Chapter 3 : Reservoir models - KU Leuven

instantaneous runoff coefficient
1
0.9
number of occurence
2000
1800
0.8
1600
0.7
1400
0.6
1200
0.5
1000
0.4
0.3
instantaneous runoff coefficient
number of occurrence
800
600
0.2
400
0.1
200
0
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
rainfall intensity (mm/h)
Figure 3.23 : Instantaneous runoff coefficients (mean values
for every rainfall intensity) for a depression storage model
with a maximum storage of 1.5 mm (markers) and the
corresponding number of occurrence of the rainfall intensities (line).
In order to make the runoff model continuous, the depression storage must also be
emptied. This can for instance be implemented by applying evaporation.
The evaporation is varying over the year (equation 1.1). The influence of this variation
on the total evaporation volume is however much smaller than the large variation of the
evaporation capacity itself, because of the limited availability of water in the
depressions on the surface. Figure 3.24 shows the monthly evaporated volumes
obtained with a continuous long term simulation of 27 years using different constant
values for the evaporation capacity (full lines) and the effect (bold dashed line) using
a variable evaporation capacity (thin dashed line). The variation of the real evaporation
(i.e. based on the water availability) is much smaller than the evaporation capacity.
For this reason a constant evaporation capacity could be used as an approximation
without having too much effect on the prediction of the overflow events. In the winter
period the evaporation is overestimated if a constant evaporation capacity of 0.1 mm/h
is used, while in the summer this is a good approximation. The overestimation of the
evaporation during the winter is about 10 mm/month. The overflow volumes in winter
could be slightly underestimated in this way. However, most (and most severe)
combined sewer overflows occur in the summer period. Moreover, the evaporation
only influences the antecedent conditions and thus has only a second order effect on the
overflow emissions.
Chapter 3 : Reservoir models 3.23