An Unsteady Subsurface Drainage Equation Incorporating Variability of Soil Drainage Properties

Almost all unsteady subsurface drainage equations developed so far use constant value of drainable porosity and hydraulic conductivity which may not be representative of entire drainage flow region. A drainage equation was, thus, developed incorporating depth-wise variability of drainable porosity (f) and hydraulic conductivity (K) of saline soils of Haryana state in India. The drain spacing with measured hydraulic heads at different periods of drainage were estimated by the developed equation and compared with the corresponding drain spacing estimated by commonly used unsteady drainage equations. The study revealed that the developed equation estimated the drain spacing that was nearest to the actual drain spacing of the existing subsurface drainage system, when a generally used design criterion of 30 cm water table drop in 2 days is considered. For a criterion of desired water table drop in 3 days and beyond, Glover equation was found to be the most superior. Hence, both the developed equation and Glover equation can be readily used with the associated design criteria for designing unsteady subsurface drainage systems in saline soils of the state of Haryana, India.