Water table rise in arid urban area soils due to evaporation impedance and its mitigation by intelligently designed capillary chimney siphons

Waterlogging of urban area soil in a hyperarid climate, caused by impedance of evapotranspiration due to land cover by an impervious pavement, is studied by a multidisciplinary team of researchers (hydropedeologists, hydrogeologists, groundwater engineers, soil physicists and mathematical modelers). In this paper, a study unique for an arid/hyperarid MENA region has been conducted: from soil pedons’ data, a thin vadose zone superjacent to a shallow water table of a coastal aquifer in Oman is described with emphasis on soil profile morphology layering and determination of the van Genuchten hydraulic parameters, used in HYDRUS modeling of evaporation-driven saturated/unsaturated flows. On a large scale, for capillarity-free groundwater flow, the Dupuit–Forchheimer model is used and an analytical solution is obtained. Intensive evaporation from the water table to a bare unpaved soil surface is impeded by an impermeable surface strip (land pavement) with an ensued rise of the water table. Waterlogging is quantified by the “dry area,” S d , under the strip. This integral is explicitly evaluated as a function of the model parameters: aquifer’s size and evaporation-normalized conductivity, the width of the strip, d, and its locus with respect to the shoreline, u 1 . Nontrivial extremes of S d ( d,u 1 ) are found. Contrary to the surface pavement, intensification of evaporation by capillary siphons, i.e., structural heterogeneities of a porous massif, is proposed as an engineering mitigation of groundwater inundation. Composite porous media with siphons (small-size rectangular inclusions of a contrasting finer texture) are numerically tackled by MODFLOW and HYDRUS2D. A constant flux or a constant pressure head condition is imposed on the top of the flow domain. The water table is shown to drop and S d to increase as a result of such “passive moisture pumping” from the aquifer. A potential model for 2D tension-saturated flow is used to solve a mixed boundary-value problem in a rectangular wick. Its flow rate is analytically evaluated as a function of evaporating width and the height of the “window” through which the aquifer feeds the wick. Conformal mapping of a rectangle in the physical domain onto a rectangle in the complex potential plane is realized via two reference planes and elliptic functions.