Urban areas differ from rural areas in the fraction of total area that is impervious. Construction of buildings, parking lots and paved roads increases the impervious cover in a watershed and reduces infiltration. With development, the spatial flow pattern of water is altered and the hydraulic efficiency of flow is increased through artificial channels, curbing, and storm drainage and collection systems.

This fraction includes directly and indirectly connected impervious areas.

The name of the urban land type is a primary key referenced by urban in landuse.lum. All names in the urban.urb database must be unique.

The names in the urban database are also used by QSWAT+ to link the grid codes in land use/land cover maps to SWAT+ urban land types.

Impervious areas can be differentiated into two groups, the area that is hydraulically connected to the drainage system and the area that is not directly connected. As an example, assume there is a house surrounded by a yard where runoff from the roof flows into the yard where it infiltrates into the soil. The rooftop is impervious, but it is not hydraulically connected to the drainage system. In contrast, a parking lot whose runoff enters a storm water drain is hydraulically connected.

When modeling urban areas, the connectedness of the drainage system must be quantified. The best methods for determining the fraction total and directly connected impervious areas is to conduct a field survey or analyze aerial photographs.

Wash off is the process of erosion or solution of constituents from an impervious surface during a runoff event. The original default value for urb_wash was calculated as 0.18 mm-1 by assuming that 13 mm of total runoff in one hour would wash off 90% of the initial surface load (Huber and Heaney, 1982). Using sediment transport theory, Sonnen (1980) estimated values for the wash-off coefficient ranging from 0.002-0.26 mm-1. Huber and Dickinson (1988) noted that values between 0.039 and 0.390 mm-1 for the wash-off coefficient give sediment concentrations in the range of most observed values. This variable is used to calibrate the model to observed data.

References

Huber and Dickinson (1988)

Huber and Heaney (1982)

Sonnen (1980)

Curb length may be measured directly by scaling the total length of streets off of maps and multiplying by two. To calculate the density, the curb length is divided by the area represented by the map.

Number of days for amount of solids on impervious areas to build up from 0 kg/curb km to half the maximum amount of solids allowed (i.e. 0.5*).