The water balance is an accounting of all water inputs, outputs, and storage changes within a defined system such as a watershed, lake, or aquifer. It is the foundation of hydrology and water resources management.
The water balance (also called water budget or hydrologic budget) applies the principle of conservation of mass to track all water fluxes entering, leaving, and stored within a defined hydrologic system over a specified time period. For a watershed, the fundamental water balance equation states that precipitation equals evapotranspiration plus runoff (surface and subsurface) plus change in storage (soil moisture, groundwater, snowpack, surface water). Each component can be measured directly, estimated from empirical relationships, or modeled using process-based hydrologic models. The water balance is the foundational tool for understanding how water moves through the hydrologic cycle and for making decisions about water resource allocation, infrastructure design, and environmental management. At global scales, the water balance explains patterns of river flow, lake levels, and groundwater recharge. At local scales, it guides irrigation scheduling, stormwater design, and aquifer management. Water balance calculations require data on precipitation, streamflow, evapotranspiration, soil moisture, and groundwater levels, making it an integrating framework that draws on multiple disciplines. Climate change alters the water balance by shifting the magnitude and timing of precipitation, evapotranspiration, snowmelt, and storage, requiring ongoing reassessment of water resource availability.