This post was written by the Ad Hoc Blue Nile Forecast Group: Sarah Alexander, Paul Block (University of Wisconsin-Madison), Shu Wu, Annalise Blum, Ben Zaitchik, and Ying Zhang (Johns Hopkins University), and Shraddhanand Shukla (Climate Hazards Center, University of California Santa Barbara).

(Background: "Soon to be the largest hydropower dam in Africa, the controversial Grand Ethiopian Renaissance Dam (GERD) will allow Ethiopia greater control over the Blue Nile River, Egypt's main source of freshwater. The GERD is expected to have the capacity to generate 15,000 gigawatt hours/year (SaliniImpregilo, 2014), substantially increasing Ethiopia's power generation capacity for both use within Ethiopia and export to neighboring countries. [...] To begin generating electricity, Ethiopia must fill the 74 cubic kilometer reservoir behind the GERD, which will impact downstream countries including Sudan and Egypt (King and Block, 2014; Zhang et al., 2016). If the filling of the reservoir occurs during years without much rain, Nile flows could be substantially reduced. This possibility has fueled speculation about the potential for a “water war” in the region (BBC, 2018). In contrast, reservoir filling that occurs during wet years may be felt less acutely downstream. [...] We developed a seasonal forecast for Upper Blue Nile rainfall and streamflow at the GERD... to provide decision-makers with timely access to our findings and to further the regional conversation on filling [the reservoir]." -- from Blum et al., 2019.)

Continuing an effort we began in 2018, our Ad Hoc Blue Nile Forecast Group has generated an ensemble seasonal forecast for rainy season precipitation in the Upper Blue Nile basin of Ethiopia and Blue Nile River flow at the site of the Grand Ethiopian Renaissance Dam (GERD; Figure 1). We make these forecasts in late May, at the onset of the primary rainy season in the Blue Nile basin, and provide updates as necessary at mid-season. Previous forecast reports can be found in the Climate Hazards Center blog archive for 2018, a July 2018 midseason update, and 2019. Additional information on the forecast motivation and technical approach are included in a 2019 paper in Frontiers in Water. 

As described in our previous posts and our paper (and in just about every other corner of the internet), the GERD will be the largest hydropower dam in Africa. Its power-generating potential has raised considerable interest in Ethiopia and in other East African nations that expect to purchase electricity generated at Ethiopian hydropower facilities. At the same time, the fact that the GERD is the first major infrastructure project on the mainstem of the Ethiopian portion of the Blue Nile River, and that the reservoir could potentially store 1.5 times the annual average flow of the river, have been a major source of concern for downstream countries—Sudan and, especially, Egypt. The reservoir-filling period is expected to be particularly tense, given the potential for Ethiopia to curtail Blue Nile flows significantly in an effort to fill the reservoir and begin producing electricity as soon as possible. Ethiopia plans to begin filling the reservoir in 2020, and there is not yet any agreement on filling policy or on GERD operations. 

As in previous years, we have generated predictions for June-September total rainfall for the Upper Blue Nile basin, defined as the Blue Nile River basin within Ethiopia, which is upstream of the GERD site. These forecasts are then applied to a water balance model to estimate June - December Blue Nile River streamflow at the GERD site. Our precipitation forecasts are derived from an ensemble that includes nine dynamically-based global seasonal forecast systems drawn from the North American Multi-Model Ensemble (NMME) and eight independently derived statistical seasonal forecast methods. Further details and evaluation are available in the Frontiers in Water paper.

 

The forecast for 2020 shows a high likelihood of normal to above-normal rainfall conditions (Figure 2); approximately half of the forecasts predict above average conditions, while only one model has a mean forecast that is “below normal”—that is, a prediction that falls in the bottom third of historic observations. For streamflow, the story is similar, but with some “above-normal” precipitation models dropping into the “normal” condition category. 

The expectation for normal to above-normal rainfall and streamflow is generally consistent across the statistical- and dynamically-based models. However, as was the case in 2018 and 2019, the dynamically-based NMME models tend to forecast wetter conditions than the statistical models (Figure 3), though not uniformly so. In 2018, this wetter forecast led the dynamically based models to overestimate rainfall, while in 2019, it resulted in a forecast that was more accurate than the statistical models. Overall, there is no clear performance advantage of one approach over the other. 

By examining each NMME model and statistical model separately (Figure 4), we see that there is considerable spread in the NMME multi-model ensemble. This applies to the spread of each model’s ensemble of realizations, as some models have a wide spread compared to other models, and also to the spread between models. Three models predict above-normal rainfall in the median (four predict above-normal rainfall in the mean), four predict normal rainfall, and one (GFDL-CM2p1-aer04) predicts below-average rainfall. The statistical models have smaller spread—all but two models forecast “normal” rainfall, while two forecast “above-normal” rainfall (note that the Nicholson2014 model falls on the bottom end of the “normal” range, with the mean within “normal” but the median “below normal”). All of this points to weak large-scale influence on the 2020 forecast, with both statistically- and dynamically-based models tending towards average predictions that are relatively widespread.  

This weak control on the forecast is, in large part, a reflection of current El Niño conditions and projections. El Niño events are associated with drought in the Blue Nile basin, and drought is generally unlikely to occur under El Niño-Southern Oscillation (ENSO) neutral or La Niña conditions. At present, tropical Pacific Sea Surface Temperatures (SSTs) and the tropical atmospheric circulation are consistent with ENSO neutral conditions, and these conditions are forecast to persist through the summer rainy season (Figure 5).

Figure 5: Probabilistic ENSO forecast, issued April 9, 2020 and included in the May 11 NOAA ENSO report, available from the NOAA Climate Prediction Center.

Based on historical precedent, then, it would appear to be unlikely that the forecast for normal to above-normal rainfall and streamflow in the Blue Nile basin will shift significantly over the course of the season. We will continue to track these conditions as the rainy season proceeds.

Questions can be addressed to [email protected]