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Understanding and managing new risks on the Nile with the Grand Ethiopian Renaissance Dam

When construction of the Grand Ethiopian Renaissance Dam (GERD) is completed, the Nile will have two of the world’s largest dams—the High Aswan Dam (HAD) and the GERD—in two different countries (Egypt and Ethiopia). There is not yet agreement on how these dams will operate to manage scarce water resources. We elucidate the potential risks and opportunities to Egypt, Sudan and Ethiopia by simulating the filling period of the reservoir; a new normal period after the reservoir fills; and a severe multi-year drought after the filling. Our analysis illustrates how during filling the HAD reservoir could fall to levels not seen in recent decades, although the risk of water shortage in Egypt is relatively low. The new normal will benefit Ethiopia and Sudan without significantly affecting water users in Egypt. Management of multi-year droughts will require careful coordination if risks of harmful impacts are to be minimized.


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Filling the GERD is expected to happen in three stages: (i) retain 4.9 bcm to test 2 low-head turbines 98 at 565m in filling year 1, (ii) retain approximately 13.5 bcm to test the remainder of the turbines at 99 595 masl during filling year 2, and (iii) gradually fill the remaining active storage space to raise the 100 reservoir to 625 masl at the beginning of a flood season (or 640 masl at the end of a flood season) 101 over some negotiated duration of time.
Although the low-head turbines could in principle allow the GERD to be drawn down to 565 masl, recent statements from the ongoing negotiations suggest that 595 masl would be the lowest 105 permissible level, therefore we consider the initial 18.4 bcm as a one-time 'cost' to fill the reservoir, 106 which is roughly deducted from the inflows to the HAD Reservoir. More precisely however, a 107 portion of this 18.4 bcm would never have reached the HAD Reservoir due to evaporation and 108 seepage between the GERD and the HAD Reservoir. In addition, some of this water could in theory 109 be consumed by water users in Sudan, though the consumption lost in Sudan is likely to be minor 110 given existing water uses. Thus, the correspondence between the water held in storage in the GERD 111 and the reduction in storage in the HAD Reservoir is somewhat more than one-to-one. In other 112 words, the net storage reduction in the HAD Reservoir will be somewhat less than 18.4 bcm.

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The downstream consequences of filling the GERD will depend on several natural and human-115 controlled factors. The first important factor will be the magnitude of flows throughout the basin 116 during the filling period. High flows that occur in the Blue Nile above the GERD location are likely to 117 shorten the period required for filling the GERD Reservoir, but this duration will also depend on 118 operational decisions for the GERD as described below. Even if higher than average flows occur 119 during filling below the GERD location on the Blue Nile or in the other tributaries, the level of the 120 HAD Reservoir is still likely to fall due to the water captured by the GERD Reservoir. Our results 121 indicate however that under wet conditions, Egypt likely will not need to reduce releases to avoid   countries. The less water that is released from the GERD during the filling process, the faster the 140 retained water will accumulate in GERD Reservoir, but this will also result in more abrupt declines 141 in the HAD Reservoir. Egypt would prefer more water to be released from the GERD throughout the 142 filling period, resulting in a longer time required to fill the GERD Reservoir and a more gradual 143 decline in the elevation of the HAD Reservoir. This would benefit Egypt by allowing more water 144 under their immediate control to meet downstream needs and minimizing losses of power 145 generation due to higher head at the HAD. However, a slower filling also comes at a cost of lower 146 power generation from the GERD, as well as greater evaporation loss from the HAD Reservoir 147 relative to that from the GERD Reservoir. If the GERD Reservoir is filled quickly, the filling period 148 will end sooner, and storage in the HAD Reservoir can begin to recover earlier. Furthermore, a 149 longer filling period exposes this process to a longer duration that could be helpful or harmful, since 150 it becomes increasingly likely that a high flood or a major drought might facilitate filling or increase Reservoir versus allowing more time for the GERD to fill, which prolongs low-level risks to the HAD Reservoir and Egyptian water supplies.

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A third factor that will significantly influence the downstream impacts of the GERD during the 157 filling process will be how rapidly the power generated by the GERD can be integrated into the 158 regional power grid. Insufficient power demands can limit the ability of the GERD turbines to 159 discharge water downstream, leaving releases to be limited by the hydraulic capacity of bottom 160 outlets (542.25 m). A temporary 'low block' spillway will be used during at least the first two years 161 of the filling process, which will be elevated each year to allow the reservoir capture additional

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We also emphasize that Sudan, unlike Egypt, has no over-year water storage to buffer reduced 203 upstream flow, so the filling and operation of GERD may cause concern if the intra-annual timing of 204 releases is not known. With coordination between the GERD and the Sudanese dams and sufficient 205 flows released from the GERD to meet current Sudanese irrigation withdrawals, the GERD will 206 provide Sudan with a wide range of economic benefits, including flood control, increased 207 hydropower from more stable water levels at dams in Sudan, increased summer water supply, and 208 reduced sedimentation of its storage facilities 11,19,20 . These risks and benefits will occur 209 immediately when filling begins and continue into the foreseeable future. Sudan is also likely to be 210 a major market for the power that will be produced from the GERD and could eventually provide During the filling of the GERD Reservoir, the HAD Reservoir will operate at considerably lower GERD Reservoir, system evaporation will only remain lower so long as levels in the HAD Reservoir 217 remain at considerably lower levels than in the pre-GERD era, which will only occur if the basin 218 enters a period of drought and low flows, and/or if upstream abstractions of water increase. Thus, 219 evaporative savings are greatest when downstream water scarcity is highest. Over the long term 220 and assuming current levels of upstream water use, the addition of the GERD, with the substantial 221 surface area of its reservoir, will increase the net evaporation of the system. After the GERD

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Reservoir filling is complete and the HAD Reservoir has recovered, the total additional evaporation 223 from the GERD Reservoir will be 1.7 bcm per year, and the evaporation savings from a lower HAD