Water flow and solute spreading in heterogeneous soils with spatially variable water content

Although the spatial variability of water content is expected to be signifi cant in heterogeneous, unsaturated media, its effect an solute transport ha s been neglected in most stochastic analyses. In this work we develop, eval uate, and numerically verify an analytical model describing the statistical moments of water content, soil water flux, soil water velocity, and solute spreading in the unsaturated zone under conditions of variable soil water content. Results are presented for a wide range of soil conditions. It is s hown that soil water content variability increases with soil water tension and decreases with soil textural tortuosity parameter m. As expected, the w ater content variability also increases with those of soil texture and satu rated hydraulic conductivity. However, at or above field capacity (near sat uration) the water content variability is generally negligible. The water c ontent variability is very large under dry conditions, even if the soil is relatively homogeneous. In the same soil, longitudinal macrodispersivity is generally smaller when including than when neglecting the variability in w ater content. Variable water content enhances longitudinal macrodispersivit y only in soils with large horizontal anisotropy or soils with m < 2. In th is case the enhancement is more significant in drier soils and at the early stages of plume displacement (less than 10 lambda(f)). With travel distanc e the longitudinal macrodispersivity in the presence of variable water cont ent converges to that in the absence of it.