Evapotranspiration

Only part of the water infiltrating into the ground keeps moving downward into the saturated groundwater zone. An important part of infiltrated water is transferred back into the atmosphere. Two major mechanisms are involved evaporation and transpiration, together called evapotranspiration. 

The mode of rainfall and degree of evaporation influence groundwater composition. High degrees of evaporation result in enrichment of the heavier stable isotopes of hydrogen and oxygen (sections 9.3 and 9.6) and in higher concentrations of dissolved salts. 

Transpiration is the process by which plants lose water, mainly from the surfaces of leaves. Plants act as pumps, their roots extracting water from the soil and the leaves transpiring it into the atmosphere. Thus the depth of 

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Nonreplenishable (fossil) groundwater can be tapped, but such extraction depletes reserves in much the same way as extractions from oil wells do. The terrestrial renewable freshwater supply, RFITTj, equals precipitation on land, which then subdivides into two major segments evapotranspiration from the land, and mnoff to the sea, T. Because groundwater and surface water are often hydrauhcaHy coimected, soil infiltration and groundwater... 

Agriculture consumes by far the most of any use category to which the accessible mnoff worldwide is appHed (Table 6). Postel and co-workers estimate that human uses make up 26% of total terrestrial evapotranspiration and 54% of the mnoff geographically and temporally accessible (9). Increased use of evapotranspiration will confer minimal benefits globally because most of the land suitable for rain-fed agriculture is already in production. New dam constmction could increase accessible mnoff by about 10% over the next 30 years however, population increase during that period is projected to be more than 45%. 

The synthesis of 1 kg of dry plant biomass requires the evapotranspiration of about 300 L of water, although smaller amounts of water are needed by some plants such as desert cacti. Approximately one-third of the annual continental rainfall (100 cm/yr) is returned to the atmosphere by evapotranspiration. Although it accounts for only about 10-15% of global evaporation, plant evapotranspiration can play a major role in local climates. For example, a molecule of water falling on the upper Amazon Basin is recycled on average five times during its eventual return to the Atlantic Ocean. 

Water returns to the atmosphere via evaporation from the oceans and evapotranspiration from the land surface. Like precipitation, evaporation is largest over the oceans (88% of total) and is distributed non-uniformly around the globe. Evaporation requires a large input of energy to overcome the latent heat of vaporization, so global patterns are similar to radiation balance and temperature distributions, though anomalous local maxima and minima occur due to the effects of wind and water availability. 

Evapotranspiration (ET) is the collective term for land surface evaporation and plant transpiration, which are difficult to isolate in practice. Transpiration refers to the process in which water is transported through plants and returned to the atmosphere through pores in the leaves called stomata, and is distinct from direct evaporation of intercepted precipitation from leaf surfaces. Some land surface processes and the roles of vegetation in the water and energy balances are illustrated in Fig. 6-5. Due to... 

Runoff sensitivity, particularly in arid and semi-arid climates, is largely a result of sensitivity in soil moisture response. If rainfall amount and frequency decrease, more soil moisture is lost to evapotranspiration, creating a soil moisture deficit that must be replaced before surface runoff or significant ground-water flow returns. The converse also tends to... 

Eor saturated surfaces, the Bowen ratio can then be used to calculate evapotranspiration as a residual of the surface energy balance (Penman, 1948). Since direct measurement of ET is difficult and expensive, the energy balance method is fairly common. 

Dickinson, R. E. (1984). Modeling evapotranspiration for three-dimensional global climate models. Geo-physical Monographs Q. E. Hansen and T. Takahashi, eds.) 29,58-72. American Geophysical Union. 

Another family of feedbacks involving biota arise via the process of evapotranspiration in which the rate of water vapor is transferred from the land surface to the atmosphere is mediated by plants. Several consequences have been proposed that include influences of biota on the greenhouse effect of water vapor as well as relative humidity and clouds. Lovelock (1988) suggested that tropical forests might be kept cool by increasing cloud cover in response to higher relative humidity released through enhanced evapotranspiration (via the clouds influences on albedo). Yet another connection arises because tree-covered land has different turbulence properties above it than bare soil, which also influences the cloud cover above. 

However, some of our deer individuals from the arid Joshua Tree National Park in California indicate unusual D-enrichment. This may derive from evapotranspiration in local plants that were part of the diet of the deer and/or in the body fluids of the animals themselves, as is expected in extremely diy environments (Cormie et al., 1994c Bowen et al., 2005). Deer occupy an ecological niche that is relatively simple from the perspective of hydrogen, as their diet consists of leafy vegetation and their water is obtained from surface waters (lakes and streams) that in many cases have D values closely representing mean annual precipitation. In contrast, omnivorous and carnivorous animals consume more diverse diets with more widely varying... 

Drought is perhaps one of the most complex examples to choose but it illustrates well the possibilities of, and pitfalls to, progress. Drought affects almost every facet of plant function and we are faced with the paradox that yield and evapotranspiration are intimately linked. In general, increases in yield when water supply is limiting are likely to result from characteristics which increase the available water supply, increase water use efficiency or increase biomass allocation to the economically useful plant parts (Pass-ioura, 1986). Additionally, features which maintain cell viability and protect metabolism in water-stressed tissue and allow rapid recovery after dry periods will contribute yield under some circumstances. 

If basic calculations such as those presented are to be conducted, it is important to collect enough weather parameters to calculate reference evapotranspiration ETf). An on-site weather station should be considered a basic requirement minimum sensor requirements to calculate a Penman equation would include solar radiation, wind speed, relative humidity or actual vapor pressure, and air temperature. An on-site rain gauge is essential but it is also a good idea to have a rain gauge on the weather station even if it is not directly on-site. The most accurate variations of the Penman equation calculate Tq on an hourly basis. However, Penman routines using daily summaries are typically satisfactory for the purpose of calculating soil-water recharge. 

If evapotranspiration of the system exceeds precipitation, it is possible to capture water that is moving vertically through soil. Areas that receive precipitation in the wintertime (the dormant season for deciduous trees) must be modeled to determine if the soil will be sufficiently dry to hold water for the next spring s growth period. 

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