Simulating upward soil-water flow from buried pipes with variable hydraulic characteristics

Abstract

Differences in the boundary condition near buried corrugated pipes used for combined drainage and irrigation show that the hydraulic resistance due to perforations are not identical under both drainage and subsurface irrigation modes. No studies to date have sufficiently examined how perforation characteristics such as size and configuration affect the exit head loss of buried corrugated pipes used for subsurface irrigation. This research proposes a new dimensionless parameter called the exit resistance (αx) that accounts for variable perforations and can be used to compute the exit head loss due to capillary rise under subsurface irrigation. Datasets obtained from 189 numerical simulations were used to investigate how the size and configuration of rectangular slots affect αx. The results show that the number of perforation lines (N) and longitudinal spacing of slots (ay) on the pipe wall have the largest impact on αx. The difference in αx can be as large as 50% when N varies between 4 and 10, while slots placed in every corrugation valley (ay = 1.645 cm) provide less than 60% of the exit resistance compared to those spaced in every third valley (ay = 4.935 cm). Exit resistance was also found to vary with the pressure head on the subsurface irrigation pipes. The impact of heterogeneity from layered soils on αx of buried perforated pipes was also investigated. The results indicated that αx is not influenced by soil heterogeneity. In general, the findings from this study implies that it is better to perforate buried corrugated pipes with slots in every corrugation valley to reduce exit head losses and maximize upward soil-water fluxes in subsurface irrigation systems.