Water balance, also called hydrologic balance or water budget, is an accounting of all the water entering, leaving, and being stored within a defined system over a specified period of time. The system may be as small as a plant or reservoir, or as large as a watershed, aquifer, lake, river basin, or the entire Earth. The purpose of a water balance is to determine whether water storage is increasing, decreasing, or remaining constant by comparing all water inputs with all water outputs. It is one of the basic principles of hydrology, water resources, environmental, agriculture, and groundwater studies.
Water Balance Formula |
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\( P \;=\; Q + ET + \Delta S \) (Water Balance) \( Q \;=\; P - ET - \Delta S \) \( ET \;=\; P - Q -\Delta S \) \( \Delta S \;=\; P - Q - ET \) |
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| Symbol | English | Metric |
| \( P \) = Percipitation | \(in\) | \(mm\) |
| \( Q \) = Streamflow / Runoff | \(in\) | \(mm\) |
| \( ET \) = Evaportranspiration | \(in\) | \(mm\) |
| \( \Delta S \) = Change in Storage | \(in\) | \(mm\) |
Water inputs commonly include precipitation (rain, snow, hail), stream inflow, groundwater inflow, imported water, and irrigation. Water outputs commonly include evaporation, transpiration by plants, stream outflow, groundwater outflow, water withdrawals, and consumptive use. Storage may occur in surface water bodies, soil moisture, groundwater aquifers, snowpack, glaciers, wetlands, reservoirs, or even within vegetation.
Engineers use a water balance to evaluate the availability and movement of water, design reservoirs and drainage systems, estimate groundwater recharge, analyze flooding and drought conditions, manage irrigation systems, and assess the sustainability of water resources. Environmental scientists use water balances to study ecosystems, wetlands, and climate interactions, while municipalities use them to manage drinking water supplies and wastewater systems.
