Drain Spacing Based on Dynamic Equilibrium

Superposition is used to obtain an equation for the water-table elevation between parallel drains in response to a periodic sequence of instantaneous recharge events of arbitrary volume and timing. The mathematical developments are especially useful for calculating the spacing of parallel-relief drains required to achieve a prescribed maximum water-table elevation under conditions of dynamic equilibrium. A drain-spacing calculation, compared with the results obtained by the Bureau of Reclamation’s (USDI) approximate, dynamic-equilibrium method, shows the latter to result in a drain spacing that is on the safe side. The spacing method of this paper provides a mathematically correct alternative to the pioneering dynamic equilibrium calculation and maintains the flexibility required to handle conditions of variable recharge and drainout times.