Behzad,
You might look into a couple of models.
EPIC and APEX models have provisions for estimating PET using any of
five methods including Hargreaves and Samani (1985), Penman (1948),
Priestley-Taylor (1972),Penman-Monteith (Monteith, 1965), or Baier-
Robertson (1965). Selection is often based on how much and what types
of data you have.
From the EPIC documentation:
"The Penman and Penman-Monteith methods require solar radiation, air
temperature, wind speed, and relative humidity as input. If wind
speed,relative humidity, and solar radiation data are not available,
the Hargreaves or Priestley-Taylor methods provide options that give
realistic results in most cases. The Baier-Robertson method
developed in Canada performs well in cold climates.

The model computes evaporation from soils and plants separately, as
described by Ritchie (1972). Potential soil water evaporation is
estimated as a function of potential evaporation and leaf area index
(LAI, area of plant leaves relative to the soil surface area).
Actual soil water evaporation is estimated by using exponential
functions of soil depth and water content. Plant water evaporation
is simulated as a linear function of potential evaporation and leaf
area index."
Here's a link:
http://www.brc.tamus.edu/epic/


The SWAT model incorporates three PET estimation methods: the Penman-
Monteith, Priestley-Taylor, and Hargreaves. Think the developers have
been doing quite of work with trees and forests recently, although
I'm not sure if the work was related to ET. Here's a link:
http://www.brc.tamus.edu/swat/swattheo.html

Good luck,
Steve


--- In hydrologymodel@..., behzad@u... wrote:
> Dear friends,
> I am looking for (mathematical) models for evapotranspiration rate.
> A literature review regarding evapotranspiration methods also is
> useful.
> In reality, I need to develop one model for evapotranspiration rate
> of trees which should be related to soil suction and properties.
> Would you please if you know any useful references let me know?
> Regards,
> Behzad