New insights into hydrologic sources and sinks in the Nile Basin: A multi-source satellite data analysis

Tuesday, 16 December 2014
Gabriel B Senay, USGS EROS, Sioux Falls, SD, United States, Naga Manohar Velpuri, ARTS Contractor, 2ASRC InuTeq LLC, Contractor to U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD, USA, Sioux Falls, SD, United States, Stefanie Bohms, SGT Inc., Contractor to USGS EROS Center, Sioux Falls, SD, United States, Yonas Demissie, Washington State University, Department of Civil and Environmental Engineering, Richland, WA, United States and Mekonnen Gebremichael, University of California Los Angeles, Civil and Environmental Engineering, Los Angeles, CA, United States
The Nile River is the longest in the world with a length of 6,800 km. However, the contrast between the length of the river or the size of the basin and the comparatively small volume of basin runoff generated is a unique feature of the Nile Basin. Due to non-availability of in-situ hydrologic data, we do not clearly understand the spatial distribution of hydrologic sources and sinks and how much they control input–output dynamics? In this study, we integrated satellite-derived precipitation, and modeled evapotranspiration data (2000–2012) to describe spatial variability of hydrologic sources and sinks in the Nile Basin. We also used long-term gridded runoff and river discharge data (1869–1984) to understand the discrepancy in the observed and expected flow along the Nile River. Results indicate that over 2000–2012 period, 4 out of 11 countries (Ethiopia, Tanzania, Kenya, and Uganda) in the Nile basin showed a positive water balance while three downstream countries (South Sudan, Sudan, and Egypt) showed a negative balance. The top three countries that contribute most to the flow are Ethiopia, Tanzania and Kenya. The study revealed that ~85% of the runoff generated in the Equatorial region is lost in an inter-station basin that includes the Sudd wetlands in South Sudan; this proportion is higher than the reported loss of 50% at the Sudd wetlands alone. The loss in runoff and flow volume at different sections of the river tend to be more than what can be explained by evaporation losses, suggesting a potential recharge to deeper aquifers that are not connected to the Nile channel systems. On the other hand, we also found that the expected average annual Nile flow at Aswan is larger (97 km3) than the reported amount (84 km3). Gravity Recovery and Climate Experiment (GRACE) mass deviation in storage data analysis showed that at annual time-scales, the Nile Basin shows storage change is substantial while over longer-time periods, it is minimal (<1% of basin precipitation). Due to the large variations of the reported Nile flow at different locations and time periods, the study recommends increased hydro-meteorological instrumentation of the basin. This study improves our understanding of the spatial dynamics of water sources and sinks in the Nile basin and identified emerging hydrologic questions that require further attention.