Application of WEAP in the Volta basin to model water allocation to the Akosombo hydropower scheme under different scenarios
D. de Condappa1, A. Chaponnière2, W. Andah3 and J. Lemoalle1
1 Institut de Recherche pour le Développement, UMR G-EAU, Montpellier, France
2 International Water Management Institute, Regional Office for Africa, Accra, Ghana
3 Water Research Institute, Accra, Ghana
Corresponding author: D. de Condappa, [email protected] Introduction
The upper part of the Volta Basin lies in semi-arid Burkina Faso and the rivers flow toward the more humid Ghana. Priority in Burkina Faso is to develop water conservation, with numerous small dams and some medium size reservoirs, while the Akosombo hydroelectric compound lies in the lower part of the basin, a high priority for Ghana. These different uses of water set the grounds for consultative water allocation, in a context of climate change and population increase at a rate of 2.5 to 3 % per year (Andah et al. 2004).
The objective of this work was to apply the Water Evaluation And Planning (WEAP) to the Volta basin (WEAP-Volta) and provide insight into possible consequences of climate change and development of upstream small reservoirs on water allocated to the downstream Akosombo hydropower scheme.
Material and method
Parameterisation of WEAP-Volta
The Volta basin was subdivided into 19 sub-units using the digital elevation model (Figure 1).
WEAP-Volta simulated simultaneously two processes within each sub-basin: (i) the available surface water resource and (ii) the allocation of this resource among the various demands. The first process modelled the surface hydrology while groundwater behaviour was ignored due to scarcity of hydrogeologic data.
The second process considered five categories of water use:
- irrigation from large reservoirs, which are formal/state arrangements,
- hydropower at the large hydroelectric reservoirs Bagre, Kompienga and Akosombo,
- water stored in small reservoirs which are mostly informal/private arrangements and mainly located in Burkina Faso,
- domestic for urban zones and,
- river-side irrigations and livestock consumptions.
We assumed for allocation priorities of the water demands that there is no upstream – downstream cooperation in management of the Basin water resource.
Figure 1: Schematic of the Volta basin in WEAP.
Figure 2: Storage of the Lake Volta: observed vs. simulation.
We calibrated the hydrologic process with available observed river flows between 1951 and 2000, taken partly from Bodo (2001) and provided partly by the project Volta HYCOS. The data set covers a variable part of the whole 1951-2000 period, and thus allow for more or less
satisfactory calibrations according to the importance of the data set.
We tested the parameterisation of WEAP-Volta by comparing simulated and observed water storage in the Lake Volta (Figure 2). The Nash coefficient equals 0.85, which is satisfactory.
Scenarios
We developed scenarios for 20 years, with the year 2000 as the year 0 of the simulations (where no change occurs yet). Three types of scenarios have been considered from year 1:
- reference, where no change occurs, - climate change,
- and a further development of small reservoirs at the sharp rate +10%/year of increase in number of small reservoirs.
Climate change scenarios were based on the report Christensen et al. (2007) which concludes that there is no consensus on the change in rainfall in West Africa. Hence, the climate change
scenarios simulated a drier and wetter climate by mimicking the reduction in rainfall that
occurred in the Basin in the 70’s (Lemoalle, 2007). We shifted the meteorological grids observed during the period 1980-2000 so as to simulate:
- a drier period, by a shift of 1 degree southwards, - a wetter period, by a shift of 1 degree northwards.
Hydropower turbine flows tended to increase upto the year 2000 (Obeng-Asiedu 2004) but having no information on the dams’ operational rule, the demand for hydropower was taken equal to the average observed value. This led to a constant value smaller than turbine flows observed in 1985-1998.
Results
Climate change
The fluctuation in annual meteorological data in each scenario produced inter-annual storage variations that were of the same magnitude than effects of climate change (Figure 3). Situation is (i) comfortable in the wetter scenario, (ii) vulnerable in the reference scenario and (iii) critical in the drier scenario.
0 20 40 60 80 100 120 140 160
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Year of simulation
Storage (km3)
Scenario of reference Scenario +10%/year
Top of inactive (70 km3) Storage capacity (148 km3)
Figure 3: Simulation of the water stored in Lake Volta for the climate change scenarios.
Figure 4: Simulation of the water stored in Lake Volta for the scenario of development of small reservoirs (+10%/year).
Development of small reservoirs
The chosen sharp rate +10%/year of increase in number of small reservoirs led after 20 years to a number 7 times greater which may appear ambitious but would be required to improve the resilience of small farmers to rainfall variability. Impact on the Lake Volta is small as storage is almost the same as in the reference scenario (Figure 4). We estimated that the reduction in inflows to the Lake is about 3% at the end of the scenario.
Outcomes and prospects
First outcome is that, with respect to all our limitations / assumptions, climate changes similar to that which had been observed in the recent past have a critical impact on the hydropower
generation at Akosombo. Electricity generation up to the level observed in late 1990’s could only be sustainable in a wetter climate, which call for careful management of the Akosombo
hydropower scheme.
The second conclusion is that a growth +10%/year of increase in number of small reservoirs during 20 years would reduce inflows by about 3%, an impact on hydropower production which may be noteworthy but which is small compared to possible consequences of a climate change.
Such result call for trade-off analysis: a significant improvement of livelihood upstream of the Basin, in Burkina Faso, versus a slight but probably noteworthy negative effect on the Akosombo hydropower scheme downstream of the Basin, in Ghana.
The way forward is to collaborate with stakeholders in the Volta basin, in particular universities, and refine further WEAP-Volta. A future implementation within the framework of the Volta Basin Authority may foster transboundary dialogue for the integrated management of the Basin's water resources.
Acknowledgements
Observed river flows were partly provided by the project ‘Volta HYCOS’.
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