In a tracer experiment TDR transect measurements were made to study percolation behaviour in a 120-year-old pine stand (Pinus sylvestris) on a water-repellent sandy soil (Haplic Arenosol). The experiment (with potassium iodide) showed an 80% labelling of the total flow in organic layers, whereas the area of transport in the mineral soil was sharply reduced to 12-30%. The average diameters of these preferential flow paths were about 8-15 cm. The TDR measurements indicate a homogeneous flow only for a short period from February until April. At this time of the year preferential flow is insignificant, because the soil is at approximately field capacity and not repellent to water. During summer (May to September) the soil dries out, and most precipitation results in preferential flow during this period. For any daily rainfall exceeding 10 mm, water infiltrates down to 1 m depth in the soil, which nevertheless, is still within the root zone. This kind of deep percolation results in the subsoil's wetting to field capacity (pF 1.8) earlier than the topsoil. A one-dimensional numerical model (SWAP) was used to simulate mean water balance with hydraulic functions with and without a water-repellency term. From the results of our tracer experiment we showed that the de-watering process in spring could be simulated well using the traditional piston flow concept, while the rewetting behaviour could be described more realistically using the mobile-immobile concept for water repellency.

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In this paper, we tried to find interrelations between water retention properties, surface characteristics, and structural features of sandy soils rich in organic matter. Raw humic, epihumic, and endohumic horizons of four acidic sandy forest soils were selected for this study. Specific areas and water adsorption energies were estimated from water vapor adsorption isotherms, micropore (nanometer range) parameters from desorption isotherms, mesopore (micrometer range) parameters from mercury intrusion porosimetry, and macropore (millimeter range) parameters from water retention curves measured using combined suction plate and pressure chamber methods. In the studied soils, pore volumes in all pore ranges were proportional to soil organic matter content. Thin column wicking technique was used to determine migration velocity vs. time dependence in the samples beds for a range of liquids of various surface tensions. From these dependencies the surface free energy and its components were estimated that were used for calculation of water contact angles and forces of interparticle interaction via a water meniscus. The dominant interactions in the studied soils were dispersive Lifshitz-Van der Waals forces. In the two upper horizons polar acid-base interactions were absent, however in the deepest horizons, high input of polar interactions occurred, due practically to electron-donor component of the surface free energy. The electron-acceptor contribution was low. The wettability of the soils was low in upper horizons as indicated by high water contact angles. Wasserretention und strukturelle Eigenschaften von sandigen WaldhumushorizontenVon vier sandigen, sehr stark sauren bis äuûerst sauren Waldstandorten wurden die Wasserretention und strukturellen Eigenschaften der Humusauflagen und der darunter befindlichen Ah-Horizonte untersucht. Die Untersuchungen zur spezifischen Oberfläche und zur adsorptiven Wasserbindung erfolgten mittels Wasserdampf-Adsorptionsisothermen. Der Mikroporenraum (Nanometerbereich) wurde mittels Desorptionsversuchen erfasst, während zur Charakterisierung des Mesoporenraums (Mikrometerbereich) die Quecksilberporosimetrie eingesetzt wurde. Die Beschreibung des Makroporenraums (Millimeterbereich) erfolgte anhand der pF-Charakteristik mittels keramischer Platten und Drucktöpfe. Der Energiestatus an den Oberflächen und zwischen den Partikeln wurde durch Migrationsversuche mit Flüssigkeiten unterschiedlicher Oberflächenspannung in kleinen Bodensäulen bestimmt, woraus auf die Kontaktwinkel und Bindungskräfte geschlossen werden konnte. Mit steigendem Gehalt an organischer Substanz nahm das Porenvolumen in allen Bereichen zu. Im Vergleich zu den Ah-Horizonten traten in den Humusauflagehorizonten nur wenig polare Bindungskräfte auf. Als Folge davon war die Benetzbarkeit in den Humusauflagen deutlich herabgesetzt, was wiederum groûe Wasserkontaktwinkel verursachte. Die dispersiven Lifshitz-Van der Waals-Kräfte spielten eine dominante Rolle.