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Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, width and depth of 1.2, 0.6, and 0.52 m, respectively, were sampled in the Ouddorp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential flow patterns were clearly visible in soil blocks sampled after distinct rain events. Additional TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Further, preferred pathways recurred at the same sites during all rain events. Simulations with a two-dimensional flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the critical soil moisture content. Based upon the obtained results, indications are given on how to incorporate this preferential flow and transport process in current one-dimensional simulation models.

The present study suggests that many soils in the Netherlands 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. The persistence of water repellency was examined in topsoils using the water drop penetration time (WDPT) test. 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. 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 cm3 steel 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 study 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.

Depth, degree and spatial variability of water repellency were examined in the surface layers of dune sands along the coast of the Netherlands. Soil samples were collected at six depths of up to 50 cm at 865 dune sand sites in nature reserves. The potential water repellency was measured on dried samples using the water drop penetration time (WDPT) test. The vegetation at the sites consisted of marram grass, buckthorn, grey hair grass, pine, oak, other grasses and heather. The 5190 samples were dried at the laboratory, after which the potential water repellency was measured using the WDPT test. About 60–70% of the samples taken at several depths in the young dunes with a sparse vegetation of marram grass were wettable, whereas the other samples were slightly to strongly water repellent. The samples taken at a depth of 0–5 cm in the surface layer at the sites with different vegetations were all strongly to extremely water repellent. At all of these sites, the severity of water repellency decreased with depth. The decrease was most evident at the grey hair grass sites. No significant differences in severity of water repellency were found between the samples taken under a cover of buckthorn, pine and oak, any of the grasses and heather. The large variability over short distances in the water repellency and water content of the soil in the dune sands is shown by the intensive sampling of soil blocks at the Ouddorp, Westduinen, Schoorl and Zwanenwater sites. Drier as well as wetter soil areas were visualized in contour plots of the soil water content distributions in transects from the blocks. Large differences in wetting capacity between samples taken at several depths at the Ouddorp site were assessed by measurements of the wetting rate. In all cases, wetter samples wetted faster than their drier counterparts.

Soil water repellency generally tends to increase during dry weather while it decreases or completely vanishes after heavy precipitation or during extended periods with high soil water contents. These observations lead to the hypothesis that soil water repellency is common in dry climates and rare in humid climates. The study objective is to test this hypothesis by examining the occurrence of soil water repellency in an arid and humid climate. The main conclusion of this study is that the effect of climate on soil water repellency is very limited. Field observations in the arid Middle Rio Grande Basin in New Mexico (USA) and the humid Piedras Blancas Watershed in Colombia show that the main impact of climate seems to be in which manner it affects the production of organic matter. An extremely dry climate will result in low organic matter production rates and, therefore, less potential for the development of soil water repellency. On the other hand, a very humid climate is favorable for organic matter production and, therefore, for the development of water repellency.

Laboratory infiltration experiments were carried out to quantify the effects of soil water-repellency on infiltration rate and the wetting front instability. A two-dimensional transparent chamber (41.5 cm wide, 50 cm high and 2.8 cm thick) was constructed for infiltration experiments using three water-repellent Ouddorp sands (The Netherlands) and a wettable silicon sand. The results showed that if the water-ponding depth (h0) at the soil surface was lower than the water-entry value (hwe) of repellent sands, infiltration would not start until the water drop penetration time (WDPT) is exceeded; and contrary to infiltration in wettable soils, the infiltration rate increased with time. However, infiltration could immediately start at any time whenh0 . hwe: The wetting front was unconditionally unstable for h0 , hwe; resulting in fingered flow. However, the flow was conditionally stable for h0 . hwe if the soil was not layered in a fine-over-coarse or wettable-over-repellent configuration, and if soil air was not compressed during infiltration. The occurrence of stable and unstable flow in repellent soils was consistent with the prediction based on a linear instability analysis. The findings can be used to improve irrigation efficiencies in water repellent soils, e.g. using high-ponding irrigation methods. q 2000 Elsevier Science B.V. All rights reserved.