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Water is one of the most important limited natural resources. Declining water resources and water quality problems have resulted in dramatic increase in the need for water-conserving methodologies on a field, watershed, and regional scale and this makes efficient use of freshwater resources an obligation of each user. During the 30-year period from 1950 to 1980, the actual level of per capita water supply decreased significantly in many countries due to population increases. It has been projected that in early year 2000 considerably low water availability per capita is anticipated in many regions of the world. As water becomes increasingly scarce and the need becomes more pressing, newer and more complete methods of measuring and evaluating techniques of handling water resources are necessary. In terms of agricultural production, approximately 17% of the cropped area of the world is irrigated and contributes more than one-third of the total world food production. In the United States, about 12% of the cropped area is irrigated and contributes about 25% of the total value of the United States crops. In the United States and around the world, irrigated agriculture uses most of the water withdrawals from the surface and groundwater supplies. Thus, accurate quantification of plant water use (evapotranspiration) is crucial for better management and allocation of water resources.

The process known as evapotranspiration (ET) is of great importance in many disciplines. Accurate quantification of ET in agroecosystems is critical for better planning, managing, and efficient use of water resources, especially in arid or semiarid environments where lack of precipitation usually limits plant growth and yield and negatively affects ecological balances. Quantification of ET is also crucial in water allocation, irrigation management, evaluating the effects of changing land use on water yield, environmental assessment, and development of best management practices to protect surface and groundwater quality.

ET can be defined as the loss of water from the ground, lake or pond, and vegetative surfaces to the atmosphere through vaporization of liquid water. In agroecosystems, ET is the sum of two terms: (1) transpiration, which is water entering plant roots and used to build plant tissue or being passed through leaves of the plant into the atmosphere in the vapor form, and (2) evaporation which is water evaporating from soil and water surfaces, or from the surfaces of plant leaves. Evaporation from buildings, streets, parking lots, etc., after a rain event also contributes to the total ET in the hydrologic cycle.

Evaporation and transpiration processes occur simultaneously and there is no easy method to separate these two processes. Evaporation in the field can take place from crop canopies, from the soil surface, or from a free water surface. When the soil surface is bare, evaporation will take place from the soil directly. In the absence of vegetation, and when the soil surface is subject to radiation and wind effects, evaporation can result in considerable loss of water in both irrigated and nonirri-gated agriculture, and other ecological landscapes. In the semiarid and arid western regions of the United Sates, evaporation can be as high as 40% of the total ET.

Transpiration increases with increasing leaf area until complete closure of the canopy occurs. For agricultural crops such maximum transpiration is usually attained at a leaf area index (LAI) of about 3-3.5. In the transpiration process, stomata opening and closure depends on water uptake rate which in turn depends on the density and distribution of roots and their effectiveness to uptake water and nutrients from the soil. Stomata would close when roots cannot uptake water from soil with sufficient rate to keep up with the transpiration. In irrigated agro-ecosystems, the goal should be decreasing the evaporation component of the total ET for optimum crop production because yield and transpiration are strongly related and evaporation does not have any contribution to the crop growth and yield. Thus, the evaporation falls into the 'unbeneficial water use' category.

The ET rate and amount for different vegetation surfaces (i.e., agronomical crops, which are mostly 'annual crops' vs. trees and shrubs, which are mostly 'perennials') show significant variation from one location to another and are strong functions of climatic, soil conditions, and management practices. For example, the seasonal crop ET for corn (maize, Zea mays) can range from 500 to 800 mm depending on climate. The ET for typical alfalfa (Medicago sativa) plant can range from 800 to 1600 mm per growing period depending on climate and length ofgrow-ing period. For a tropical plant such as banana (Musa spp.), this value is between 1200 mm in the humid tropics and 2200 mm in the dry tropics. Water requirement of trees can also show wide variations depending on climate, soil type, and root structures. For example, an orange tree (Citrus aurantium) can use as much as between 900 and 1200 mm of water per year whereas olive tree (Olea euro-pacal) can use only between 400 and 600 mm of water per year. The expected water use of natural vegetations can also show significant variation. For example, the seasonal water use of cattail (Typha) can range from only 890 mm to as much as 2500 mm. Water use for foxtail (Lycopodium clavatum) is about 140 mm and for pine tree (Pinus) water use can range from 480 to 1190 mm. Water use of different natural vegetation and agronomical plants are important and necessary for accurate determination of hydrologic balance components.

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