it is simple...ETc=Kc.ET0; where ETc is actual evapotranspiration, Kc is crop coefficient and ET0 is Referece evapotranspiration.
You might get Kc values based on the specified crop as well as its stage of growth. Some times, one might apply single or dual crop coefficient. For more information you may see "Allen et al., 1998. Guidelines for ...." which is Irrigation and Drainage paper, no 56 of FAO. Just go to WWW.FAO.ORG and search for irrigation and drinage papers...There, there are lots of papers dealing with evapotranspiration subjct....also you can find Paper 56 (Allen et al., 1998)...
Nevertheless, I would apply "Reference Evapotranspiration" insted of "potential evapotranspiration"...Having Reference Evapotranspiration (ET0) you will simply use the mentioned equation to calculate actual evpotranspiration.
Keep me updated if these information are sufficient and clear.
May be you are asking about the relationship between ETm and ETa, not ETo and ETa.
If that is the case, then in the same publication referenced by Jalal Shiri, you can find a way to relate ETa with ETm and the soil water content (a required data). Another parameter affecting to this calculation is the allowable depletion level, in other words, the soil water threshold that determines the moment when the crop is affected by water deficit (ETa
@ HAMIMED Abderrahmane , @ U. Sahin @Alfonso Domínguez ·@ Jalal Shiri I have a question If I calculate
- Evaporation = Precipitation - Runoff
- Potential Evaporation by FAO method
than what will be relation between both?
I mean Evaporation will be smaller than Evapotranspiration? and can i have PET< ET ?
@ HAMIMED Abderrahmane , @ U. Sahin @Alfonso Domínguez ·@ Jalal Shiri I have a question If I calculate
- Evaporation = Precipitation - Runoff
- Potential Evaporation by FAO method
than what will be relation between both?
I mean Evaporation will be smaller than Evapotranspiration? and can i have PET< ET ?
The potential ET is the amount that the plant+soil will loose if the amount of moisture is present there ... but the actual ET is always less as there are limiting factors such as moisture condition and plant characteristics ... Using reference Et value and crop coefficients could be one of the best way to estimate actual Et however it doesnot take account of soil moisture.
this links may gives you some hint
http://www.sciencedirect.com/science/article/pii/S1687850715000321
http://www.hydrol-earth-syst-sci.net/17/1331/2013/hess-17-1331-2013.pdf
Dear Bibhuti
I recommend, like others, the method proposed in FAO56. This is based on the Kc culture coefficient,
ETa = Kc * ETr
where ETa is the actual evapotranspiration and ETr is the reference evapotranspiration calculated by Penman-Monteith or Hargreaves Samani (FAO56).
Kc is estimated from the NDVI, you should check articles on the web to find the ones you need. I recommend you check the articles by Isidro Campos of the University of Castilla La Mancha in Spain. He has worked on simple linear statistical relationships to estimate the Kc from the NDVI for several crops. It is an approximate method, but for some crops it gives reasonable values.
Hello Dear Bibhuti Bhusan Sahoo
Actual evapotranspiration is usually estimated using different methods.The Penman–Monteith method (Allen et al.1998), considering aerodynamic resistance and surface resistance, has been successfully used to calculate actual evapotranspiration from different land covers. Hydrological models estimate actual evapotranspiration on a basin scale. In recent years, remote sensing data has been gradually used to estimate actual evapotranspiration.
References
Allen RG, Pereira LS, Raes D, Smith M (1998) Crop evapotranspi-ration—guidelines for computing crop water requirements— FAO Irrigation & Drainage Paper 56, FAO. ISBN 92-5-104219-5
Best regards
Fatah BOUTELDJAOUI
it is simple... ETc=Kc. ET0; where ETc is actual evapotranspiration, Kc is crop coefficient and ET0 is Reference evapotranspiration. You might get Kc values based on the specified crop as well as its stage of growth.
Actual ET is something different from Actual ET which is soil and plant dependent parameters of soil moisture and physiological parameters of plant. Crop ET is estimated from PET and Kc values which vary from climate to climate and plants. basically for AET, we need to monitor soil moisture at regular intervals through out crop growth period.
AET can be estimated through several methods, there are traditional methods which are based on weather station data, and remotely sensed methods which needs a lot of data and calculations. Some local scale methods are Thornthwaite-Mether, Grindley's Model, and by Crop ET (FAO56) method. Surface energy balance methods are SEBAL, METRIC and several others.
There are several empirical and process based models for estimating AET.
Evapotranspiration is an aggregate of the two processes, Transpiration and evaporation from the plant and surface respectively. To estimate AET different methods will be used based on the objective of the investigations. The main question is how to calculate Evaporation and transpiration separately.