Determination of peak runoff rate
ILWIS Applications Guide 133
tc= Time of concentration [min].
L = Length of main river [m].
S = Distance weighted channel slope [m/m].
Available data The materials used in this exercise are:
− Contour and drainage map of the catchment.
− Land use map.
− Soil map with soil attribute table.
− Two tables (Table 11.1 and Table 11.2) with typical C coefficients. Reference
is made to the hydrology text books (Schwab et al., 1993).
Table 11.1: Runoff coefficient C for agricultural watersheds (Soil group B)
Coefficient C for rainfall rates of
Crop and hydrologic condition 25 mm/h 100 mm/h 200 mm/h
Row crop, poor practice 0.63 0.65 0.66
Row crop, good practice 0.47 0.56 0.62
Small grain, poor practice 0.38 0.38 0.38
Small grain, good practice 0.18 0.21 0.22
Meadow, rotation, good 0.29 0.36 0.39
Pasture, permanent, good 0.02 0.17 0.23
Woodland, mature, good 0.02 0.10 0.15
Table 11.2: Hydrologic soil group conversion factors
Factors for converting the runoff coefficient C
from Group B soils to
Crop and hydrologic condition Group A Group C Group D
Row crop, poor practice 0.89 1.09 1.12
Row crop, good practice 0.86 1.09 1.14
Small grain, poor practice 0.86 1.11 1.16
Small grain, good practice 0.84 1.11 1.16
Meadow, rotation, good 0.81 1.13 1.18
Pasture, permanent, good 0.64 1.21 1.31
Woodland, mature, good 0.45 1.27 1.40
Determine time of concentration
In order to determine the time of concentration, the catchment area, for which the
peak runoff rate is going to be determined, has to be inspected on size, land cover
and topography.
F
• Study the catchment area and determine the size of the total
catchment area.
• Study the topography of the catchment area and determine the limits
