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scenarios, riverine alluvial dugouts are negligible in the catchment. In the
last scenario where riverine alluvial dugout irrigation is significant, the water
loss in the WEAP Model is higher than that of the water balance.
Thus to have a valid comparison, scenarios with negligible riverine alluvial
dugout irrigation can best be used to explain the differences between the two
methods. Such scenarios are the first two scenarios (5% and 10% growth) of
the Yarigatanga catchment. From these two scenarios the water losses
deduced from the water balance analysis are slightly higher than that of the
WEAP model. This is because the WEAP model results report the average
over the entire study period which has varying climatic conditions, while the
water balance analysis is based on the long-term mean hydrological year.
This average hydrological year is not representative for the entire period,
because frequent slightly dry years are alternated by few very wet years (see
Figure 8.4). This variation explains the higher water losses estimated by the
water balance analysis.
From the above comparison and the reasons given for the differences between
the two methods, it may be concluded that the WEAP model is more
reliable than the simple water balance analysis. This is because the WEAP
model is able to capture the complexities with the riverine alluvial dugouts
irrigation and also provides results commensurate with the climatic condition
over the study period. However, the water balance analysis also confirms a
weakness of the WEAP model, namely that it considers groundwater to be
unconnected to surface water, implying that water abstracted from
groundwater for irrigation is not considered to have any impact on
streamflow.
8.5 D
ISCUSSION OF THE
R
ESULTS
The objective of this chapter is to assess the impact of increasing irrigation
development in the White-Volta sub-basin. This section summarises the
impacts on consumptive water use of the current and possible future
irrigation developments. In a following section the impact on competing
water uses downstream of the White Volta sub-basin will be assessed.
8.5.1 Current Level of Exploitation of Water
Resources for Irrigation
Current irrigation covers 1% of the Anayari catchment area, 0.8% of
Atankwidi and 2.6% of Yarigatanga. Because of the presence of the Vea dam
in the Yarigatanga catchment, irrigation development depending on surface
water is 90% of the current total with the remaining 10% depending on
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