The Okanagan region in British Columbia, Canada, is a semi-arid watershed of 8,200 km2 area. The region's water resources will be unable to support an increase in demand due to projected climate change and population growth, so a broad portfolio of adaptive measures will be needed (Cohen and Neale, 2006; Cohen et al., 2006). Irrigation accounts for 78% of the total basin licensed water allocation.
Figure 3.7 illustrates, from a suite of six GCM scenarios, the worst-case and least-impact scenario changes in annual water supply and crop water demand for Trout Creek compared with a drought supply threshold of 30 million m3/yr (36% of average annual present-day flow) and observed maximum demand of 10 million m3/yr (Neilsen et al., 2004). For flows below the drought threshold, local water authorities currently restrict water use. High-risk outcomes are defined as years in which water supply is below the drought threshold and water demand above the demand threshold. For all six scenarios, demand is expected to increase and supply is projected to decline. Estimated crop water demand increases most strongly in the HadCM3 A2 emissions scenario in which, by the 2080s, demand exceeds the current observed maximum in every year. For HadCM3 A2, high-risk outcomes occur in 1 out of 6 years in the 2050s, and in 1 out of 3 years in the 2080s. High-risk outcomes occur more often under A2 than under the B2 emissions scenario due to higher crop water demands in the warmer A2 world.
Table 3.3 illustrates the range of costs of adaptive measures currently available in the region, that could either decrease water demand or increase water supply. These costs are expressed by comparison with the least-cost option, irrigation scheduling on large holdings, which is equivalent to US$0.35/m3 (at 2006 prices) of supplied water. The most expensive options per unit of water saved or stored are metering and lake pumping to higher elevations. However, water treatment requirements will lead to additional costs for new supply options (Hrasko and McNeill, 2006). No single option is expected to be sufficient on its own.
Trout Creek supply/demand HadCM3-A2
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