Concepts and empirical relations are taken directly from the pervious land segment water balance module of the Hydrological Simulation Program - FORTRAN (PERLND PWATER HSPF). For more information see the HSPF Version 12 User's Manual, pages 49 to 69. Variable naming is lengthened for more clarity, but if you are more familiar with HSPF variable names, you can see those by hovering your mouse over any element.
The model is designed for predicting discharge from ungaged pervious catchments. Modeled discharge is the sum of surface runoff, interflow, and active groundwater outflow, which can also be viewed separately. The model is typically run with time steps of an hour or less for better resolution of infiltration and overland flow processes.
This model is currently written for deterministic modeling of runoff as a function of fixed parameters and rainfall and pan evaporation rates. It could easily be extended to include snowmelt or time variable or stochastic parameters. It can also be cut and pasted into a localized container in another model to simulate a catchment within a larger system.