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well which is from the Burkina Faso fields. The reason is that the other
technologies are more prevalent (approximately 90%) in the Ghana section of
the three catchments while the permanent shallow well is more prevalent
(approximately 85%) in Burkina Faso.
Irrigation water requirement in Table 8.1 is derived from CROPWAT.
Transpiration is equated to the sum of irrigation water requirement and
effective rainfall during the irrigation season. The irrigation efficiency is the
ratio between the irrigation water requirement and water abstracted. The
Evaporation represents the non-productive water losses from the soil and
canals. Return flows were not measured. Following Burt (1999), the return
flow is estimated at 50% of excess water.
8.2.3
Groundwater characteristics
According to studies conducted by Martin (2006) Regolith aquifers in the
weathered zone overlying Precambrian basement rocks serve as the main
source of groundwater in the study area and large parts of the Volta basin.
The depth of the aquifer varies from 3m to 51m and averages 18m. The long
term groundwater recharge rate is approximately 6% of average annual
rainfall and equals 66 mm/a. Total current groundwater abstraction is 4
mm/a in the study area. Martin (2006) also showed that a 20% reduction in
rainfall results in a 30% reduction of groundwater recharge.
Long-term monthly rainfall data was obtained for Vea, located in the
Yarigatanga catchment, over the period 1961-2005. The groundwater
storages for the three catchments were estimated using an average aquifer
depth of 18m, porosity of 8% and the catchment area.
8.2.4
Rainfall-Runoff Co-efficient
Runoff generation in semi-arid areas typically follows a distinct pattern. The
first rains at the start of the rainy season wet up the soil that is dried out
after the dry season. These rains produce very little runoff. Only after the
soils are above field capacity does water flow from the watershed to streams.
Studies conducted in the Volta Basin showed that the rainfall runoff
coefficient for the Volta Basin varies between 7% and 57% (Van de Giesen et
al., 2001). The water balance of the Atankwidi catchment by Martin (2006)
shows that, in the Atankwidi catchment 23% and 11% of annual rainfall ends
up as surface run-off in a wet year and a dry year, respectively, the long term
average of surface-runoff is 13%, approximately 40% of surface run-off
consists of interflow (shallow sub-surface flow), while the main fraction is
direct surface run-off and base flow from groundwater is negligible.
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