Moisture variability resulting from water repellency in Dutch soils

The present study suggests that many soils in the Netherlands, in natural as well as in agricultural areas, may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. Preferential flow has wide-ranging significance for rapid transport of solutes, such as agrichemicals, towards the groundwater and surface water, making it essential to understand this phenomenon.The persistence and degree of water repellency was examined in topsoils of nature reserves and cultivated soils, using the water drop penetration time (WDPT) and alcohol percentage tests. The severity of water repellency measured on dried soil samples, the so-called "potential" water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. In some cases, however, the severity of potential water repellency was found to be sensitive to the initial moisture content of the soil and the temperature during drying. Measurement of the "actual" water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The "critical soil water content", below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies.Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm 3steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This thesis provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.