On the Shortcomings of Modelling Downstream Water Budget Deficit Under Nile Upstream Damming

Accurate hydraulic modelling and sharing utilized datasets are crucial for resolving Nile water usage rights disputes and building trust enabling sustainable and collaborative transboundary water management.
On the Shortcomings of Modelling Downstream Water Budget Deficit Under Nile Upstream Damming

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Examining the Accuracy of Using a Single Short-Term Historical Flow Period to Assess the Nile’s Downstream Water Deficit from GERD Filling: A Technical Note - Earth Systems and Environment

Increasing water and energy demands, hydroclimatic fluctuations, damming, and usage rights disputes present major challenges in managing transboundary rivers worldwide. Of particular interest is the Eastern Nile River Basin (ENRB), which is subject to broad debate over which modeling approach should be applied to resolve the disparities in transboundary water management among the river’s riparian nations, under increased upstream damming. Several studies have simulated the downstream High Aswan Dam (HAD) storage change during the Grand Ethiopian Renaissance Dam (GERD) filling under different hydrological conditions. However, their findings diverge regarding the impacts of GERD filling on HAD storage, especially when considering a specific, naturalized, historical 10-year period to represent the average flow condition used, as a benchmark for assessing potential downstream impacts. Our extended analysis of the Nile flow historical records demonstrates that considering a single historical 10-year period to simulate the downstream water budget, as performed in Wheeler et al. (Nat Commun 11:5222, 2020, https://doi.org/10.1038/s41467-020-19089-x), widely communicated to policymakers as a robust proof that GERD filling will not generate a deficit at HAD under average flow conditions, is inconclusive as it strongly depend on the selection of the historical inflow period, due to the river high interannual flow variability. Our simulation results of 20 average/near average historical flow periods in Wheeler et al. (Nat Commun 11:5222, 2020, https://doi.org/10.1038/s41467-020-19089-x)’s datasheet indicates that 60% of them generates a downstream water deficit ranging from 0.5 to 14.5 BCM under the same modeling conditions and filling policy. Therefore, considering the simulation results of multiple flow sequences is crucial for accurately reflecting the impact of the Nile’s high interannual flow variability on downstream water deficit assessments, thereby settling the disparities in transboundary water management forecasts for the impacts of GERD filling and operation.

The Nile River is a unique hydro-ecological system and an indispensable water resource for riparian basin countries. Population growth, changes in precipitation patterns, and increased damming present extreme challenges in managing the basin's primary water resource. Policymakers can forecast and mitigate these challenges using realistic modelling outcomes of the altered flow. Hence, ensuring the accuracy  of the hydraulic modelling of the altered flow in the Nile Basin is crucial for settling disputes over usage rights, fostering transboundary trust, and creating a fair and long-lasting collaborative framework. This is particularly decisive for assessing the impacts of upstream damming and mitigating any potential downstream water budget deficit during filling or operation phases, such as in the Grand Ethiopian Renaissance Dam (GERD) case.

GERD is a hydropower project aiming to triple the existing supply of Ethiopian electricity to mitigate the country's electricity shortage, as 83% of Ethiopia's population currently lacks access to electricity. Moreover, GERD will regulate downstream flow in Sudan, curbing the risks of floods and extending the longevity of its hydropower dams. However, GERD's impact on the downstream water availability in Egypt has been subject to debate, mostly in the case of the change in water volume in the High Asswan Dam (HAD) reservoir, which is the nation’s primary strategic reserve against droughts. Hydraulic modelling efforts have been used to predict GERD impacts downstream on HAD under several filling and operation policies during different flow conditions (i.e., wet, average, and dry). While the dam is expected to reach its total capacity in the summer of 2024, the accuracy and limitations of hydraulic modelling forecasts are subject to broad debate due to their implications for future operation phases under fluctuating climate conditions. The latter is expected to yield unpredictable flow sequences alternating between extreme droughts and floods.

Undermining these downstream risks in scientific investigations impede downstream mitigation measures and hinders informed decisions, leading to stakeholder disagreements and mistrust in hydraulic modelling efforts. As such, reexamining the investigation of Wheeler et al., 2020 published in Nature Communication (https://doi.org/10.1038/s41467-020-19089-x), widely communicated to policymakers as robust modelling for assessing downstream risks on the HAD reservoir, is crucial to building consensus among the scientific community on the reported impacts. In particular, it is vital for decision-makers to assess the validity and reproducibility of Wheeler et al.'s 2020 findings during the average flow period, which policymakers perceive as the benchmark suggesting GERD’s low potential downstream damages.

 Shortcomings in Sampling flow periods in hydraulic Modelling

In their analysis, Wheeler et al., 2020 defined downstream impacts from GERD filling as the potential water budget shortage at HAD reservoir measured during the Nile average flow condition. As such, if hydraulic modelling shows that the HAD reservoir storage will be above the critical level of 60 Billion Cubic Meter (BCM), it will imply that the impacts are negligible under average flow conditions. Below this critical storage level of 60 BCM, Egypt would have to trigger its Drought Management Policy (DMP), which consists of a series of nationwide water conservation measures impacting the agricultural and industrial sectors. To achieve this objective, Wheeler et al., 2020 analyzed the records of the last 100 years of the Nile flow. Their analysis used criteria to select a representative average flow period based on the annual flow volume and avoid extreme events such as floods or droughts. They chose the period of 1943–1952 as a representative sequence for the Nile average flow condition. Their results indicated that filling GERD under the suggested National Independent Scientific Research Group (NISRG) filling policy will not result in the HAD reservoir reaching its storage level below 60 BCM under the hydraulic conditions of this near-average historic flow period. As such, Wheeler et al., 2020 suggested that Egypt will not have to trigger the DMP under average flow conditions, as shown in Figure 4.

However, while the study presented only one near-average flow period, their datasheet identified nineteen other compliant near-average historic Nile flow periods with similar average flow volumes (85–86.9 BCM) that have not been plotted in the published analysis. Wheeler et al., 2020 only simulated the single 10-year near-average period of 1943–1952, suggesting that the results are representative of the other average periods. The main average period of 1909–1918 and other compliant nineteen near-average periods representing the Nile average flow conditions were overlooked in the published paper without any justifications.

Based on this single period of 1943-1952, in Figure 4, Wheeler et al., 2020 concluded that HAD will not drop below the DMP level of 60 BCM, and as such, there would be no downstream deficit under average flow conditions. The above was used to support that the impact of GERD filling on Egypt’s water budget is relatively low. Furthermore, the study asserted that scientific investigations contradicting these findings are meant to create a water panic using social media.

Replicating findings under different average flow periods           

In our technical note, Heggy et al., 2023 (https://doi.org/10.1007/s41748-023-00355-z), we reexamine Wheeler et al., 2020 results using their datasheets, Eastern Nile RiverWare Model (ENRM), assumptions, and NISRG filling policy applied in their published analysis. We extend the simulations to include (1) the main average period of 1909–1918 and (2) other compliant nineteen near-average periods that represent the Nile average flow conditions, which are identified in the datasheet and not displayed in Wheeler et al. (2020).

Our reexamination identified the following inconsistencies:

  • The simulation results of the main average period of 1909–1918 indicate that HAD water storage volume will drop below the critical level of the DMP for three consecutive years, causing a measurable cumulative water deficit of 14.5 BCM downstream, contradicting the result of the single simulated near average period of 1943–1952 presented in Wheeler et al., which showed no downstream water deficit during the average conditions.
  • Only 8 of the 20 considered near-average periods will not trigger the DMP. In contrast, the other 12 periods will show invoking of the DMP, leading to a measurable downstream water deficit ranging from 0.5 to 14.5 BCM in 1-3 years, as shown herein in Figure 1. Again, these results contradict the simulated single 10-year near-average period presented in Figure 4 of Wheeler et al., 2020, showing no downstream water deficit will be triggered during the average condition.
  • In Wheeler, et al.’s 2020 simulated period of 1943–1952, the year 1946 inflow, assumed to be the fourth year of the GERD’s initial filling, witnessed a significant flood of 106.7 BCM. That contradicts Wheeler et al.’s (2020) own assumptions for selecting representative periods that must avoid abnormal events such as single-year floods or droughts during the initial filling years. Because of this flaw, Wheeler et al., 2020 Figures 3 and 4 show controversial results where the GERD reservoir would be filled faster—in 4 years—under average flow conditions and slower—in 5 years—under the wet one with a higher inflow rate to the reservoir.


Fig. 1: Simulated HAD storage during the GERD’s initial filling under four representative periods of average Nile flow conditions from Wheeler et al. (2020) datasheet. (a) near-average period 1943–1952 used in Wheeler et al. (2020), (b) main average period 1909–1918, (c) near-average period 1902–1911, and (d) near-average period 1988–1997

Fig. 1: Simulated HAD storage during the GERD’s initial filling under four representative periods of average Nile flow conditions from Wheeler et al. (2020) datasheet. (a) near-average period 1943–1952, used in Wheeler et al. (2020),  (b) main average period 1909–1918, (c) near-average period 1902–1911, and (d) near-average period 1988–1997

The above discrepancies between the calculated downstream deficit of the single 10-year period in Wheeler et al. (2020) and multiple other ones in their datasheet, as shown in our technical note (Heggy et al., 2023), illustrate the dependency of their modelling outcome on the selection of the flow period under a given hydraulic condition. This is due to the high interannual variability of the Nile River system that will be potentially accentuated by future climatic fluctuations. Neglecting the above by selecting a particular single near-average historical flow period to represent all average/near-average flow periods led to the finding of Wheeler et al., 2020 being irreplicable under other compliant flow periods. Such an inconclusive result calls into question the robustness of the carried hydraulic modelling in Wheeler et al., 2020 and its claimed utility for decision-makers, the scientific community, and civil society, as stated in the promotion campaigns associated with this publication.

Outlook on Future Modelling Efforts

Today, GERD's four fillings have been completed with 42 BCM (56% of the announced total reservoir volume), and the fifth one is projected to be completed in summer 2024. The current level of the HAD is almost full due to the filling of GERD occurring during wet flow conditions. As such, no direct water budget deficit has been observed downstream to this date. However, the growing disparities between riparian nations, causing disruptions in negotiations to reach a joint collaborative framework, are exacerbated by how existing hydraulic models represent the uncertainty surrounding the Nile's future flow. Both the GERD and HAD will likely come to or near their dead storage during prolonged drought periods such as the 1978–1987 period. Simultaneously refilling both reservoirs would be more perplexing than the current GERD's initial filling circumstances during favorable wet conditions. Thus, improving model reliability and data sharing is crucial to address concerns about GERD's operation in the coming years under both average and dry flow conditions. The ability to replicate results is fundamental for the credibility of the calls for building a collaborative framework among the river riparian. Authors and Editors of published investigations aimed to support policymakers should provide all efforts toward improving transparency in data analysis and availability to build trust in resolving water rights disputes.


Additional Material: A step-by-step detailed reproduction of Wheeler et al.’s 2020 results can be found in this video: https://www.youtube.com/watch?v=MG-TVJGSnsI

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