Integration of Dagan's Steady State Drainage Equation to Transient Conditions for Subsurface Drainage Design

The steady state drain spacing equation of Dagan (1964) with flow above drain level was integrated to predict transient water table height midway between the drains by applying Bouwer van Schilfgaarde (1963) approach. The developed drain spacing equation was compared with field experimental data and existing drainage theories of Glover-Dumm reported by Dumm (1960), Integrated Hooghoudt by Bouwer and van Schilfgaarde (1963), modified Glover equation by van Schilfgaarde (1964), van schilfgaarde (1963) and Integrated Dagan equation with no flow above drain level by Singh et al. (1992). The present Integrated model predicted quite closer values of water table height midway between the drains in 50 m and 75 m drain spacing with corresponding average deviations of 6.81 and 2.67%, respectively. The equation does not require any correction for radial flow and avoids trial and error method to calculate drain spacing. The equation may be used for predicting transient midway water table height or for drain spacing calculation.