In hydrology, the water balance is an equation which describes how water can be accounted for within a drainage basin.

The equation is P = Q + E ± S

Where

P= Precipitation

Q= Runoff

E= Evapotranspiration

S= Storage (in soil or the bedrock)

The water balance can be used to manage water supply ad suggest where there may be water shortages. It is also used in irrigation, flood control and pollution control.

The water balance can be illustrated using a water balance graph which plots levels of precipitation and evapotranspiration often on a monthly scale.