Hydrological mass variations in the Nile River Basin from GRACE and hydrological models

Terrestrial water storage(TWS)change is a key component of the global water cycle and hydrologic cycle.Therefore,it is of significance to quantify TWS variations at large scale and understand how the climate changes affect in the water systems,particularly in the Nile River Basin.In this study,TWS variations in the Nile River Basin are estimated and investigated from January 2003 to August 2016 by using the Gravity Recovery and Climate Experiment(GRACE).Also,the Global Land Data Assimilation System(GLDAS)and Tropical Rainfall Measuring Mission(TRMM)are used to understand the causes of the TWS variations in the Nile River Basin(NRB).The peak of precipitation has happened in July and August.After one month the appearance of the influence of precipitation on soil moisture is clear.On the other hand,after two months the same effect of precipitation in TWS is clear.While the peak of soil moisture has occurred in August and September,the maximum of TWS has observed in September and October.The maximum annual TWS from GRACE measurements is 42.66 mm in NRB during September-November(autumn),and its minimum is-33.77 mm during March-May(spring).Also,the maximum annual soil moisture is 25.22 mm in NRB during September-November(autumn),and its minimum is-23.34 mm during March-May(spring)while the peak precipitation is 100.11 mm during June-August(summer)and its minimum is 14.62 mm during December-February(winter).The trend of TWS variations in NRB is 0.04 mm/yr from GRACE.Furthermore,the TWS variations are mainly dominated by the soil moisture in the Nile River Basin with the correlation coefficient of 0.88.In addition,the correlation factor between the non-seasonal TWS and ScPDSI is 0.60,indicating that GRACE-derived TWS well capture the most severe droughts occurred in 2006 and 2011.