APPLICATION: FLOOD FORECASTING MODEL FOR KLODZKO CATCHMENT (POLAND) - Modelling Uncertainty in Flood Forecasting Systems - page 86

I a =0.2 S

(5.2)

The maximum retention, S, is a function of the catchment characteristics. An
intermediate parameter called curve number (CN) is used to compute S (in SI unit) as:

(5.3)

The CN values range from 100 for water bodies to approximately 30 for permeable soils
with high infiltration rates.

Direct-runoff model
Clark's unit hydrograph (UH) method is used for the transformation of excess
precipitation (runoff volume) to runoff. This method explicitly represents two critical
processes of transformation: translation and attenuation . The former is the movement of
the excess precipitation from its origin throughout the drainage to the catchment outlet, and
the latter is the reduction in the magnitude of the discharge as the excess water is stored
throughout the catchment. The transformation is defined in the model by two parameters:
time of concentration, t c , and the storage coefficient, R . The basin storage coefficient, R,
is an index of the temporary storage of precipitation excess in the catchment as it
drains to the outlet. Both of these parameters are normally estimated via calibration.

Baseflow model
The baseflow is computed using the exponential recession method (Chow et al. 1988). It
defines the relationship of Q t , the baseflow at any time t, to the initial baseflow (at time
zero), Q 0 , as:

Q t = Q 0 # t

(5.4)

where # is an exponential decay constant of the baseflow recession. In HEC-HMS, # is
defined as the ratio of the baseflow at time t to the baseflow one day earlier. In HEC-1
the # is represented by the parameter called RTIOR, which is defined as the ratio of the
current recession flow to the flow one hour later. The following equation can be used to
convert an HEC-1 recession constant for use in HEC-HMS:

(5.5)