The United Nations Food & Agriculture Organization (FAO) standard methods for modeling Evapotranspiration (ET) use a Penman-Monteith equation. The ASCE standard methods modify that Penman-Monteith equation for use with an hourly time step. The SWAT model is one of many GIS integrated hydrologic models estimating ET using Penman-Monteith equations.
Evapotranspiration contributions are very significant in a watershed's water balance, yet are often not emphasized in results because the precision of this component is often weak relative to more directly measured phenomena, eg. rain & stream flow. In addition to weather uncertainties, the Penman-Monteith equation is sensitive to vegetation specific parameters, eg. stomatal resistance or conductance. Gaps in knowledge of such are filled by educated assumptions, until more specific data accumulates.
Various forms of crop coefficients (Kc) account for differences between specific vegetation modeled and a Reference Evapotranspiration (RET or ET0) standard. Stress coefficients (Ks) account for reductions in ET due to environmental stress (eg. Soil saturation reduces root zone O2, low soil moisture induces wilt, air pollution effects, and salinity). Models of native vegetation cannot assume crop management to avoid recurring stress.
Note: Often resistances are used rather than conductivities.
Also note that varies over each day, and in response to conditions as plants adjust such traits as stoma openings. Being sensitive to this parameter value, the Penman-Monteith equation obviates the need for more more rigorous treatment of perhaps varying within each day. Penman's equation was derived to estimate daily ET from daily averages.
A derivation of this equation may be found at http://biomet.ucdavis.edu/evapotranspiration/PMDerivation/PMD.htm
This also explains relations used to obtain & in addition to assumptions key to reaching this simplified equation.