The following table contains a list of various advantages and disadvantages regarding the aforementioned evapotranspiration methods available for use within HEC-HMS.  However, these are only guidelines and should be supplemented by knowledge of, and experience with, the methods and the watershed in question.

Method

Advantages

Disadvantages

Hamon
  • Simple, parsimonious method.
  • Mean air temperature is the only required meteorologic input.
  • Based on an empirical relationship between air temperature and net radiation.
Hargreaves
  • Simple, parsimonious method.
  • Mean air temperature is the only required meteorologic input.
  • Based on an empirical relationship between air temperature and net radiation.
Priestley Taylor

Penman Monteith
  • Energy balance with mass transfer method.
  • Widely used and documented through United Nations Food and Agricultural Organization (FAO).
  • Method is less parsimonious than simpler ET methods; it requires many more meteorologic boundary conditions.
Monthly Average
  • Simple, parsimonious method.
  • Pan evaporation data is widely available.
  • Differences in the water and cropped surface can produce significant differences in the water loss from an open water surface and the crop.
  • Empirical coefficients used to relate evapotranspiration to pan evaporation.