Water table behaviour in drained lands: effect of evapotranspiration from the water table

Subsurface drain spacing is underestimated by the equations that do not account for evaporation-evapotranspiration (ET) lowering the water table in drained lands. An analytical solution is proposed to evaluate water table behaviour in subsurface drained lands in the presence of ET. A piecewise linear model is proposed and used to describe any realistic functional relation between ET and depth to water table. Characteristics of the solution have been highlighted with the help of numerical examples for which drainage parameters have been chosen from two actually operating drainage systems installed in semi-arid regions. The accuracy of the proposed solution has been verified with the existing numerical scheme as well as by comparing the water table behaviour with the observed field data. Application of the solution in subsurface drainage design has been illustrated which suggests that drain spacing at this particular site could be increased by 9 to 18% if the contribution of ET in lowering the water table is taken into account.