A water budget is a quantitative accounting of water inflows, outflows, and changes in storage within a defined system over a specific time period. It is essential for water resources planning and management.
A water budget (also called a water balance) provides a systematic, quantitative accounting of all water entering, leaving, and stored within a defined hydrologic or management unit, such as a watershed, aquifer, reservoir, city, or irrigation district, over a specified time period. The fundamental equation states that inputs minus outputs equals the change in storage. Inputs include precipitation, surface water inflow, groundwater inflow, imported water, and return flows from wastewater and irrigation. Outputs include evapotranspiration, surface water outflow, groundwater outflow, water exports, consumptive use, and discharge to the ocean. Storage components include soil moisture, groundwater, surface water in lakes and reservoirs, and snowpack. Water budgets are prepared at scales ranging from individual farm fields to entire nations, and at time steps from daily to annual or longer. They serve multiple purposes: identifying whether a system is gaining or losing water, quantifying the relative importance of different water sources and losses, detecting long-term trends in water availability, and evaluating the impacts of proposed management actions or climate change scenarios. The USGS has published comprehensive water budgets for major aquifer systems and river basins across the United States, providing foundational data for water resources planning.