Water Balance

This is the balance between the inputs and outputs of a drainage basin.

Water balance is expressed as:

P = Q + E (+/- change in storage)

where

P = precipitation

Q = run-off

E = evapotranspiration

Rivers will always have a regime which they follow, in that some months the discharge of the river will be higher than others. The water balance looks at how the amount of precipitation compares with the water leaving the system as runoff or as evapotranspiration. This balance will change throughout the year and will be affected by the overall climate of the area near to the river.

For example, under ‘usual’ conditions the precipitation will be matched by run-off and evapotranspiration giving a ‘normal’ river level. If evapotranspiration becomes greater for a few months in summer while precipitation and run-off remain the same, the river will flow below normal level.

Key terms to describe various water balance conditions are:

• Water Surplus: there is excess water available to the system. This occurs when precipitation exceeds evapotranspiration and the excess is not being used by plants.
• Deficiency: there is a reduction of water available within the system. This occurs when evapotranspiration exceeds precipitation.
• Recharge: after a period of deficiency precipitation will occur and replace the lost water in the soil. This needs to occur before a period of surplus can reoccur.

• Field capacity: the maximum amount of water that soil can hold before it becomes saturated.

A water surplus can result in wet soils, high river levels and additional run-off whereas a deficit leads to dry soil, falling river levels and possibly a drier micro-climate.