A defined area's water storage changes with total inflow and outflow.
Key knowledge
A water budget accounts for water entering, leaving and being stored within specified boundaries over a stated interval. Storage change equals inflows minus outflows. Surface, subsurface and atmospheric exchanges must be included consistently rather than assumed to balance at every moment.
Conditions and scope
Define the area, interval and storage compartments; use compatible volume or equivalent-depth units.
Unmeasured fluxes and measurement errors can leave an estimated residual; that residual is not automatically groundwater recharge.
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References
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Darcy's law relates macroscopic specific discharge through a porous medium to hydraulic-head gradient and hydraulic conductivity. Conductivity depends on both medium and fluid properties. Darcy flux is discharge per bulk cross-sectional area, not the speed along each pore path.
Groundwater flux is one contribution to an appropriately bounded flow budget.
Streams carry bed load, suspended material and dissolved solutes through different processes. Entrainment and deposition thresholds depend on grain size, cohesion, flow and bed conditions. Fine cohesive sediment can require stronger entrainment than coarser loose sand, so smaller does not always mean easier to erode.
Changing flow and discharge connect to catchment water supply.
Porosity measures pore-space fraction, while hydraulic permeability/conductivity concerns connected flow under stated fluid conditions. Saturated aquifers receive and lose water through recharge, discharge and extraction. Confined and unconfined settings have different storage and head responses; high porosity alone does not guarantee rapid flow.
Recharge/extraction must satisfy a defined control-volume balance.
For a one-dimensional incompressible reach, ∂(A+A_s)/∂t+∂Q/∂x=q_L balances conveying area A, additional nonconveying storage area A_s and discharge Q. Positive q_L adds water per unit channel length. Integrating between sections gives the rate of water-volume storage equal to upstream-minus-downstream discharge plus lateral input.
A reach is a spatially resolved control-volume water balance.
For a well-mixed reservoir with solution mass M,positive salt fraction s and conserved salt mass Ms, freshwater gain/loss gives ds/dt=−(s/M)dM/dt. This follows from d(Ms)/dt=0. Rainfall dilutes and salt-free evaporation concentrates the reservoir; seawater exchange or salt-bearing input requires a separate salt flux rather than this closed-salt formula.
A surface-water drainage basin, catchment or watershed is the land area contributing surface drainage to a specified outlet. Neighboring basins are separated by drainage divides, commonly associated with topographic high ground. Changing the outlet changes the basin; smaller basins can nest within larger ones. Infiltration, storage and evaporation mean precipitation does not all immediately become streamflow. Groundwater divides describe subsurface flow systems and need not coincide with surface-water divides.
A water budget must state its domain and include cross-boundary exchanges rather than assume topographic closure.