Dear Asmadi,

The evapotranspiration (ET) is the amount of water transferred to the atmosphere by evaporation from the soil and plant transpiration: Evapotranspiration = Evaporation + transpiration.

With my best regards

Prof. Bachir ACHOUR

I suggest to define the footprint of your study and to get infos about soil coverage by season and free surface activities, climatic variation and surface hydrology. A broad reference can be http://www.fao.org/docrep/x0490e/x0490e04.htm. Whenever there is vegetation micromets measures E and T.

You will either have to model bare soil only (in which case evapotranspiration equals evaporation), or consider root water uptake by plants. Root water uptake by plants is limited either by (1) the available water in the root zone, (2) weather variables (more wind and more sun= more transpiration), (3) the size and type of plant.

Search for "evapotranspiration partitioning" to see how people usually go about this.

See these two "old" but interesting contributions

Ritchie J.T. (1972) Model for predicting evaporation from a row

crop with incomplete cover. Water Resources Research 8,

1204-1213.

Ritchie J.T., Rhoades E.D. & Richardson C.W. (1976) Calculating

evaporation from native grassland watersheds. Transactions

of the American Society of Agricultural Engineers 19,1098-1103

Dear Asmadi:

Your question might be either simple or complex to answer. First of all, we have to keep in mind that most people wish to convert evaporation to evapotranspiration, rather than the opposite.

Assuming the easiest answer, if you know the reference evapotranspiration (ETo), it is sufficient to divide by the pan coefficient (Kp). This coefficient may vary between 0.55 and 0.80 and can be estimated by meteorological data according to Doorenbos and Pruitt (1977) (FAO paper 24).

In our area, we have estimated Kp as 0.79 using lysimeter data (see attached file).

A complex answer involves additional information: What kind of ET you have? (Reference? Crop? Real?). What prevents you to directly measure pan evaporation instead of estimating?

Actually I got the evapotranspiration data from climate change model. Since my rainfall-runoff model used pan evaporation data ,I think  it will not appropriate to used future Evapotranspiration data directly into the model. i need to find a way to make some adjustment to the ET data so it can be used in RR model.

I appreciate all the answer for my question in this forum

TQ in advance.

Now, I see your point. Well, there are at least three probable approaches:

1) Since you have reference ET, so just divide by Kp found in the Paper 24 of FAO

2) Try to find evaporation data from one of the existing stations. Maybe there is one nearby your location.

3) Collect your own data by installing a pan or similar. Of course, you need to gather enough readings

Dear

Three methods:

1- By mulching soil surface. 2. by using drip irrigation methods (subsurface drip irrigation. 3- time of irrigation (at night)

Based on your question, I assume you have estimated the ET through an equation that uses climatic data (Penman-Monteith, Hargreaves). If you did it, then I believe that there is no equation that can convert this value into evaporation. I recommend that you try to estimate only the evaporation by other methods. 

You can estimate evaporation simply by PAN A evaporo-meter or you can use some mathematical models (refer FAO procedure). transpiration component is directly link with vegetation type, growth stage and climatic condition et al. it can determined by direct method or indirect method. now a day indirect methods have been developed up to sufficient level of accuracy. you can use digital probes like stomatal conductivity testing, sap flow estimating like that. if you don't want to such a accurate data, roughly it can be calculated using crop factors. better to read FAO 56 irrigation and drainage materials.