Water balance

Water balance is the accounting of water entering, leaving, and accumulating within a defined system over a specified period. It is expressed as inputs minus outputs, with any difference reflected in storage change.

Water budget: An accounting of water inputs, outputs, and storage changes for a defined place and period. This is the practical accounting framework used to calculate a water balance.
Catchment hydrology: The study of water movement and storage within a drainage area bounded by a watershed divide. A catchment is a common spatial unit for calculating water inputs and outputs.
Control volume: A defined region in space across whose boundary matter and energy may flow. The chosen control volume determines which water flows count as inputs or outputs.
Energy balance: An accounting of energy entering, leaving, and changing storage within a defined system. It uses a similar conservation framework but accounts for energy rather than water.
Continuity equation: A conservation equation stating that changes in stored quantity equal inflows minus outflows. Water balance is the continuity equation applied to water in a chosen system.
Irrigation scheduling: Planning when and how much to irrigate crops to meet their water needs. Field water balances help estimate irrigation requirements and avoid excess application.
Hydrologic flux: The rate at which water moves across a surface or boundary. Fluxes quantify the transfers summed in a balance.
Water footprint: An indicator of freshwater consumed or polluted to produce goods and services. It attributes water use to products and activities rather than closing a physical system balance.
Evapotranspiration: The combined transfer of water to the atmosphere through evaporation and plant transpiration. It is often a major water loss from land and agricultural systems.
Reservoir operation: The management of reservoir releases and storage to meet water, energy, and flood-control objectives. Reservoir inflows, releases, and storage changes form an operational water balance.

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