Preferential flow paths in a water repellent clay soil with grass cover

Grass-covered heavy basin clay soils in the Netherlands appeared to be water repellent. Water repellency in the top layers of these soils occurred mainly as a coating on the aggregates. The variation in soil moisture content over short distances was studied by sampling the soil 10 times during the period August 31, 1993, to December 22, 1994. Each time, 35 samples (100 cm(3)) were taken in close order over a distance of 195 cm at depths of 0-5, 10-15, 20-25, and 30-35 cm. Differences between minimum and maximum soil moisture contents were high in all layers sampled, occasionally as much as 28 vol %. When the clay soil is dry, a major proportion of the water from precipitation or sprinkler irrigation may flow rapidly through shrinkage cracks to the subsoil, bypassing the matrix of the clay peds. However, preferential flow is not limited to macropore flow: irregular wetting patterns are also formed through the small pores of the matrix. The relationship between dry bulk density and volumetric water content was found to be positive when the clay soil was relatively dry and negative when it was relatively wet.