The Precipitation-Runoff Modeling System is a deterministic, distributed-parameter, physical process based modeling system used to evaluate the response of various combinations of climate and land use on streamflow and general watershed hydrology. Response to normal and extreme rainfall and snowmelt can be simulated to evaluate changes in water-balance relations, streamflow regimes, soil-moisture relations, and groundwater recharge. Each hydrologic process is represented within PRMS by an algorithm that is based on a physical law or an empirical relation with measured or estimated characteristics.
Distributed-parameter capabilities are provided by partitioning a watershed into hydrologic response units (HRUs) that are based on the physical attributes of the watershed such as land-surface elevation, slope and aspect, vegetation type, soil type, and spatio-temporal climate patterns. The physical attributes and hydrologic response of each HRU are assumed to be homogeneous. A water balance and an energy balance are computed daily for each HRU. The sum of the responses of all HRUs, weighted on a unit-area basis, produces the daily watershed response in the most basic configuration. In addition, PRMS can provide more sophisticated methods of internal routing that are available for more complex modeling applications.