Use of GRACE, Rainfall-Runoff and Global Circulation Models to Assess the Primary Source(s) of the Nile Floods and their Intensity in the 21st Century

Extreme precipitation and flooding events are are becoming more common in many places worldwide necessitating developing a thorough understanding of this phenomena and implementing sustainable management scenarios of these added water resources. One of those settings is the source areas of the Nile River. Twice (1998-2003; 2019-2022) in the last two decades, high precipitation over the Nile River source areas has led to flooding in northern and central Sudan, the filling of Lake Nasser (LN), and the diversion of the overflow to depressions in the plateau bounding the Nile River from the west, where it is lost to evaporation. We adopted an integrated approach using temporal GRACE and GRACE-FO data, precipitation, and continuous rainfall-runoff models (Soil Water Assessment Tool; SWAT) and global circulation model (GCMs;   CCSM4, HadGEM3, and GFDL-CM4.0) projections to investigate the primary source area contributing to the observed increased in runoff that reached LN in the past two decades, and assessed the impact of climate change on the LN’s runoff throughout the 21^st century. Findings include: (1) the primary contributor to increased downstream runoff reaching LN is the Blue Nile subbasin (BNS), (2) we simulated the BNS runoff  in the 21st century using a calibrated (calibration period: 1965-1992) rainfall-runoff model with CCSM4, HadGEM3, and GFDL-CM4.0 projections as model inputs, (3) the extreme value analysis that we conducted for the projected runoff driven by GCMs’ output indicates extreme floods are more severe in the 21^st-century, (4) based on the predicted median values of stream flows for the 21st century, the stream flows are expected to increase by 2%, 5%, 9%, and 13%, corresponding to 25-, 50-, 100-, and 200-year events, respectively, under RCP 4.5 and 2%, 7%, 11%, 15% and 20% for 10-, 25-, 50-, 100-, and 200-year events, respectively, under RCP 8.5, and (5) one adaptation for the projected 21^st-century increase in precipitation (25%-39%) and flood (2%-20%) extremes is to recharge Egypt’s fossil aquifers during high flood years that are being depleted (2002-2023) by 0.98 km³/yr.