For precipitation-runoff-routing simulation, the program provides the following components:
Precipitation methods which can describe an observed (historical) precipitation event, a frequency-based hypothetical precipitation event, or an event that represents the upper limit of precipitation possible at a given location.
Snow melt methods which can partition precipitation into rainfall and snowfall and then account for accumulation and melt of the snowpack. When a snow method is not used, all precipitation is assumed to be rain.
Evapotranspiration methods which are used in continuous simulation for computing the amount of infiltrated soil water that is removed back to the atmosphere through evaporation and plant use.
Loss methods which can estimate the amount of precipitation that infiltrates from the land surface into the soil. By implication, the precipitation that does not infiltrate becomes surface runoff.
Direct runoff methods that describe overland flow, storage, and energy losses as water runs off a watershed and into the stream channels. These are generally called transform methods because the "transform" uninfiltrated precipitation into watershed outflow.
Baseflow methods that estimate the amount of infiltrated water returning to the channel. Some of the included methods conserve mass through the infiltration process to baseflow; others do not have the same conserving properties.
Hydrologic routing methods that account for storage and energy flux as water moves through stream channels.
Models of naturally occurring confluences (junctions) and bifurcations (diversions).
Models of water-control measures, including diversions and reservoirs.
These methods are similar to those options included in HEC-1. Significant methods not included in HEC-1 include:
A distributed transform model for use with distributed precipitation data, such as the data available from weather radar.
A simple one-layer and more complex five-layer soil-moisture-accounting model for use in continuous simulation. They can be used to simulate the long-term response of a watershed to wetting and drying.
The program also includes a number of tools to help process parameter data and computed results, including:
An automatic calibration tool that can be used to estimate parameter values and initial conditions for most methods, given observations of hydrometeorological conditions.
An analysis tool to assist in developing frequency curves throughout a watershed on the basis of storms with an associated exceedance probability.
Links to a database management system that permits data storage, retrieval and connectivity with other analysis tools available from HEC and other sources is also included.