Depression Storage (Initial Losses) Model

Initial losses are defined as the quantity of rainfall required to cause overland runoff. Hence, the initial loss for catchment surfaces can be taken as the intercept on the rainfall axis of a plot of rainfall verses runoff.

(1) where D - Average depth of initial losses (mm). Note depression storage is defined in metres in runoff parameters files. S - Average slope of the sub-catchment (%)

This model was included in the original Wallingford Procedure software (WASSP). For pitched roofs where less data was available an initial loss of 0.4mm was advised.

Subsequent experimental studies have shown that initial losses are also dependent on surface type, with variation within surface type dependent on surface slope as previously. Accordingly, the current approach uses an extension of Eq. 1 with units changed to SI:

(2) where D - Average depth of initial losses (m) S - Slope (m/m) k - Coefficient (m)

The value of k reflects such factors as the surface microtopography and layout. The default value used for paved areas is 0.000071 (m).

Within the simulation engine, InfoWorks ICM will impose a minimum slope value of 0.002. This prevents divide by zero errors should a smaller value be entered for Catchment Slope.

The initial state of the initial loss storage is defined as an equivalent depth of rainfall associated with a rainfall event. If higher than the depth D given by Eq. 2 this value is equated to D. Furthermore, the initial loss store can also be depleted by evapotranspiration at a constant rate.

It is also possible within the software to define an absolute initial loss value independent of surface slope. This facility is used to override the initial loss values defined for pervious surfaces when using the SCS and New UK Percentage Runoff (variable PR) equation models.

The normal values for k are:

Paved and roof surface	0.000071 (m)
Pervious surface	0.00028 (m)

Pitched roof surfaces have the same coefficient as road surfaces but use a slope of 0.05.

Depression storage may be partly or wholly taken up by rainfall immediately before an event; that is, the antecedent rainfall.