Evapotranspiration estimates

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%. 

ET is the evaporation of water from the soil surface and by plant transpiration (primarily through the stomata on the plant s leaves). ET should be carefully considered during all stages of design since it will be the largest mechanism of water removal in the water balance for an ET cover. With current knowledge, it is necessary to estimate potential evapotranspiration (PET) first and then using the PET estimate the actual evapotranspiration (AET) for the site. 

In assessing evapotranspiration, it was sometimes assumed that the temperature profile over a canopy, when extrapolated to z — d = 0, gave a measure of the surface temperature. More correctly the profile should be extrapolated to the roughness length for heat transfer, which is nearly the same as zv for water vapour. In typical mid-day conditions, this increases the estimated surface temperature by several degrees (Thom, 1972). 

Evapotranspiration returns distilled water into the atmosphere. The salts, dissolved in the original precipitation water, remain in the soil zone, and at the end of dry seasons some of these salts accumulate on the surface. The next rain event redissolves these salts, and eventually they are brought into the saturated zone. Thus recharge water is richer in atmospherically derived salts than the original precipitation. The factor by which the chlorine concentration in the recharge water is greater than its concentration in the respective precipitation serves as a semiquantitative tool to estimate the amount of water transferred to the atmosphere by evapotranspiration (section 6.15). 

With the crude age one can estimate the paleoenvironmental conditions in terms of glacial or interglacial period, sea level, distance of paleorecharge area from the seashore, season of major recharge, altitude of the former water table, former evapotranspiration intensity, or the type of plant community that prevailed at the studied paleorecharge site. These considerations should lead to the selection of the pertinent paleo-input functions. 

Katerji, N, and Rana, G. 2008. Crop Evapotranspiration Measurements and Estimation in the Mediterranean Region. CRA-SCA, Bari, Italy. 

A further estimate of evapotranspiration was made (Leopoldo et al., 1982b) for high forest on a model basin of 23.5 km, 60 km north of Manaus, which is drained by a large stream. This area has an average rainfall of 2000 mm. Measurements were made of precipitation, inter-... 

The solar energy which falls on the central Amazon region i.e. near Manaus is on average 425 cal cm day" , which is used principally for evaporation of water and the evapotranspiration of plants (Villa Nova et d., 1976). It is estimated that 50-60% of the solar energy is used in this way. 

Two box models are used for the parameterization of water as conservative material and salts, including sulfur, as non-conservative material. For example, the first estimates were made up for the wet season. For the coastal water body budget, the following fluxes were monitored and calculated precipitation, evapotranspiration, runoff, groundwater, and residual flow. These budget estimates give the fresh water residence time as 6.2 days with residual flow equal -822.34 x 10- For Klong... 

A water balance detailing the water inputs and outputs from the field can be used to estimate recharge. Inputs to this balance include precipitation and irrigation. The outputs which can be estimated are runoff and evapotranspiration, and the remainder can be assumed to be recharge. 

PRZM may be overestimating the amount of water to percolate frcm the top zones to lower zones. This would occur if the water holding capacities measured in the field and assigned in PRZM are too low, resulting in less evapotranspiration and more percolation. This could also occur with inacurrate estimations of runoff - if more runoff occurs than is being simulated, then percolation will be overestimated. 

Earlier estimates show that the annual rainfall on Lake Eyre amounts to " 125 mm (4.92 in) with an average annual pan-evaporation of 3,800 mm (149.6 in). The difference between these two quantities gives the net specific evaporation rate. The measure of evaporation used in a recent report (McMahon et al., 2005) is the mean annual area potential evapotranspiration (APET). APET values vary from 1,000 mm/year in the south of the Lake Eyre Basin to >1,500 mm/year in the north (Fig. 89.5 for monthly evaporation rates). For an average value of 1,250 mm/year, one finds g(evap,i) = 0.396 10 m /m /s. 

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