In rainfall/runoff modelling rainfall intensities play an essential
role. Therefore, data recorded at short intervals have to be processed.
Daily observations, which were available in sufficient number, were
assumed as sufficient for our purpose.
The mathematical concept for splitting rainfall into soil retention,
direct runoff and deep infiltration is based on the CN-method of the US
Soil Conservation Service (US-SCS). It uses the following
with Q = direct runoff
P = precipitation
Ia = initial retention of rainfall in the soil
S = maximum retained humidity up to saturation limit
all units in mm.
This non-linear approach leads to near zero runoff values for low
precipitation. Larger values are obtained for increasing rainfall
volumes, and thus reveals a
phenomenon observed in real world.
As generally the required information about soil characteristics is not available, it is often assumed
Furthermore, in order to facilitate the use of nomograms, the following substitution has been introduced:
with S expressed in inches (25.4 mm).
This has led to the term "CN-method".
CN-values are tabulated for four different soil types (from deep well drained sands to
poorly permeable soils with high clay content or with high groundwater
level / or impermeable layer) and a considerable number of various
combinations of land use and vegetation cover.
However, these tabulated values can only serve as an approximation.
Particularly for large-scale investigations they have to be adjusted by calibration.
Furthermore, according to the vegetation period and the accumulated
rainfall height of the preceding five days, three different antecedent
moisture conditions (AMC) are distinguished. The SCS handbook contains
tables permitting the transformation of the CN-values tabulated for AMC
II (medium moisture) into values valid for AMC I (dry) or AMC III
(wet). However, for computer aided application the transformation with
mathematical functions is more appropriate.
As a novelty, the approach of the CN-method has not only been used
for determination of the direct runoff, but also for assessment of deep
infiltration. For this purpose, the amount of daily rainfall was
reduced by the portion of direct runoff.
The modelling consisted of the stages model preparation (manually) and model calculation (automated and in daily steps).
For model preparation, first a grid map "hydrological soil
classification" has been created. It is based on the map "surface geology"
which had been prepared earlier for the purpose of groundwater
modelling. Influences due to urbanisation and land use have been considered
through correction factors. In order to facilitate calibration, the
assignment of standardised CN-values for direct runoff and deep
infiltration can be easily modified.
For each day, the actual model calculation consists of the following steps: