Basic Concepts and Equations

Priestley and Taylor (1972) developed a simplified energy balance approach to modeling evapotranspiration. Data from multiple sites, each with saturated surface or open water conditions and without advection, were used to develop a widely applicable formula for the relationship between sensible and latent heat fluxes. For partially saturated land surfaces, the reference evapotranspiration is related to the saturated evaportranspiration rate by a coefficient:

$//<![CDATA[ \begin{array}{l}ET_o = \alpha (R - G) \frac {\Delta}{\Delta + \gamma}\end{array} //]]>$

where $//<![CDATA[ \begin{array}{l}\alpha\end{array} //]]>$ is the dryness coefficient, R is the net incoming radiation, G is the heat flux into the ground (R - G = LE + H) where H is sensible heat and LE is latent heat, $//<![CDATA[ \begin{array}{l}\Delta\end{array} //]]>$ is the slope of the saturation vapour pressure curve, and $//<![CDATA[ \begin{array}{l}\gamma\end{array} //]]>$ is the psychrometric constant.

Required Parameters

The only parameter required to utilize this method within HEC-HMS is the dryness coefficient. In addition, air temperature and net radiation must be specified as a meteorologic boundary condition. Net radiation should be computed, entered in the program as a radiation time-series gage, and selected as the shortwave radiation method. 

A Note on Parameter Estimation

While HEC-HMS provides a default coefficient value of 1.26, this value must be calibrated and validated. The default value corresponds to a saturated surface, or wet conditions. Specifically, lower values of the dryness coefficient should be used in humid regions while higher values should be used in arid regions.