A water balance, like a storm runoff estimation, establishes volumes and rates of flow. Storm runoff and water balance estimations supplement each other as tools for evaluation and design. Storm runoff estimation is needed to protect against, control and utilise runoff during individual storm events. Water balance estimations show the effects of land use and stormwater control on the local ecosystem. The underlying principle of the water balance is the change-of-volume concept, expressed in the following equation applied to any component of a landscape during any given increment of time:

storage = inflow - outflow

Any difference between inflow and outflow must be accounted for by a change in the amount of water stored. The inflow can include precipitation, stream flow or artificial irrigation. The outflow can include evapotranspiration, direct runoff, base flow and additional outflows such as withdrawals for water supply. Figure 6 illustrates these different flows and the water balance concept. The water balance is a way to evaluate the aggregate effect of the hydrologic regime.

Figure 6: Water balance concept (Source: Ferguson, 1998)