Runoff coefficient

The runoff coefficient or runoff is the ratio between the sheet of water precipitated on a surface and the sheet of water that runs off the surface, (both expressed in mm).

Calculation

k=EsPr{displaystyle k={frac {E_{s}}{P_{r}}}}

Symbol	Name	Unit
k{displaystyle k}	Coefficient of drainage
Pr{displaystyle P_{r}}	Precipitation	mm
Es{displaystyle E_{s}}	Lamina escurrida	mm

The value of the parameter k varies greatly depending on the type of land use. The following table presents some generally accepted values for long-term rainfall.

Looking at these values determined through field trials, it is easy to see why forest destruction and urbanization cause much higher floods.

Observation

The runoff coefficient equation must be corrected, since this is the relationship between the flow that runs off over the precipitated flow (which is always greater due to the losses that occur during runoff, such as infiltration and evaporation), which means that the runoff coefficient is always less than unity. The greater the losses of the precipitated flow, the lower the runoff coefficient will be, and vice versa. Therefore:

k=EP{displaystyle k={frac {E}{P}}}