# 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:

 Units English Metric $$\large{ Q }$$ = runoff $$\large{ft^3}$$ $$\large{m^3}$$ $$\large{ I_a }$$ = initial abstraction, the amount of water before runoff $$\large{ft^3}$$ $$\large{m^3}$$ $$\large{ S }$$ = maximum soil moisture retention after runoff begins $$\large{dimensionlessa}$$ $$\large{ P }$$ = rainfall $$\large{in}$$ $$\large{mm}$$