Potential ET

The concept and definition of potential ET (PET) is not universal among hydrologists and other scientists. Most recognize that the maximum, or potential, ET largely depends upon the energy available for the liquid-to-vapor phase change, and that this energy source is primarily solar radiation. But in addition, the time-spatial distribution of this energy, the advection or energy transfers, and local reception characteristics such as slope, aspect, albedo, surface roughness, thermal and psychometric properties, and biological flora and fauna all modify this basic input. Therefore, methods based on radiation or radiation plus air properties have been the most widely used for short-term estimates. These methods, such as Penman, Jensen-Haise, and Kanemasu, all define PET from atmospheric conditions with little surface influence. Pan evaporation is an indirect, integrated method of estimating PET.

Another common method to define PET used primarily for irrigation-related estimates isas that water lost from a well-watered reference crop such as grass or alfalfa. These measured values are then related to other crops with different growth stages and water requirements. While quite practical, this method incorporates both meteorological and reference crop variables and generally requires some local calibration. Summaries and references to a variety of ET methods are given by Saxton and McGuinness (1982), Saxton (1971), Saxton (1981) and Jensen,(1973).

PET for the SPAW model may be obtained from one of the several meteorological methods. It is currently written for either daily pan evaporation modified by a monthly adjusted pan-to-PET coefficient or values calculated by meteorological measurements using the Penman equation. Other meteorological methods are equally applicable and can be accommodated with a variety of coefficients with either daily or monthly values. However, there may need to be some local calibration if the method contains surface or site conditions unique to the method or measurements.


Field Hydrologic Processes | Precipitation | Infiltration
Interception | Soil Water Evaporation | Plant Transpiration | Crop Water Stress
Root Water Uptake | ActualET | Soil Water Redistribution | Irrigation