Flow and transport in water repellent sandy soils

English. Water repellency in soils is currently receiving increasing attention from scientists and policy makers, due to the adverse and sometimes devastating effects of soil water repellency on environmental quality and agricultural crop production. Soil water repellency often leads to severe erosion and runoff, rapid leaching of surface-applied agrichemicals, and loss of water and nutrient availability for crops.In general, soils become water repellent through the coating of soil particles or structural elements with water repellent organic substances originating from decaying plant material. Soil water repellency manifests itself when the water content of the soil drops below a critical level. Water flow and solute transport patterns are complex under such conditions. The present study deals with flow and transport processes in an untilled, grass-covered water repellent sandy soil consisting of three layers.Extensive tracer experiments indicate that distribution flow dominates in the humous top layer, preferential flow in the water repellent sand layer, and diverging flow in the underlying wettable zone. Preferential flow paths or fingers occur almost throughout the year. Fingers develop rapidly during severe rain storms, causing significant portions of the infiltrating water to be preferentially transported to the deep subsoil. Fingers form at sites with relatively low degrees of water repellency, and finger diameters range from 10 to 25 cm.Model simulations show that fingered flow results from hysteresis in the water retention function, and the nature of the formation depends on the shape of the main wetting and drainage branches of that function. Once fingers are established, hysteresis causes them to recur along the same pathways during subsequent rain events. In the long term, recurrence of fingers may lead to changes in physical and/or chemical properties of the soil within the fingered flow pathways. It is only under initially dry conditions, with soil water contents below the critical level, that fingers will be formed during infiltration. Under wetter conditions, with soil water contents above the critical level, wetting fronts will remain stable and no fingers will develop.Future research should focus on improving our understanding of the origins, occurrence, hydrological responses and agricultural functioning of water repellent soils.

Dutch; Flemish. Volgens vele onderzoekers is waterafstotendheid van gronden een van de slechtst begrepen verschijnselen in de bodemkunde en de hydrologie van de onverzadigde zone. Er is dan ook grote behoefte aan intensief onderzoek op dit gebied. Dit is des te belangrijker omdat in de komende tientallen jaren bodems in allerlei nieuwe gebieden binnen en buiten Nederland waterafstotend kunnen worden als gevolg van veranderingen in het landgebruik, bodem- en waterbeheer en weersomstandigheden.