Evaluating the carrying capacity of a subsurface drainage network based on a spatially varied flow regime

The carrying capacity of a subsurface drainage network based on spatially varied flow with increasing discharge and the presence of a free surface is examined and reported. The discharge capacity of a subsurface drainage network depends on the hydraulic and geometric parameters of the network components which should be clearly identified. In this regard a prototype scale experimental set‐up was prepared to determine the major head loss of the corrugated pipes employed in these networks and to investigate the minor head losses of different network junction types. According to the compiled data average values for the major and minor head loss coefficients were proposed and employed for modelling the flow condition within a subsurface drainage network. The model determines the network discharge capacity based on the real flow condition i.e. a free surface spatially varied flow regime. Comparison between the outputs of the applied model with that of the traditional relationships indicated a large discrepancy. The results revealed that applying existing formulations in the design process would likely pressurize significant portions of the network which could affect the water depletion period from the plant root zone.