Info Sheets

Potential Evapotranspiration (PET or ETo)

Potential Evapotranspiration is widely used as a way to manage water use. Because PET is an estimation of water loss, it is important to understand how it is calculated and how it might relate to actual plant stress.

As the name suggests, PET is the potential, not the actual evapotranspiration from the surface of a plant. It is calculated by one of a few complex algorithms from standard weather variables.One of the most common of these algorithms is the Penman-Monteith equation (at right).


1. What is evapotranspiration?

Evapotranspiration is the combined total of water loss from a plant through surface evaporation and transpiration. It is normally measured in mm.

(Some would argue that that it is inaccurate to see evapotranspiration as different from evaporation from any surface whether it be plant, animal or mineral, however we will leave that to the scientists to discuss among themselves.)


2. Why do we not measure evapotranspiration directly?

A direct measurement of a variable is always better than an estimation. Unfortunately, no one has really found a way to measure evapotranspiration accurately in an economically feasible way. That is why we estimate it.


3. What variables are used to calculate PET?

The Penman-Monteith algorithm requires input of the following measurements:

  • Air Temperature
  • Relative Humidity
  • Wind Speed
  • Solar Radiation

These can all be measured relatively economically, and then the readings put into the algorithm to calculate PET. This is automatically done by our datalogging systems.


4. What are the limitations of calculated PET?

As with any model, a calculation of PET is likely to vary, at least a little, from the real world. The kind of things that may cause variations are the height of measurement, the type of crop being grown, the growth stage of the crop and the "roughness" of the terrain. There are ways to account for all of the above factors,  to get more accurate estimates however, for most normal farming operations, this amount of detail is not required.

Probably more important than the assumptions above, to the accuracy of the calculation, is the quality of the equipment being used to measure temperature, humidity, wind speed and radiation. If any of these parameters are inaccurate due to faults or poor maintenance, the estimation of PET will be inaccurate. For this reason, we recommend keeping your weather station in good condition, with regular maintenance.



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