Developing a recharge model of the Nile Basin to help interpret GRACE data

The Gravity Recovery and Climate Experiment (GRACE) data provide a new opportunity to gain a direct and independent measure of water mass variation on a regional scale. Processing GRACE data for the Nile Basin through a series of spectral filters indicates a seasonal spatial variation to gravity mass (±0.005 mGal). To understand how this gravity mass variation relates to the different components of the hydrosphere within the Nile Basin, and particularly what proportion, if any, relates to groundwater recharge, a recharge model was developed for the Nile catchment using the ZOODRM model and remotely sensed data. Results indicate: a significant proportion of the annual water mass variation indicated by GRACE in the Nile Basin (~5.0x1011 m3/yr) is related to seasonal groundwater mass change (~1.2x1011 m3/yr); annual water mass variation in the Nile Basin as indicated by GRACE is an order of magnitude less than observed rainfall input minus river outflow; evaporative losses of surface water are significant within the Nile Basin; and relatively little water is lost by outflow from the Nile delta. The results of this study show the value of using groundwater models to interpret subtle variations in GRACE data, to gain an insight into the partitioning of water cycle within river basins, and also the ability to develop a plausible recharge model using remotely sensed data.