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The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. [ 2 ] [ 3 ] In hydrology , a water balance equation can be used to describe the flow of water in and out of a system.
By considering a long-term (e.g. seasonal) average depth of the water table (Dw) in combination with the long-term average recharge rate (R), the net storage of water in that period of time is negligibly small and the steady state condition is satisfied: one obtains a dynamic equilibrium. Derivation of the equation
Figure 7. Runoff from the water balance. The recharge, also called effective rainfall or rainfall excess, can be modeled by a pre-reservoir (figure 6) giving the recharge as overflow. The pre-reservoir knows the following elements: a maximum storage (Sm) with unit length [L] an actual storage (Sa) with unit [L] a relative storage: Sr = Sa/Sm
The energy required per unit mass of water vaporized. (J g −1) L v = Volumetric latent heat of vaporization. The energy required per unit volume of water vaporized. (L v = 2453 MJ m −3) E = Mass water evapotranspiration rate (g s −1 m −2) ET = Water volume evapotranspired (mm s −1) Δ = Rate of change of saturation specific humidity ...
The manual provides detailed instructions on the audit process, which occurs at three levels a) top-down approach, b) component analysis, and c) bottom-up approach. For top-down approach, all water flow data are collected in order to calculate the water balance between input volume and the sum of consumption and losses.
Likewise the safe yield of wells, extracting water from the aquifer without overexploitation, can be determined using the geohydrologic water balance or the overall water balance, as defined in the section "Combined balances", depending on the availability of data on the water balance components.
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The water balance equation relates the change in water stored within the basin (S) to its input and outputs: = In the equation, the change in water stored within the basin (ΔS) is related to precipitation (P) (water going into the basin), and evapotranspiration (ET), streamflow (Q), and groundwater recharge (D) (water leaving the basin). By ...