Potential ET (ETp)
Many methods have been developed for direct and indirect measurement of ET. The water loss (evaporative losses) from different surfaces such as turf, bare soil, and water was originally measured in large tanks (lysimeters). The term 'lysimeter' was derived from the Greek words 'lysis' and 'metron' meaning dissolving and measuring, respectively. The term is applicable to any device utilized that measures the rate, amount, and composition of percolation of water through soil. In a simple term, the lysimeter can be defined as large containers packed with soil located in the field to represent field and environmental conditions, with bare soil or vegetated surfaces (field crops, trees, shrubs, grass, etc.) for measuring the ET of plants or evaporation from bare soil through a mass-balance approach. Lysimeters are expensive and labor-intensive tools to measure evaporation or ET. Thus, other meteorological approaches have been developed over the years to simplify the measurements of ET. One of the commonly used methodologies to determine ET will be discussed later.
The original ET equation was based on evaporation from free water surface as measured with lysimeters. The definition of potential ET that emerged from an earlier work implied a maximum value of ET when there was adequate amount of water to be transpired or evaporated. Formally, potential ET has been defined as the evaporation from an extended surface of short green crop which fully shade the ground, exerts little or negligible resistance to the flow of water and is always supplied with water.'' Potential ET cannot exceed free water evaporation under the same weather conditions. However, in the definition of potential ET, the condition of nonlimiting supply of water is never achieved because the resistance of water flow through plants and soil has a finite value greater than zero. Another problem with this definition is the phrase 'short green crop'. The short green crop has been defined as 8-15 cm tall grass cover, but it has also been defined as a 30-50 cm tall crop of alfalfa. It is important to distinguish between the short green vegetations because the ET rates from well-watered agricultural crops may be as much as 1030% greater than that occurring from short green grass. This dichotomy in the definition of a 'short green crop' has led to the use of the term 'reference crop ET (ETref)'. To eliminate the confusion, in late 1970s and early 1980s, engineers and practitioners introduced and started using the 'reference ET' concept rather than 'potential ET'. The use of the term 'potential ET' is diminishing rapidly and the term 'reference ET' has been gaining significant acceptance by the water resources community.
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