Runoff coefficient

The runoff coefficient , sometimes also called the total runoff coefficient , is a term from hydrology and describes the ratio of that part of a precipitation event that directly reaches the runoff (effective precipitation) to the total precipitation : ${\displaystyle \Psi }$ ${\displaystyle Ao}$ ${\displaystyle N}$

${\displaystyle \Psi ={\frac {Ao}{N}}}$

The difference between precipitation and runoff is due to the following reasons:

• At the beginning of a rain event, local hollows in the terrain are first filled (loss of wetting and hollows). This coincides with the observation that at the beginning of a rain event there is no runoff at all, the first part of a precipitation event is lost to the runoff, so to speak
• part of the precipitation seeps away or fills the soil pores (seepage loss)
• another part evaporates (evaporation loss).

The runoff coefficient is used to dimension rainwater and mixed water canals , to calculate flood runoff in rivers based on observed (historical) rain events and for flood forecasts .

The runoff coefficient depends on the characteristics of the catchment area (proportion and permeability of paved areas, soil characteristics, vegetation , terrain slope, pre-moisture, ice cover) as well as the intensity and duration of the precipitation event. The effect of the area characteristics can change with the intensity and duration of precipitation.

Further discharge parameters

In addition to the (mean) runoff coefficient defined above, other values are used in hydraulic engineering , v. a. for the planning and dimensioning of wastewater systems:

• The peak runoff coefficient: This is determined as the quotient of the maximum rain runoff donation of a considered area to the designed / measured donation.
• Another important value is the final runoff coefficient. This is defined as the runoff coefficient, which results as a constant value after a lengthy rain event after filling up all temporary storage tanks and losses (particularly relevant here: trough loss, loss of wetting) - the runoff coefficient of an area is zero at the beginning of a rain event, it increases afterwards the duration of the rain, when all the intermediate storage tanks are gradually filled in order to finally reach a constant (maximum) value: the final runoff coefficient. Such runoff coefficients are determined and used to determine the precipitation water charge:

Type of area	Final discharge coefficient
attached and sealed like concrete or asphalt	1.0
fastened with partially water-permeable materials	0.3 to 0.6
Concrete and natural stone paving , paving slabs and composite stones	0.6
Lawn paving stones , gravel , crushed stone , green roofs	0.3

In contrast to the runoff coefficient , the runoff coefficient indicates the ratio of precipitation to the measured runoff over a longer period of time. It also contains the parts of the precipitation that are initially stored in the area, but only later reach the drain. There is only a difference here from the evaporation that has taken place .

See also

• Discharge figure

Individual evidence

1. ↑ Geiger, Dreiseitl; New ways for rainwater; Oldenbourg 1995; P. 267.
2. ↑ ig: The more rainwater seeps away, the cheaper it is. Eschbach. In: Badische Zeitung. Dr. Christian H. Hodeige, November 24, 2010, archived from the original on March 17, 2011 ; Retrieved March 17, 2011 .