Quoted from: https://hydrologicmodels.tamu.edu/regional-hydroclimate-model-reghcm/hydrology/ 

      RegHCM is a physically-based, distributed, regional scale, fully-coupled atmospheric-hydrologic model.  The model is used to simulate coupled atmospheric-land hydrologic processes in time and space continuously at climatic scales (10s – 100s of years) for the quantitative assessment of water balances under changing climatic conditions and land surface conditions over continental, country and watershed domains in terms of precipitation, air temperature, snowmelt, evapotranspiration, soil water storage, and streamflow. RegHCM is made up of the atmospheric component of the nonhydrostatic 5th Generation Mesoscale Model of Penn State-NCAR (MM5) that is coupled in a two-way-interaction to a physically-based regional-scale land hydrology model. The regional hydrology model component is based upon upscaled hydrologic conservation equations that are scalable with the spatial grid scales.

      Mathematical Basis: RegHCM is made up of the atmospheric component of the nonhydrostatic 5th Generation Mesoscale Model of Penn State-NCAR (MM5) that is coupled in a two-way-interaction to a physically-based regional-scale land hydrology model. The regional hydrology model component is based upon upscaled hydrologic conservation equations that are scalable with the spatial grid scales.