waterbalans

Water balance, or waterbalans, is a method used in hydrology to quantify the flow of water into, through, and out of a defined system over a specified time interval. It rests on the conservation of mass: the amount of water entering the system plus any change in stored water equals the amount leaving the system. The chosen system boundary—such as a watershed catchment, a reservoir, or a soil column—determines which fluxes are included.

Key fluxes include precipitation (P), evapotranspiration (ET), runoff or discharge (Q), and processes such as infiltration and groundwater recharge. The change in storage (ΔS) accounts for gains or losses in soil moisture, groundwater, and surface water within the boundary. For a catchment over a period, a common form is P = ET + Q + ΔS, with Q representing streamflow leaving the area.

Groundwater balance and reservoir balance are variations; more detailed models separate surface runoff, baseflow, and recharge. Applications of the water balance approach include assessment of water resources, drought monitoring, irrigation planning, hydropower, flood risk management, and ecological flow calculations.

Data sources and methods include precipitation measurements, evaporation estimates, river discharge data, and storage changes from surveys or remote sensing. Calculations can be performed at different scales using simple water balance equations or more complex hydrological models. Uncertainties arise from data gaps, spatial variability, and changing boundary conditions.