Talk:Potential evapotranspiration

Three different types of evapotranspiration exist: Reference evapotranspiration (ET_ref), Potential evapotranspiration (ET_pot) and Actual evapotranpiration (ET_act). The reference evapotranspiration is (based on the definition of FAO publication 56; written in an attempt to standardize the evapotranspiration equations) defined as the evapotranspiration of a standard grass (with defined albedo, bulk surface resistance and aerodymanic resistance) with an unlimited supply of water. Since the evapotranspiration rates of ground surfaces with a soil cover different than grass can very from the standard grass cover, a potential evapotranspiration can be calculated for a specific soil cover. For most agricultural crops, the relation between grass evaporation (under unlimited water supply conditions) and the specific crop has been defined through the use of crop coefficients. For a cotton crop, at full development, the potential evapotranspiration is approximately 1.2 times higher than that for grass.

The step from potential evapotranspiration to actual evapotranspiration includes multiple stress factors, the main one ususally being the shortage of water supply (others like salinity stress, temperature stress, fertility shortage). A stress factor thus reduces the potential evapotranspiration of a crop to an actual evapotranspiration.

Reference evapotranspiration is commonly calculated using the climatic inputs for solar radiation, temperature, humidity and windspeed. Several relations exist between these parameters (among which the Blainy-Criddle, the Penman, the Penman-Monteith equations) and the reference evapotranspiration, and all of these relations are empirical. A more theoretical definition of the reference evapotranspiration was developed using the theoretical grass cover [equation published in FAO 56 {eq3?}].

The actual evapotranspiration of an area can be calculated based on a water balance (common approach) or an energy balance. Based on the energy balance, the potential evapotranspiration can also be calculated, thus providing an alternative method of obtaining a measure for the crop coefficient.

Rwos 11:33, 15 April 2007 (UTC)

Potential or reference evapotranspiration

Thus, if the above definitions of reference ET and potential ET can be agreed upon, then the current equations on the main page should be under the heading "reference ET". The definition in text is more or less correct, refering to the evaporation assuming a limitless supply of water. Rwos 11:37, 15 April 2007 (UTC)

External link

The definition of ET_pot in the external link (ag.arizona.edu) does not appear correct. It appears that they have calculated the reference evapotranspiration, since actual ground cover does not seem to be included in the definition.

Another world-wide calculated map of reference evapotranspiration (based on work of the University of Sussex) is also available on the IWMI website (http://www.iwmi.cgiar.org/WAtlas/atlas.htm)

Rwos 11:50, 15 April 2007 (UTC)

Thornthwaite Equation

All parameters of the equation should be defined in the article without leaving cryptic N's, L's and Ta's lying around. 99.253.108.67 (talk) 02:28, 8 January 2009 (UTC)