Alfalfa reference ET is defined as the ET rate from an extensive, uniform surface of dense, actively growing alfalfa, 0.30-0.50 m tall and not short of soil water. In the literature, the terms 'reference ET' and 'reference crop ET' have been used interchangeably and they both represent the same ET rate from a short, green alfalfa or grass surface. Unlike the potential ET definition, in the alfalfa reference ET definition, an alfalfa crop is specifically noted as the reference crop.
One of the other important differences between potential and reference ET is that the weather data collection site is well defined in the reference ET definition. It is important to note in the reference ET definition that the climate data that are used to estimate reference ET need to be collected in a well-watered and has certain characteristic (reference) environment. Therefore, based on the definition, the weather data for the reference ET estimations should be collected in a well-irrigated and well-maintained grass or alfalfa field. The irrigated grass area of the weather data collection site should be fairly large (e.g., at least 4 ha) to have enough fetch distance between the instrumentation to measure the climatic variables and the edge of the field because the quality of the weather data will ultimately affect the final estimated reference ET value. Enough fetch distance allows the air to travel on the reference crop surface and represent the aerodynamic, humidity, and temperature characteristics of the reference crop before it is sampled at the weather station. In a hot, dry month the average air temperature may be as much as 5-6 °C higher in a dryland (non-irrigated) area than for a nearby well-irrigated area. The differences in the air temperature will also affect the relative humidity and vapor pressure deficit values, and these differences will ultimately cause differences in the reference ET calculated using the weather data collected from the two sites (dry vs. well-irrigated).
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