IMPACT OF INTENSIFYING IRRIGATION DEVELOPMENT AND IMPLICATIONS FOR SUSTAINABLE WATER RESOURCES MANAGEMENT IN THE WHITE VOLTA SUBBASIN - Sustainable Irrigation Development in the White Volta Sub-Basin - page 203

Input - Output = Change in Storage Equation 8.3

Water Balance at Catchment level:
P - (E R + E SI + E GI + E V + Q S + Q G ) = Δ S/ Δ t Equation 8.4
Water Balance at groundwater irrigation level:
P + I G + R I - E GI = Δ G R / Δ t

Equation 8.5

Water Balance at surface water irrigation level:

P + I S - E SI - E R = Δ S R / Δ t Equation 8.6

Table 8.10 : Monthly Evaporation from reservoirs
Months

Jan Feb March April May June July August Sept Oct Nov Dec

Reference
Evaporation
(mm/month)

177.0 184.0 210.0 190.0 180.0 153.0 141.0 130.0 131.0 143.0 147.0 156.0

Long-term
mean
Rainfall
(mm/month)

2.0 9.7 51.9 101.0 134.0 147.2 158.7 151.9 207.7 105.4 23.1 8.8

Net
Evaporation
(mm/month)

175.0 174.3 158.1 89.0 46.0 5.8 -17.7 -21.9 -76.7 37.6 123.9 147.2

The water balance calculations were performed to determine the water losses
due to irrigation activities. From Equation 8.5, the water losses due to
groundwater irrigation are the evaporation (E GI ) from irrigated fields. From
Equation 8.6, the water losses due to surface water irrigation equal the direct
evaporation from the surface reservoir (E R ) and the evaporation (E SI ) from
the irrigated fields. Evaporation from the surface reservoir equals the net
evaporation between the reference evaporation and direct rainfall on the
surface of the reservoirs (see Table 8.10).
The evaporation from irrigated fields and evaporation from surface reservoirs
for all scenarios are shown in Table 8.11.