16 Potential Transboundary Impacts of the Grand Ethiopian …
363
3 GERD Impact Assessment Methodology, Scenarios,
and Assumptions
The study of the impacts of the first filling and operation of the GERD is based on
simulating more than 100 possible scenarios (AbuZeid 2017b). The amount of the
first seepage losses, annual seepage, and annual evaporation from the GERD were
estimated based on the volume of storage and the water surface area of the GERD
Reservoir. The study presents scenarios for the first filling volume of up to 15, 25,
and 62 BCM over a period of 6 years, 10 years, and in some cases, 4 or 5 years. The
study is based on simulating the historical series (1911–2015) of Blue Nile flows on
which GERD is being built in Ethiopia (Abu-Zeid 2010; Said 1993). Different water
levels in the High Aswan Dam (HAD) reservoir of 150 meters (m), 160, 165, 170,
and 175 m (above mean sea level) were used as simulations for the HAD level at the
beginning of the first filling of the GERD reservoir in the Upper Blue Nile. Different
combinations of these variables created the scenarios that were studied and modeled
into the future when the GERD is in place. The impact of the GERD on the flows
of the Blue Nile downstream and the water levels and storage volumes of the HAD
reservoir was predicted by the model, taking into account the other Nile tributaries
flows and Egypt’s and Sudan‘s shares of the Nile water. The results were compared
to the baseline scenario (without GERD). The number of years of water deficit for
Egypt and Sudan, and the volume of the deficits were assessed for every scenario.
Since there is no agreement yet on the operating rules of the GERD, assumptions for
operation scenarios were made based on lowering the reservoir levels of GERD by
the end of the hydrological year back to different scenarios for the first filling volume
of 15, 25, and 62 BCM.
The “first filling” is defined as filling up the dead storage volume in addition to
a safety height of about 20 m above the level of the main turbines, to 590 m above
mean sea level (amsl), which is equivalent to total storage of around 15 BCM. The
relationship between the water level in the GERD reservoir and the water surface
area, and the storage volume in the GERD reservoir is depicted in Fig. 4.
The “first seepage” and “annual seepage” from GERD were assumed as per the
“average scenario” in the charts given in Figs. 5 and 6.
According to Wale (2008), the evaporation rate from Lake Tana in Ethiopia, the
surface water level of which is 1786 m (amsl), is about 4.63 mm/day. According to
Bashar and Mustafa (2009) , the evaporation rate from the Roseires Dam Reservoir
in Sudan, the water level of which is 480 m (amsl), has an average evaporation rate
of about 6.62 mm/day. As the GERD highest water level is about 640 m (amsl),
by interpolation the evaporation rate of the GERD reservoir may be estimated to
be 6.38 mm/day. This estimated evaporation rate is reflected by the “interpolation
scenario” for GERD “annual evaporation” as shown in Fig. 7. The assessment in this
study uses the “average” scenario.
The relationship among the water level in the HAD reservoir, the water surface
area, and the storage volume is presented in Fig. 8.
363
3 GERD Impact Assessment Methodology, Scenarios,
and Assumptions
The study of the impacts of the first filling and operation of the GERD is based on
simulating more than 100 possible scenarios (AbuZeid 2017b). The amount of the
first seepage losses, annual seepage, and annual evaporation from the GERD were
estimated based on the volume of storage and the water surface area of the GERD
Reservoir. The study presents scenarios for the first filling volume of up to 15, 25,
and 62 BCM over a period of 6 years, 10 years, and in some cases, 4 or 5 years. The
study is based on simulating the historical series (1911–2015) of Blue Nile flows on
which GERD is being built in Ethiopia (Abu-Zeid 2010; Said 1993). Different water
levels in the High Aswan Dam (HAD) reservoir of 150 meters (m), 160, 165, 170,
and 175 m (above mean sea level) were used as simulations for the HAD level at the
beginning of the first filling of the GERD reservoir in the Upper Blue Nile. Different
combinations of these variables created the scenarios that were studied and modeled
into the future when the GERD is in place. The impact of the GERD on the flows
of the Blue Nile downstream and the water levels and storage volumes of the HAD
reservoir was predicted by the model, taking into account the other Nile tributaries
flows and Egypt’s and Sudan‘s shares of the Nile water. The results were compared
to the baseline scenario (without GERD). The number of years of water deficit for
Egypt and Sudan, and the volume of the deficits were assessed for every scenario.
Since there is no agreement yet on the operating rules of the GERD, assumptions for
operation scenarios were made based on lowering the reservoir levels of GERD by
the end of the hydrological year back to different scenarios for the first filling volume
of 15, 25, and 62 BCM.
The “first filling” is defined as filling up the dead storage volume in addition to
a safety height of about 20 m above the level of the main turbines, to 590 m above
mean sea level (amsl), which is equivalent to total storage of around 15 BCM. The
relationship between the water level in the GERD reservoir and the water surface
area, and the storage volume in the GERD reservoir is depicted in Fig. 4.
The “first seepage” and “annual seepage” from GERD were assumed as per the
“average scenario” in the charts given in Figs. 5 and 6.
According to Wale (2008), the evaporation rate from Lake Tana in Ethiopia, the
surface water level of which is 1786 m (amsl), is about 4.63 mm/day. According to
Bashar and Mustafa (2009) , the evaporation rate from the Roseires Dam Reservoir
in Sudan, the water level of which is 480 m (amsl), has an average evaporation rate
of about 6.62 mm/day. As the GERD highest water level is about 640 m (amsl),
by interpolation the evaporation rate of the GERD reservoir may be estimated to
be 6.38 mm/day. This estimated evaporation rate is reflected by the “interpolation
scenario” for GERD “annual evaporation” as shown in Fig. 7. The assessment in this
study uses the “average” scenario.
The relationship among the water level in the HAD reservoir, the water surface
area, and the storage volume is presented in Fig. 8.