I m using Hec-Hms Snowmelt module based on temperature index. I would like to know what is the exact formulation used by Hec-Hms,? does heat brought by rain is included? if yes how?
You need to see this USACE document from 2000 (it has the underlying theory of the model):
HEC-HMS Technical Reference Manual (CPD-74B) Discussion of the technical underpinnings of the program including theoretical basis, parameter estimation techniques, and application suggestions.