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Abstract: A shortage of rain and water resources exits in semi-arid and arid zones, necessitating the use of marginal water such as treated sewage water (TSW) for irrigation. These waters may contain substantial amounts of mixed-sodium- calcium salts, which, in turn, interact with the soil matrix. The soil solution-soil matrix (SS-SM) interactions, particularly in the presence of smectite minerals (e.g., montmorillonite), may change the soil pore-size distribution, and, consequently, could affect the soil hydraulic properties. Since the magnitude of the SS-SM interactions depends on time-dependent flow-controlled attributes, i.e., water content, pressure head, and solute concentrations, the resultant hydraulic properties are also time-dependent. Numerical simulations of flow and transport in a three- dimensional (3-D), spatially heterogeneous, variably saturated soil, for the case in which the flow is coupled to the transport through the dependence of the soil hydraulic properties on solute concentrations, were employed in order to analyze flow and transport in salt-affected soil. The case under consideration was a citrus orchard planted on a structured clay soil in a region characterized by a distinct rainy period during the winter and a relatively long dry season, requiring irrigations. Results of the analyses suggest that enhanced SS-SM interactions, induced by increasing soil exchangeable sodium during the irrigation season and dilution of the soil solution during the rain season, may considerably reduce soil hydraulic conductivity during the rain season, particularly, close to the soil surface. Consequently, enhanced SS-SM interactions, may slow down the solute movement, may substantially increase the tailing of the mean solute breakthrough at a given horizontal control plane (CP), and may lead to mean and standard deviation solute profiles which exhibit bimodal distributions, characterized by a substantial, secondary peaks close to the soil surface. Practical implications of this study regarding the long-term use of TSW for irrigation are briefly discussed. [Copyright &y& Elsevier]

• The main impact of long-term irrigation with TWW on water flow is due to salinity. • Alternating TWW with desalinized water enhanced water uptake of trees over FW irrigation. • Trenches with finer substrate enhanced water uptake by the trees more than coarse TUF. • Pulse irrigations with coarse TUF enhanced water uptake of trees irrigated with TWW. • Irrigation with fresh water, and ADW, minimized solute leaching below the root zone. The yield of avocado trees planted on clayey soils decreases due to irrigation with treated-waste water (TWW). We hypothesized that the main cause for this yield reduction is the reduction in water uptake by the trees roots. The aim of this numerical study was to identify the main soil factors that control the reduction in water uptake by the trees roots, and to test various soil substrate-based and water-based management schemes design to counterweigh the water uptake reduction. The study relies on physically based, three-dimensional (3-D) simulations of flow and transport in variably saturated, spatially heterogeneous, flow domain, conducted for three successive years. The main findings of this study suggest that: (i) the long-term effect of irrigation with TWW on the response of the flow system is attribute to the salinity of the TWW, and not to its sodium adsorption ratio, SAR; (ii) with respect to improving water uptake by the trees' roots, the water-based scheme that alternates irrigation water quality between TWW and desalinized water, DSW, (ADW) performed better than the water-based scheme that uses fresh water only (FW). The soil substrate-based schemes, TUFp, that used trenches with highly coarse-textured soil material and pulse irrigations, and, particularly, SAop, that used trenches with finer soil texture, performed substantially better than the soil substrate-based scheme that used trenches with highly coarse-textured soil material only (TUF); (iii) with respect to minimizing solute leaching below the root zone, the water-based schemes, FW, and, particularly, ADW, performed substantially better than the soil substrate-based schemes. [ABSTRACT FROM AUTHOR]