SOIL-MOISTURE AND DRY BULK-DENSITY PATTERNS IN BARE DUNE SANDS

Recent research has shown that irregular wetting and preferential flow may occur in heterogeneous field soils. Bare dune sands, which at first sight show the greatest homogeneity, were sampled volumetrically at nine locations in temperate and arid areas to determine the spatial distribution of soil moisture content, theta, and dry bulk density. Soil moisture patterns were irregular, even in extremely dry situations. As k - theta functions for dune sands are very steep in the dry theta range, small differences in water content result in huge differences in hydraulic conductivity, k. These differences, induced by soil moisture variability, may result in anisotropy of the system, and hysteresis tends to magnify this phenomenon. So, if soil moisture differences exist, they will be consolidated and even amplified during new rain events. At one site on a recently deposited dune sand, infiltrating water was able to transform a one-layer soil system into a two-layer soil system, inducing preferential flow at the layer interface. At another site, preferential flow paths appeared at the soil surface as 'sand columns' as a result of the displacement of the dry sand between the flow paths, caused by wind erosion. In all the dunes studied, dry bulk density patterns were irregular and tended to be slightly vertically orientated. Spatial distributions of volumetric and gravimetric soil water content showed great similarity.