Impervious area storage capacity: The impervious area of a catchment consists of those impervious surfaces which are directly connected to the catchment drainage system. Typical impervious surfaces are roadways, parking areas, paths, roofs, and occasionally rock outcrops. The volume of runoff from the impervious areas of the catchments is governed by impervious area storage capacity.

This represents the maximum volume of water that may be trapped in small depressions in the impervious areas. Typical values are:

Gentle to steep slopes – 0.6 to 1.2 mm

Flat slopes – 1.2 to 1.5 mm

The value chosen will generally affect the runoff volume generated by small storms only.

The impervious areas are likely to be the only source of surface runoff for small storms and the predominant source of surface runoff for many larger storms as well.

Interception Storage Capacity: Interception is precipitation captured by vegetation cover. Intercepted water will not reach the soil. Typical values are as follows:

Grass cover: 1 – 1.5 mm

Forest cover: 2 – 2.5 mm

This parameter may have some effect on runoff volumes most notably when storms are of relatively low intensity.

Depression Storage Capacity: This is the maximum volume of water that may be trapped in small depressions in the surface of the catchment. The value is a function of catchment topography:

Mild to steep slopes: 2-4 mm

Flat slopes: 4-10 mm

Some typical values are:

Typical Depression Storage Values
Land CoverDepression Storage (mm)
Impervious Surfaces1.25 - 2.5
Lawns2.50 - 5.00
Forest Litter7.5
Source: Design and Construction of Urban Stormwater Management Systems, New York, NY, ASCE (1992)

The effect of this parameter on runoff from small storms may be considerable. However, its effect on large storms will be small.

Upper Soil Storage Capacity: This is the capacity of the upper soil zone storage. The upper soil zone may be considered to correspond to the top soil layer, which has a depth of between 2 and 10 cm and comprises the grass root zone (Chapmen 1970).

The capacity of the soil zone store may be evaluated using the available water per unit depth for the approximate soil type. The value of USC chosen will affect the infiltration characteristic. However, once a reasonable estimate is obtained it should not have to be altered during the calibration process. Typical values for some soil types are given by Eagleson (1970) and are reproduced in the following table:

Soil TypeAvailable Moisture
at Field Capacity
Fine Sand85
Sandy Loam110
Fine Sandy Loam150
Silty Loam165
Light Clay Loam165
Clay Loam165
Heavy Clay Loam150

Values adopted by Goyen (1981) were very much lower than the values given above and were found to be in the range of 12.5 – 25.0 for urban pervious surfaces. These lower values gave considerable better results with the particular Canberra data analyzed.

Lower Soil Storage Capacity: Capacity of the lower soil zone storage capacity. This is evaluated using the same considerations that were used to determine upper soil storage capacity.

The lower soil zone may be considered to correspond to the root zone of larger vegetation such as bushes and trees.

When determining the depth of the lower soil zone it should be noted that the lower soil store is depleted by evapotranspiration but the ground storage is not. Thus the lower physical bound of the lower soil zone represents the upper limit to which the groundwater may rise before evapotranspiration acts on it.

The depth of the lower zone is typically 1-2 m unless constrained by shallow soil. Again, once a reasonable estimate is obtained for LSC the model is generally not sensitive to small changes in its value.

Initial: Initial moisture values in individual stores at start of storm event in mm.