Drainage equation

[3] where: Steady (equilibrium) state condition In steady state, the level of the water table remains constant and the discharge rate (Q) equals the rate of groundwater recharge (R), i.e. the amount of water entering the groundwater through the watertable per unit of time.

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.

Hooghoudt published tables for the determination of the equivalent depth (d), because the function (F) in d = F (L,Di-Dd,r) consists of long series of terms.

The availability of a computer program also helps in quickly assessing various alternatives and performing a sensitivity analysis.

The blue figure shows an example of results of a computer aided calculation with the amplified drainage equation using the EnDrain program.