Climate change is another potential influence on the demand for water. Municipal demand is related to climate to a certain extent. Shiklomanov (1998) notes different rates of use in different climate zones, although in making comparisons between cities it is difficult to account for variation in nonclimatic controls. The sensitivity of municipal demand to climate change is likely to be very dependent on the uses to which the water is put. The most sensitive areas are increased personal washing and—more importantly in some cultures—increased use of water in the garden and particularly on the lawn. Studies in the UK (Herrington, 1996) suggest that a rise in temperature of about 1.1°C by 2025 would lead to an increase in average per capita domestic demand of approximately 5%—in addition to nonclimatic trends—but would result in a larger percentage increase in peak demands (demands for garden watering may be highly concentrated). Boland (1997) estimated the effects of climate change on municipal demand in Washington, D.C., under a range of different water conservation policies. Table 4-5 summarizes percentage change in summer water use under the range of scenarios considered. Boland (1997) concludes that the effect of climate change is “small” relative to economic development and the effect of different water conservation policies.

Industrial use for processing purposes is insensitive to climate change; it is conditioned by technologies and modes of use. Demands for cooling water, however, may be affected by climate change. Increased water temperatures will reduce the efficiency of cooling, perhaps necessitating increased abstraction (or, of course, changes in cooling technologies to make them more efficient).

Agricultural demand, particularly for irrigation water, is considerably more sensitive to climate change. There are two potential effects. First, a change in field-level climate may alter the need for and timing of irrigation: Increased dryness may lead to increased demands, but demands could be reduced if soil moisture content rises at critical times of the year. Döll and Siebert (1999) applied a global irrigation water-use model with a spatial resolution of 0.5°x0.5° to assess the impact of climate change on net irrigation requirements per unit irrigated area, with a climate change scenario based on the ECHAM4 GCM. Figure 4-3 shows the relative change of net irrigation requirements between the present time (1961–1990) and 2025 in all areas equipped for irrigation in 1995. Under this scenario— and similarly under the corresponding HadCM3 scenario—net irrigation requirements per unit irrigated area generally would decrease across much of the Middle East and northern Africa as a result of increased precipitation, whereas most irrigated areas in India would require more water. The extra irrigation requirements per unit area in most parts of China would be small; the HadCM3 scenario leads to a greater increase in northern China. Other climate models would give different indications of regional changes in irrigation requirements. On the global scale, increases and decreases in net irrigation requirements largely cancel, and there is less difference between different climate models; under two scenarios considered by Döll and Siebert (2001), global net irrigation requirements would increase, relative to the situation without climate change, by 3.5–5% by 2025 and 6–8% by 2075. Actual changes in withdrawals would be dependent on changes in the efficiency of irrigation water use.

The second potential effect of climate change on irrigation demand is through increasing atmospheric CO₂ concentrations (Chapter 5). Higher CO₂ concentrations lower plant stomatal conductance, hence increase WUE; but as indicated in Section 4.3.3, this may be offset to a large extent by increased plant growth.

Hatch et al.(1999) assessed irrigation water requirements in Georgia, USA, using a climate change scenario derived from HadCM2. This scenario produced increased rainfall in most seasons, which, together with a shorter growing season and the assumed effect of CO₂ enrichment, resulted in a decrease in irrigation demand, ranging from just 1% by 2030 for soybean to as much as 20% by 2030 for corn. Along the Gulf Coast of the United States, however, the same scenario implies an increase in irrigation demands (Ritschard et al., 1999). Strzepek et al. (1999) also simulated decreases in irrigation requirements across the U.S. cornbelt under two of three scenarios (with the decrease depending on assumed irrigation use efficiency) but an increase under the third scenario. These three studies together indicate considerable uncertainty in estimated future irrigation withdrawals.

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