For potential transpiration, canopy resistance depends on maximum leaf conductance, reduced for humidity, temperature, and light penetration. Soil evaporation resistance depends on soil water potential in the top soil layer. Actual transpiration is the lesser of potential transpiration and a soil water supply rate determined by the resistance to liquid water flow in the plants and on root distribution and soil water potential in the soil layers. Snowmelt is based on a degree day factor and accounts for snowpack temperature and liquid water content. The relationships among matric potential, soil water content, and hydraulic conductivity are parameterized by a modified Clapp-Hornberger formulation with values given at field capacity.
Water is added to groundwater by gravity drainage from the deepest soil layer.
A fixed fraction of the groundwater outflow may be deep seepage. This can be compared with measured streamflow if that is available. The standard version is designed to run on any machine that has a Fortran-77 compiler. Compilation is necessary. The program may be freely modified.