Hi Jen,
You can use daily maximum temperature as a substitute for Epot. All hydromad models include a parameter "e". If you use Epot this parameter should be 1 (or close to 1), if you use maxT your estimate would be 0.15 (but it can also be a calibratable parameter). I have so far mostly used daily maximum Temperature.
I think the original publication is this one:
Dye, P.J., Croke, B.F.W., 2003. Evaluation of streamflow predictions by the IHACRES rainfall-runoff model in two South African catchments. Environmental Modelling & Software, 18(8-9): 705-712.
But it is also mentioned in this paper:
Croke, B.F.W., Jakeman, A.J., 2004. A catchment moisture deficit module for the IHACRES rainfall-runoff model. Environmental Modelling & Software, 19(1): 1-5.
And I used it similarly in this paper
Vervoort, R.W., Miechels, S.F., van Ogtrop, F.F., Guillaume, J.H.A., 2014. Remotely sensed evapotranspiration to calibrate a lumped conceptual model: Pitfalls and opportunities. Journal of Hydrology, 519, Part D(0): 3223-3236. DOI:
http://dx.doi.org/10.1016/j.jhydrol.2014.10.034
Alternatively you could use a Hargreaves equation or other ET potential equation to calculate ETpot separately.
Willem