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Applicability and Limitations of Evapotranspiration Models
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 lessparsimonious 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.