Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator.
Potential or reference evapotranspiration (PET or ETo) is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop (grass or alfalfa), completely shading the ground, of uniform height and with adequate water status in the soil profile. It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour from the ground up into the lower atmosphere. Evapotranspiration is said to equal potential evapotranspiration when there is ample water.
The reference evapotranspiration is calculated with the corrected Penmann-Monteith method described in FAO paper no 56. This feature is an implementation of that method.
As far as symbols are concerned, most of the data can be retrieved from meteorological data. There is a free collection of worldwide data provided from FAO, that will give an estimation of evapotranspiration. If you want more accurate results you will have to pay for a local and precise dataset from your local meteorological station.
In this program Ea is the actual vapour pressure. It can be calculated in two ways. The first and less precise is EaPoor. In this method you need Tmax (the daily maximum temperature in Celsius), Tmin (the daily minimum temperature in Celsius) and Rh which is the relative humidity. The second and more precise method of Ea calculation uses Tmax,Tmin, RhMax and RhMin.
When we calculate Ea in one of the two forms, we can use it to calculate ETo (mm /day). For this we need Tmax,Tmin,Ea, Air velocity, the number of the month with which we are working (normally we need the day of the month too, but since the free meteo-dataset of fao is not so accurate in this implementation we use the 15th day anyway), the degrees and minutes of the latitude, the average monthly temperature for the month we are working on (in C again), the same for the temperature of the previous month, the altitude and the hours of sun light.
After all that we can happily calculate our ETo.
Now if we have a specific crop in mind and if we have its Kc (crop coefficient) in hand for the appropriate growh period (Kc can be found in FAO web site again) we can insert it in the appropriate field and obtain the ET (mm /day) for our crop in the specific month and farm field.