Water budget equation

In hydrology, a water budget or balance equation can be used to describe the flow of water in and out of a system.

For a given catchment area in an interval of time Δt, continuity equation is given as:

   Mass inflow ― Mass outflow = Change in mass storage.

                    Vi  ―Vo = ΔS
         
             Where,
                        Vi = Inflow volume
                        Vo = outflow volume
                        ΔS = change in storage

Water Budget Equation in hydrological cycle:
It involves writing the hydrological continuity equation for the lake and determining the evaporation from a knowledge or estimation of other variables.

                          P ―R ―G ―E ―T = ΔS
          or,            P ―(R +G +E +T) = ΔS

             Where,
                         P = precipitation
                         R = surface runoff
                         G = net ground water flow
                         E = evaporation
                         T = transpiration

Rainfall, Evaporation, Surface runoff are expressed in units of depth over the catchment i.e, if annual flow from a 20 square km catchment is 10^8 cubic m. then corresponds to depth of {(10^8)/(20*10^6)}=5m.