaterGAP3 has been applied to correct estimates of seawater mass derived from the GRACE gravity mission and to account for freshwater inflow into both basins. The performance of the WaterGAP3 regional solution has been evaluated by comparing the seawater mass derived from GRACE corrected for the leakage of continental hydrology, to an independent estimate derived from steric-corrected satellite altimetry with steric correction from regional oceanographic models. The agreement is higher in the Mediterranean Sea than in the Black Sea. Results using WaterGAP3 and WaterGAP2 are not significantly different. However the agreement with the altimetry-derived results is higher using WaterGAP2, due to the smaller annual amplitude of the continental hydrology leakage from WaterGAP3. We conclude that the regional model WaterGAP3 is capable of realistically quantifying water mass variation in the region, further developments have been identified. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction In terrestrial hydrology, the separation of water in the numer- ous storage compartments is a highly complex problem. The heterogeneous nature of soil and topography results in water stor- age variations which can vary strongly in the spatial domain. At the same time, precipitation, evapotranspiration, water rout- ing and anthropogenic water use all play a strong role in the (re-)distribution of water. Often depending on integrative mea- surements, such as observed river runoff, the use of models in hydrology is indispensable in understanding the (re-)distribution ∗ Corresponding author at: Im Neuenheimer Feld 348, 69120 Heidelberg, Germany. E-mail address: ausderbeek@usf.uni-kassel.de (T. aus der Beek). of water. Their utilization is twofold; on the one hand, they pro- vide estimates of water storage where observations alone do not suffice, while on the other hand they provide an opportu- nity to study the physics of the system by means of simulations. The validation/calibration of the hydrological models depends on independent observations. River gauges are traditionally used to calibrate the models, but the data is sparse or unavailable. Since the launch of the Gravity Recovery and Climate Experiment (GRACE), large scale measurements of total water storage have been available (Schmidt et al., 2006). When used in conjunction with in situ obser- vations, GRACE data have the potential to significantly improve our understanding of hydrological processes and the global water cycle and provide opportunities for model calibration (Werth, 2010). A comprehensive overview of hydrological and land surface models dealing with GRACE derived mass variations is given by Güntner (2008). 0264-3707/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2011.11.011