When water enters a water-repellent topsoil, preferential flow paths develop and the flow bypasses a large part of the unsaturated zone. Therefore, preferential flow caused by water- repellency is expected to accelerate solute leaching to the groundwater. In soils with water-repellent toplayers and groundwater-affected wettable sublayers, a flow pattern develops that consists of three regions. In the top few centimeters, water flow converges toward the tops of the preferential flow paths. Within the preferential flow paths, water moves vertically downward. In the wettable subsoil, the flow diverges due to matric: forces. A new analytical three-region model is developed to calculate convective transport of an inert tracer in this steady-state flow system. Calculations with this model show that the wettable subsoil largely determines the residence time in the unsaturated zone and the shape of the breakthrough curve. For a sandy soil with a wettable soil of 1.20 m thickness on average, model results indicate that spreading of the solute front on the field scale caused by soil heterogeneity can be much larger than the front spreading caused by the diverging flow in the wettable subsoil.
A new type of lysimeter was built to study the spatial distribution of drainage and solute leaching with a resolution of 5 cm from an undisturbed sandy soil column with a water-repellent toplayer (1.00 m 2area, 0.55 m height). The experiments provided strong support for the concepts underlying the preferential flow model. Additionally, during an eight-month period of uninterrupted experimentation, unique observations of the long-term dynamics of unsaturated flow were made. Areas of high drainage moved over lateral distances of up to 0.25 m, and the distribution of drainage over different areas with large drainage amounts varied slowly but strongly. The three-region analytical model can reproduce without calibration the breakthrough of a chloride pulse reasonably well. In combination with the support provided by the lysimeter data for the underlying concepts, this makes the model a promising starting point for a transient, numerical model for solute transport in fields with water-repellent toplayers.
|Qualification||Doctor of Philosophy|
|Award date||5 Jun 1996|
|Place of Publication||S.l.|
|Publication status||Published - 1996|
- sandy soils
- hydraulic conductivity
- cum laude