I had the same problem with HEC-HMS while analyzing tile flow-runoff-precipitation relations in small project of hydrograph seperation. First of all, I did not have a full licanced copy of the program, but an available copy in the program site's of web. Basically what I did was to correct physically measured ETs with the actual precipitatiıns by a automated weather station in the research field and integrated to the HEC-HMS.
You can find useful tips on the HEC-HMS v4.1 user manual (page #287, etc.):
http://www.hec.usace.army.mil/software/hec-hms/documentation/HEC-HMS_Users_Manual_4.1.pdf
As Alireza Araghi said, the HEC-HMS User's Manual is a good resource. There are a number of different evapotranspiration methods available in HEC-HMS. These are added in the meteorologic model. You'll also need to add a canopy method in your basin model. The canopy extracts moisture from the soil as a function of the evapotranspiraton rate.
Just look into the manual of the HEC-HMS and search Evapotranspiration and that is it. For working with the HEC-HMS, nothing is better than its manual. However, when using the ET methods be careful of using correct empirical coefficients. For instance, the Priestley-Taylor method, the default value is 1.26 that is good for wet conditions. When using the model in dry regions be aware that it could be even higher than 2.5 or 3 depending on the season. Do not be afraid to try with values close to 2. Remember that, HEC-HMS is all about correct and physically-based parameterization of the catchment. I have seen some works that the parameters were not physically sound and were out of range.
For Evapo-Transpiration you can select Simple canopy then you need to fill the canopy activated boxes with related with initial and max storage.
instead of user manual I recommend technical reference manual (2000) so you can easy go through all HEC-hms steps.
My Question is that all user manual and technical manual not explained the (WHY) for different methods because I bit you faced many problems and multi options to which you should go or select and why. for instance, Loss method which method should be selected and why for urban area around 80% impervious ???
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