The land in a watershed is divided into catchments. Each catchment drains to only one river segment or reservoir segment.
Catchments are divided into 1 to 5 vertical layers. The number of vertical layers may be chosen by the user. There may also be snow and detention storage of water on the surface of catchments.
Water percolates downward through each layer and moves laterally into the river segment or reservoir segment downstream. Water on impervious surfaces flows directly downstream without entering the soil layers. Water quality is simulated assuming that each soil layer (and the snowpack or detention storage, as applicable) is a mixed reactor. Equilibrium is achieved with adsorption sites in the soil, and soil minerals are weathered to introduce constituents to the subsurface water.
To edit coefficients for a catchment, double-click on that catchment on the Map when in Input Mode. To edit coefficients for several catchments at once, select the catchments and then double-click on one of them with the Shift key held down. The following dialog box will appear:
The name of the catchment is shown in the caption of the dialog box (above, a single catchment named Lower Linville Valley is selected). The image in the lower left corner is a copy of the map showing the catchment(s) being edited. When each tab is selected, a new group of coefficients appears.