The Water Balance

The balance between inputs and outputs is known as the water balance or budget. The water balance can be shown using the formula:

precipitation (P) = streamflow (Q) + evapotranspiration (E) +/- changes in storage (S)

P=Q+E +/- S

The water balance affects how much water is stored in a system. The general water balance in the UK shows seasonal patterns. In wet seasons precipitation is greater than evapotranspiration which creates a water surplus. Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels. This means there is a positive water balance. In drier seasons evapotranspiration exceeds precipitation. As plants absorb water ground stores are depleted. The is a water deficit at the end of a dry season.

Figure 3. Model of the water budget in a drainage basin

Figure 1. Model of the water budget in a drainage basin