The program uses a separate method to represent each component of the runoff process that is illustrated in Figure 2, including:

  • Methods that process precipitation, snow, and potential evapotranspiration meteorologic data.
  • Methods that compute infiltration and the resulting runoff volume.
  • Methods that represent direct runoff, including overland flow and interflow.
  • Methods for describing baseflow.
  • Methods for computing channel flow.

The methods that compute infiltration and the resulting runoff volume are listed in Table 3. These methods address questions about the volume of precipitation that falls on the watershed: How much infiltrates on pervious surfaces? How much runs off of the impervious surfaces? When does it run off?
The methods for surface runoff are listed in Table 4. These methods describe what happens as water that has not infiltrated or been stored on the watershed moves over or just beneath the watershed surface. Table 5 lists the models of baseflow. These simulate the slow subsurface drainage of water from the system into the channels.
The choices for modeling channel flow within the program are listed in Table 6. These so-called hydrologic routing models simulate one-dimensional open channel flow. An exception is the kinematic wave method which is a simplified hydraulic routing model.
Table 3.Loss methods for computing infiltration.

Model

Categorization

Deficit and constant

continuous, spatially averaged, conceptual, measured parameter

Exponential

event, spatially averaged, empirical, fitted parameter

Green and Ampt

event, spatially averaged, conceptual, measured parameter

Gridded deficit and constant

continuous, distributed, conceptual, measured parameter

Gridded SCS curve number

event, distributed, empirical, fitted parameter

Gridded SMA

continuous, distributed, empirical, fitted parameter

Initial and constant

event, spatially averaged, conceptual, fitted and measured parameter

SCS curve number

event, spatially averaged, empirical, fitted parameter

Smith Parlange

event, spatially averaged, conceptual, measured parameter

Soil moisture accounting (SMA)

continuous, spatially averaged, conceptual, fitted and measured parameter


Table 4. Transform methods for computing surface runoff.

Model

Categorization

User-specified unit hydrograph (UH)

event, spatially averaged, empirical, fitted parameter

User-specified s-graph

event, spatially averaged, empirical, fitted parameter

Clark's UH

event, spatially averaged, empirical, fitted parameter

Snyder's UH

event, spatially averaged, empirical, fitted and measured parameter

SCS UH

event, spatially averaged, empirical, fitted parameter

ModClark

event, distributed, empirical, fitted parameter

Kinematic wave

continuous, spatially averaged, conceptual, measured parameter


Table 5.Baseflow methods for computing subsurface flow.

Model

Categorization

Bounded recession

event, spatially averaged, empirical, fitted parameter

Constant monthly

continuous, spatially averaged, empirical, fitted parameter

Exponential recession

event, spatially averaged, empirical, fitted parameter

Linear reservoir

continuous, spatially averaged, empirical, fitted parameter

Nonlinear Boussinesq

event, spatially averaged, conceptual, measured parameter


Table 6.Routing methods for computing open channel flow..

Model

Categorization

Kinematic wave

continuous, spatially averaged, conceptual, measured parameter

Lag

continuous, spatially averaged, empirical, fitted parameter

Modified Puls

continuous, spatially averaged, empirical, fitted parameter

Muskingum

event, spatially averaged, empirical, fitted parameter

Muskingum-Cunge

continuous, spatially averaged, quasi-conceptual, measured parameter

Straddle-stagger

continuous, spatially averaged, empirical, fitted parameter

Confluence

continuous, conceptual, measured parameter

Bifurcation

continuous, conceptual, measured parameter

Reservoir

Continuous, empirical or conceptual, measured parameter