The human population living in the Nile basin countries is projected to double by 2050, approaching one billion 1 . The increase in water demand associated with this burgeoning population will put significant stress on the available water resources. Potential changes in the flow of the Nile River as a result of climate change may further strain this critical situation 2,3 . Here, we present empirical evidence from observations and consistent projections from climate model simulations suggesting that the standard deviation describing interannual variability of total Nile flow could increase by 50% (±35%) (multi-model ensemble mean ±1 standard deviation) in the twenty-first century compared to the twentieth century. We attribute the relatively large change in interannual variability of the Nile flow to projected increases in future occurrences of El Niño and La Niña events 4,5 and to observed teleconnection between the El Niño-Southern Oscillation and Nile River flow 6,7 . Adequacy of current water storage capacity and plans for additional storage capacity in the basin will need to be re-evaluated given the projected enhancement of interannual variability in the future flow of the Nile river.The Nile river basin is an ecosystem under severe stress. The basin is shared by about 400 million people in eleven countries with economies that depend heavily on agriculture, which employs the vast majority of the labour force in most of these countries 1 . Furthermore, almost half of the Nile basin countries are projected to live below the water scarcity level, 1,000 m 3 /person/year, by 2030 8,9 . Thus, any future changes in the magnitude of the flow volume of the Nile river can lead to significant impacts on the lives of people living within the basin and may increase the already high level of water stress.To fully utilize the water resources of the basin, several dams were built in the previous century to control the seasonal and interannual variability of the Nile flow. The recent conflict over the Nile water has received significant attention in the past few years after the decision by Ethiopia to build a large dam on the Blue Nile (the Grand Ethiopian Renaissance Dam, or GERD) to produce electricity, mostly for export to neighbouring countries. The dam, currently under construction, is relatively large compared to previous designs for the same location, which raised serious concerns regarding its effect on water shares of downstream countries (that is, Egypt and Sudan). If variability of the Nile flow changes in the future, then water storage capacity in the basin will need to be re-evaluated.Until recently, attempts to project the future of the Nile flow yielded inconsistent results. Although several studies examined the impacts of climate change on the Nile basin using different approaches 10-18 , the uncertainty surrounding conclusions from these studies was high for several reasons. First, none of the previous studies presented observational evidence to support their hypotheses, as they estimated the impacts of cli...

The simulations and predictions of the hydrological cycle by general circulation models (GCMs) are characterized by a significant degree of uncertainty. This uncertainty is reflected in the range of Intergovernmental Panel on Climate Change (IPCC) GCM predictions of future changes in the hydrological cycle, particularly over major African basins. The confidence in GCM predictions can be increased by evaluating different GCMs, identifying those models that succeed in simulating the hydrological cycle under current climate conditions, and using them for climate change studies. Reanalyses are often used to validate GCMs, but they also suffer from an inaccurate representation of the hydrological cycle. In this study, the aim is to identify GCMs and reanalyses' products that provide a realistic representation of the hydrological cycle over the Congo and upper Blue Nile (UBN) basins. Atmospheric and soil water balance constraints are employed to evaluate the models' ability to reproduce the observed streamflow, which is the most accurate measurement of the hydrological cycle. Among the ECMWF Interim Re-Analysis (ERA-Interim), NCEP–NCAR reanalysis, and 40-yr ECWMF Re-Analysis (ERA-40), ERA-Interim shows the best performance over these basins: it balances the water budgets and accurately represents the seasonal cycle of the hydrological variables. The authors find that most GCMs used by the IPCC overestimate the hydrological cycle compared to observations. They observe some improvement in the simulated hydrological cycle with increased horizontal resolution, which suggests that some of the high-resolution GCMs are better suited for climate change studies over Africa.

Abstract. This study analyzes extensive data sets collected during the twentieth century and defines four modes of natural variability in the flow of the Nile River, identifying a new significant potential for improving predictability of floods and droughts. Previous studies have identified a significant teleconnection between the Nile flow and the eastern Pacific Ocean. El Niño-Southern Oscillation (ENSO) explains about 25 % of the interannual variability in the Nile flow. Here, this study identifies a region in the southern Indian Ocean, with a similarly strong teleconnection to the Nile flow. Sea surface temperature (SST) in the region (50-80 • E and 25-35 • S) explains 28 % of the interannual variability in the flow of the Nile River and, when combined with the ENSO index, the explained variability of the flow of the Nile River increases to 44 %. In addition, during those years with anomalous SST conditions in both oceans, this study estimates that indices of the SSTs in the Pacific and Indian oceans can collectively explain up to 84 % of the interannual variability in the flow of the Nile. Building on these findings, this study uses the classical Bayesian theorem to develop a new hybrid forecasting algorithm that predicts the Nile flow based on global model predictions of indices of the SST in the eastern Pacific and southern Indian oceans.