Use of impervious surface data obtained from remote sensing in distributed hydrological modeling of urban areas - Urban Remote Sensing: Monitoring, Synthesis and Modeling in the Urban Environment - page 246

Impervious
evaporation

Transpiration
Interception

Precipitation

Bare-soil
evaporation
Open water
evaporation

Impervious
evaporation

Impervious
recharge

Vegetated
recharge

Bare-soil
recharge

Total Evapotranspiration

Impervious
runoff

Transpiration

Precipitation

Bare-soil

runoff Open water

Vegetated
runoff

runoff
Groundwater recharge

Interception

Evapo.

Depression

Infiltration

Root zone

Groundwater

Slope

Percolation

Saturated
zone

FIGURE 18.1 Hydrological processes simulated in WetSpa model (Mohammed, 2009) (left), an example of a heterogeneous
urban cell (right).

Landuse Map Soil Map

Digital Elevation Model

Direction Accumulation Slope

Roughness coefficient Hydraulic conductivity

Stream net Root depth Porosity

Hydraulic radius Interception capacity Field Capacity

Wilting point

Evap. parameters

Velocity

Residual soil moisture

t 0

c D Pore distribution index

σ

WetSpa Runoff coefficient & Depression storage capacity

FIGURE 18.2 Relationship between the three main input maps, digital elevation model, land-use and soil map, and the
WetSpa model parameters. t 0 is the mean flow time, c is the celerity of the diffusion wave, D is the dispersion coefficient
coefficient, σ is the standard deviation of the flow time.

The influence of urban areas on storm runoff is self-evident.
Due to the raster resolution, cells may not be 100% impervious.
In WetSpa the remaining area is assumed to be pervious and
covered by grass, and therefore, the potential runoff coefficient
for urban areas is calculated as

area. Table 18.2 presents standard impervious cover percentages
for different land-use categories. Imperviousness percentage for
residential, commercial and industrial area is estimated based on
the information in Chow, Maidment and Mays (1988). Other
estimates are considered reasonable guesses. Zero impervious
percentage is assumed for land-use categories not listed (i.e.,
agriculture, grassland, and forest land).
The effect of rainfall duration is also included in the runoff
process, as more runoff is produced during a storm event due to

C u = IMP + (1 − IMP ) C grass (18.2)

where C u and C grass are potential runoff coefficient for urban
and grass raster cells, and IMP is the proportion of impervious