Working Group II: Impacts, Adaptation and Vulnerability

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4.3.3. Evaporation

Evaporation from the land surface includes evaporation from open water, soil, shallow groundwater, and water stored on vegetation, along with transpiration through plants. The rate of evaporation from the land surface is driven essentially by meteorological controls, mediated by the characteristics of vegetation and soils, and constrained by the amount of water available. Climate change has the potential to affect all of these factors—in a combined way that is not yet clearly understood— with different components of evaporation affected differently.

The primary meteorological controls on evaporation from a well-watered surface (often known as potential evaporation) are the amount of energy available (characterized by net radiation), the moisture content of the air (humidity—a function of water vapor content and air temperature), and the rate of movement of air across the surface (a function of windspeed). Increasing temperature generally results in an increase in potential evaporation, largely because the water-holding capacity of air is increased. Changes in other meteorological controls may exaggerate or offset the rise in temperature, and it is possible that increased water vapor content and lower net radiation could lead to lower evaporative demands. The relative importance of different meteorological controls, however, varies geographically. In dry regions, for example, potential evaporation is driven by energy and is not constrained by atmospheric moisture contents, so changes in humidity are relatively unimportant. In humid regions, however, atmospheric moisture content is a major limitation to evaporation, so changes in humidity have a very large effect on the rate of evaporation.

Several studies have assessed the effect of changes in meteorological controls on evaporation (e.g., Chattopadhyary and Hulme, 1997), using models of the evaporation process, and the effect of climate change has been shown to depend on baseline climate (and the relative importance of the different controls) and the amount of change. Chattopadhyary and Hulme (1997) calculated increases in potential evaporation across India from GCM simulations of climate; they found that projected increases in potential evaporation were related largely to increases in the vapor pressure deficit resulting from higher temperature. It is important to emphasize, however, that different evaporation calculation equations give different estimates of absolute evaporation rates and sensitivity to change. Therefore, it can be very difficult to compare results from different studies. Equations that do not consider explicitly all meteorological controls may give very misleading estimates of change.

Vegetation cover, type, and properties play a very important role in evaporation. Interception of precipitation is very much influenced by vegetation type (as indexed by the canopy storage capacity), and different vegetation types have different rates of transpiration. Moreover, different vegetation types produce different amounts of turblence above the canopy; the greater the turbulence, the greater the evaporation. A change in catchment vegetation—directly or indirectly as a result of climate change—therefore may affect the catchment water balance (there is a huge hydrological literature on the effects of changing catchment vegetation). Several studies have assessed changes in biome type under climate change (e.g., Friend et al., 1997), but the hydrological effects of such changes—and, indeed, changes in agricultural land use—have not yet been explored.

Although transpiration from plants through their stomata is driven by energy, atmospheric moisture, and turbulence, plants exert a degree of control over transpiration, particularly when water is limiting. Stomatal conductance in many plants falls as the vapor pressure deficit close to the leaf increases, temperature rises, or less water becomes available to the roots—and transpiration therefore falls. Superimposed on this short-term variation in stomatal conductance is the effect of atmospheric carbon dioxide (CO2) concentrations. Increased CO2 concentrations reduce stomatal conductance in C3 plants (which include virtually all woody plants and temperate grasses and crops), although experimental studies show that the effects vary considerably between species and depend on nutrient and water status. Plant water-use efficiency (WUE, or water use per unit of biomass) therefore may increase substantially (Morison, 1987), implying a reduction in transpiration. However, higher CO2 concentrations also may be associated with increased plant growth, compensating for increased WUE, and plants also may acclimatize to higher CO2 concentrations. There have been considerably fewer studies into total plant water use than into stomatal conductance, and most empirical evidence to date is at the plant scale; it is difficult to generalize to the catchment or regional scale (Field et al., 1995; Gifford et al., 1996; Amthor, 1999). Free-air CO2 enrichment (FACE) experiments, however, have allowed extrapolation at least to the 20-m plot scale. Experiments with cotton, for example (Hunsaker et al., 1994), showed no detectable change in water use per unit land area when CO2 concentrations were increased to 550 ppmv; the 40% increase in biomass offset increased WUE. Experiments with wheat, however, indicated that increased growth did not offset increased WUE, and evaporation declined by approximately 7% (although still less than implied by the change in stomatal conductance; Kimball et al., 1999). Some model studies (e.g., Field et al., 1995, for forest; Bunce et al., 1997, for alfalfa and grass; Cao and Woodward, 1998, at the global scale) suggest that the net direct effect of increased CO2 concentrations at the catchment scale will be small (Korner, 1996), but others (e.g., Pollard and Thompson, 1995; Dickinson et al., 1997; Sellers et al., 1997; Raupach, 1998, as discussed by Kimball et al., 1999) indicate that stomata have more control on regional evaporation. There clearly is a large degree of uncertainty over the effects of CO2 enrichment on catchment-scale evaporation, but it is apparent that reductions in stomatal conductance do not necessarily translate into reductions in catchment-scale evaporation.

The actual rate of evaporation is constrained by water availability. A reduction in summer soil water, for example, could lead to a reduction in the rate of evaporation from a catchment despite an increase in evaporative demands. Arnell (1996) estimated for a sample of UK catchments that the rate of actual evaporation would increase by a smaller percentage than the atmospheric demand for evaporation, with the greatest difference in the “driest” catchments, where water limitations are greatest.

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