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 191

Table 8.4 : Range of rainfall and modifying factors for climatic variations

Climate Variations Very Wet Wet Normal Dry Very Dry

Range of Rainfall (mm/a) 1248-1818 1151-1247 1050-1150 920-1049 762-919

Modifying factors for rainfall 1.27 1.09 1.00 0.89 0.76

Modifying factors for groundwater
recharge and streamflow

1.42 1.15 1.00 0.86 0.69

According to the study of Martin (2006) and van de Giesen et al., (2001),
groundwater recharge and streamflow varies much more from year to year
than rainfall. Therefore the climate modifying factors for the rainfall will
have to be modified to depict variations in streamflow and groundwater
recharge. Martin (2006) found that in the study area a 20% reduction in
annual rainfall resulted in a 30-60% reduction of annual groundwater
recharge. Moreover, runoff coefficients varied from 11% to 24% for dry and
wet years, respectively. This information could not provide specific modifying
factors for groundwater recharge and streamflow; however it did guide the
modification of factors due to rainfall variation. The study assumed the same
modifying factors for groundwater recharge and streamflow due to lack of
data (Table 8.4).
Apart from the domestic and urban water demands which vary from year to
year because of the population growth rate, all other demands and factors
remained the same in the reference scenario. The reference scenario results
show that all water demands are met with no significant impact on
groundwater and surface water resources.

8.4 S CENARIOS D EVELOPMENT AND A NALYSIS

In WEAP scenarios are developed to simulate likely evolution of the system
which may be due to a range of future possibilities. First, a reference
scenario was established from the current accounts which specifies the
projected period for scenario analysis (WEAP, 2009). The scenario analysis
was aimed at possible future situations in the year 2025. The reason for
choosing 2025 is based on the historical development of irrigation in the
study area. The expansion of irrigation development in the sub-basin has
been going on in the past two decades based on certain factors. ‚What if‛
scenarios representing increase in irrigation, changes in policy and technology
were modelled based on the reference scenario. The ‚what if‛ scenarios
considered in the analysis include: