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

Abstract

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,” Sd, 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, u1. Nontrivial extremes of Sd(d,u1) 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 Sd 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.