Flow and transport in water repellent sandy soils

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.</p

Describing the soil physical characteristics of soil samples with cubical splines

The Mualem-Van Genuchten equations have become very popular in recent decades. Problems were encountered fitting the equations¿ parameters through sets of data measured in the laboratory: parameters were found which yielded results that were not monotonic increasing or decreasing. Due to the interaction between the soil moisture retention and the hydraulic conductivity relationship, some data sets yield a fit that seems not to be optimal. So the search for alternatives started. We ended with the cubical spline approximation of the soil physical characteristics. Software was developed to fit the spline-based curves to sets of measured data. Five different objective functions are tested and their results are compared for four different data sets. It is shown that the well-known least-square approximation does not always perform best. The distance between the measured points and the fitted curve, as can be evaluated numerically in a simple way, appears to yield good fits when applied as a criterion in the optimization procedure. Despite an increase in computational effort, this method is recommended over the least square method

Revolution op fairway kan wereld op zijn kop zetten

Op golfbaan ‘De Pan’ in Bosch en Duin onderzochten we de effecten van het toedienen van de surfactant Revolution op de bevochtiging van een zwakke helling in een fairway met behulp van een groot aantal vochtsensoren. Ook onderzochten we of door de surfactant het ontstaan van waterafstotendheid in de bovengrond en de vorming van (voor het milieu nadelige) preferente stroming kon worden voorkomen. Bovendien werd aandacht besteed aan de omgekeerde wereld op een fairway, waar een hogere zandrug juist groener en beter bevochtigbaar was dan de lagere dele