Evapotranspiration potential

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. 

An aridity definition that takes account of the relation precipitation/potential evaporation is the Aridity Index used in the Atlas of Desertification (UNEP, 1993). Four different degrees of aridity can be recognized. These are best defined using the Aridity Index (AI), calculated as the ratio P/PET, where P stands for precipitation and PET for (potential) evapotranspiration. AI values of <1.0 indicate an average annual moisture deficit. According to this criterion, four subzones can be discussed ... 

The annual mean water percolation Qie,zb is determined by the long-term mean annual temperature (mainly determining the potential evapotranspiration, Epol) and precipitation (mainly influencing the actual evapotranspiration, act) according to... 

Fig. II. Mean values during weekly intervals of the rate of ammonia loss from a grazed ryegrass sward receiving 420 kg N ha 1yr 1 and the corresponding values for accumulated rainfall and potential evapotranspiration. The swards were grazed by yearling steers within a 28-day rotation. The numbered bars indicate each grazing period and the actual stocking rate (steers per ha) during that period.

Evaporation is influenced not only by the amount of energy available but also by the amount of water available. If unlimited volumes of water are available in the soil and on surface areas, this is referred to as potential evapotranspiration, i.e. the highest possible evapotranspiration under given climatic conditions. The actual evapotranspiration is the evapotranspiration which can effectively be observed. This is always lower than potential evapotranspiration and is dependent on water availability, plant and surface attributes, net radiation, air humidity, and wind speed. In mountains with a great deal of barren soil and large areas of debris and rock cover which are unable to store water in any great quantities, and with fastflowing water over steep terrain, actual evapotranspiration is very often limited and hence much lower than potential evapotranspiration. 

If is frequently necessary to calculate evapotranspiration on the basis of climate and land use data. On the basis of the calculation of potential evapotranspiration, various procedures have been proposed with the aim of calculating actual evapotranspiration. There are a number of methods [68, 69]. 

In Austria, which is significantly lower lying (cf. Table 3), the evapotranspiration amounts to 510 mm a . On average the actual evapotranspiration is 90% of the potential evapotranspiration for the whole of Austria [70]. 

In almost all regions, evapotranspiration essentially depends on temperature. Because of abundant precipitation, most of the time it corresponds to almost the same values as potential evapotranspiration. Indeed, a significant positive trend in all but the Inn basin can be observed. 

Shiau S.V. and Davar K.S., Modified Penman method for potential evapotranspiration from forest regions. Journal of Hydrology , 18, 349-365 (1973). 

Mean daily temperature of the study region (Fig. IB) ranges from —15°C in January to +18°C in July. Potential evapotranspiration of 530mm/yr. greatly exceeds the mean annual precipitation of 373 mm/yr. These data are from a compilation of hydrogeologic data for the area by Hackbarth (1975). The area therefore has a cold semi-arid continental climate. 

Since for the majority of ecosystems in North-Eastern Asia the ratio of precipitation to potential evapotranspiration (P PE) is equal to 1.00 or slightly exceeds 1.00 (except of ecosystems with some cambisols, histosols and andosols) the values of runoff can be neglected in calculation of critical loads of acidity, CL(Ac), and they were found as... 

The water recharge for the Maui fields was calculated with a simple hydrologic balance for each field using rainfall data from nearby rain gauges. Both fields were non-irrigated. A rainfall-runoff relationship was developed for each field using the USDA-Soil Conservation Service "curve number" approach. The validity of this approach for Hawaii conditions was demonstrated by Cooley and Lane (20). Potential evapotranspiration from pineapple fields was assumed to be one-third of pan evaporation throughout the study period, based on results of Ekern (21). but actual ET was adjusted for the water input for each month (3 ). 

Since for the majority of ecosystems in North-Eastern Asia the ratio of precipitation to potential evapotranspiration (P PE) is equal to < 1.00 or slightly exceeds the... 

The CENTURY model also includes a simplified water budget model that calculates monthly evaporation and transpiration water loss, water content of the soil layers, snow water content, and water flows between the soil layers. The main abiotic driver for the water budget model is the potential evapotranspiration rate, which is calculated as a function of the average monthly maximum and minimum air temperature. Near surface soil temperature is calculated as a function of maximum and minimum air temperatures, litter and standing plant biomass. The plant production model calculates potential plant production as a function of soil temperature and the ratio of actual water loss to potential water loss. Potential plant production is reduced when nutrients limit growth and the element that is most limiting for plant growth controls plant production. The nutrient... 

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. 

Soil drainage, dependent on soil texture, is a factor which affects the rate and degree of solute (e.g., nitrate) leaching. Soil moisture affects nitrate leaching rates. Diy soils leach little nitrate compared to wetter soils. The precipitation to potential evapotranspiration ratio (P E ratio) is used is an indicator of soil moisture conditions ... 

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