APPLICATION: FLOOD FORECASTING MODEL FOR KLODZKO CATCHMENT (POLAND) - Modelling Uncertainty in Flood Forecasting Systems - page 85

Figure 5.2 . Typical representation of HEC-1 and HEC-HMS watershed runoff.
5.1.2 Methods used to model rainfall-runoff processes
Different options are available for selecting the methods for these models. The methods
used in the present model are briefly described below. The selection of the methods in
the present model is governed particularly by the availability of the data. The analysis of
the influence of different methods is beyond the scope of this study.
Runoff volume computation model
The Soil Conservation Service (SCS) Curve Number (CN) method is used for the direct
runoff volume (precipitation excess) computation, using the following equation:
(5.1)
where P e is the accumulated precipitation excess (cumulative direct runoff volume in
mm) at time t; P is the accumulated rainfall depth at time t; I a is the initial abstraction
(initial loss); and S is the potential maximum retention, that is the measure of the ability
of a watershed to abstract and retain storm precipitation. The SCS developed the
following empirical relationship between I a and S