Evaporation from the land surface includes evaporation from open water, soil, shallow groundwater, and water stored on vegetation, along with transpiration through plants. The combined effect, commonly referred to as evapotranspiration, has a substantial influence on basin water budgets, runoff, and groundwater recharge. There is an enormous hydrologic literature regarding the nature, response, and controls of evapotranspiration under current and future climate conditions, but the interplay between atmospheric energy, moisture, and turbulence, and plant water use efficiency under different water, energy, nutrient, and CO₂ levels is complex and not yet fully understood.

A consistent prediction of climate models is that global warming will increase total evaporation.  Increases in surface temperature and higher wind speeds promote potential evaporation, while the greatest change will likely result from an increase in the water-holding capacity of the atmosphere.  While potential evaporation will almost certainly increase with temperature, its impact on precipitation in specific regions remains uncertain.  There are many balances and counter-balances in the atmosphere that aren’t fully under­stood. For example, atmospheric moisture originating from actual evaporation over oceans may help offset, and possibly even lessen, potential evaporative pressures over land.  Likewise, there are regional controls on evaporation. In humid regions where water is not limiting and actual and potential evaporation are nearly equal, evaporation is constrained by the water-holding capacity of air above the surface, so an increase in this capacity due to warming may have a large evaporative effect.  In dry regions, other factors such as surface water availability, surface temperature and wind are more important determinants of actual evaporation. A reduction in summer soil water, for example, could lead to a reduction in the rate of actual evaporative demands from a catchment despite an increase in potential evaporation. 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 between actual and potential evaporation occurring in the “driest” catchments, where water limitations are greatest (IPCC WGII 2001).

In their 1993 study on the Colorado River Basin, Nash and Gleick demonstrate the importance of evapotranspiration in determining water availability.  The study modeled the impact of several climate change scenarios on runoff.  Hypothetical precipitation changes ranged from a 20-percent increase to a 20-percent decrease, and the study considered temperature increases of 2°C and 4°C.  Their study showed that changes in precipitation would cause proportional changes in runoff, if all else remained constant.  Therefore, an increase or decrease in precipitation of 20 percent would result in runoff changing by approximately 20 percent.  However, the impact of temperature on runoff was also substantial, due to evapotranspiration.  The study found that, with no change in precipitation, a 2°C increase in temperature would reduce mean annual runoff by 4 to 12 percent.  The change in runoff for a 4°C increase would be between 9 and 21 percent.  Therefore, if temperature increased by 4°C, precipitation would need to increase by nearly 20 percent to maintain runoff at historical levels.