# Runoff

Written by Jerry Ratzlaff on . Posted in Geotechnical Engineering

Runoff, abbreviated as Q, is the water flow that occures on the ground surface when the soil is at full capacity from excess water from rainfall.

## Runoff formula

 $$\large{ Q = \frac { \left( p \;-\; I_a \right)^2 } { P \;-\; a_i \;+\; S } }$$

### Where:

$$\large{ Q }$$ = runoff

$$\large{ I_a }$$ = initial abstraction, the amount of water before runoff

$$\large{ S }$$ = maximum soil moisture retention after runoff begins

$$\large{ P }$$ = rainfall

Tags: Equations for Soil