Effect of Mixed Na‐Ca Solutions on the Hydraulic Properties of Unsaturated Soils

The effects of mixed Na‐Ca solutions on soil water diffusivity and unsaturated hydraulic conductivity were tested for a loamy soil from Gilat, Israel. Soil water diffusivity functions, D(θ), were determined by the horizontal infiltration method. Unsaturated hydraulic conductivity functions, K(θ), were determined both directly (by vertical infiltration for suction heads of 0–50 cm H2O) and indirectly (from diffusivity functions and soil water retention curves). Prior to determinations, soil samples were equilibrated with solutions containing a given concentration and composition of Ca and Na. The results show that D(θ) and K(θ) are independent of solution concentrations in a Ca‐saturated system. In mixed Na‐Ca systems, hydraulic conductivity and soil water diffusivity functions are highly dependent on the composition and concentration of the soil solution, in addition to their dependence on soil water content, θ. For any given θ, both K(θ) and D(θ) decrease as either soil solution concentration decreases or the sodium fraction in the soil solution increases. The negative effect of a combination of high Na‐to‐Ca ratio and low soil solution concentration on K(θ) is directly related to the degree of water saturation of the soil. Low values of θ can compensate for the negative effects of high Na‐to‐Ca ratio and low solution concentration.