Experimental and numerical modelling of constructed channels in the desert sand dunes for MAR applications

Hydrology of sand dunes in the deserts of arid regions, specifically, groundwater in aquifers underlying the sand fields (e.g., Wahiba Sands in Oman) has been studied since 1970th. Sustainable water resources management in the desert dunes is essential, particularly in Gulf countries, where no perennial streams exist. Interdunal areas can be explored and tested for managed aquifer recharge (MAR) if natural wadis or small-size constructed channels are used as surface water spreading systems. These channels would both transport and seep tertiary treated wastewater into the vadose zone and enhance the amount of moisture content there. In this paper, a coupled surface-subsurface flow was experimentally and numerically studied in application to MAR through small-size triangular channels. In the field, experiments were done at the crest and slope of a selected dune, a triangular channel had a sand bed and a mild slope (S = 5°). By applying a constant discharge at the inlet of the channel, the length, L, of the water “jets” propagating until their complete seepage extinction has been measured. Numerically, by HYDRUS2D, we simulated saturated–unsaturated transient infiltration by considering a cross-section of triangular channels of different bank slopes λ to find the maximum total volume of infiltrated water from the chan-nel. The boundary condition was a hydrostatic pressure head along the wetted perimeters of the channels with water depth gradually dropping downslope. The Morel–Seytoux shape factor μ = q_i / 1/₂ (K_s A₀) where q_i is the rate of infiltration as a function of λ is plotted. If q_i X << Q, where X is a characteristic size in the direction of surface flow in the channel, Q is the surface flow rate, that is, for large channels and fast conveyance of surface water, q_i can be assumed independent of X. Then there is a unique minimum of μ(λ) at a given A₀. If q_i is not small, we determine the trench shape of a given A₀, which maximizes the total volume of water infiltrated from the trench. The results in this paper can be used in studies of furrow irrigation in desert agriculture and for evaluation of “transmission losses” from wadi channels flowing after flash floods.