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Estimating Hydraulic Properties of Soil Aggregate Skins from Sorptivity and Water Retention

^a Institut für Bodenlandschaftsforschung, Zentrum für Agrarlandschafts- und Landnutzungsforschung (ZALF) e.V., Eberswalder Straße 84, D-15374 Müncheberg, Germany
^b Institut für Geologische Wissenschaften und Geiseltalmuseum, Martin-Luther-Universität Halle-Wittenberg, Fachgebiet Umweltgeologie, Domstraße 5, D-06108 Halle, Germany

View larger version (31K): [in a new window]   Fig. 1. Schematic cross-section of the capillary imbibition pressure-plate apparatus for multiple soil aggregates: elastic foam bed (1), sand bed (2), 30-µm pore-size membrane (3), plastic foil with holes for samples (4), aggregate samples (5), plastic foil (6), lateral sealing (7), foam-coated cover (8). A water-storage bottle of 11-cm i.d. placed on a balance is indicated on the left side.

View larger version (18K): [in a new window]   Fig. 2. Schematic picture of the physical system indicating soil-aggregate sample preparation and used symbols. Cubic or prismatic samples with intact (intact) and cut skins (cut) were prepared out of irregularly shaped (original) soil aggregates leaving the material of the removed skin layer of the cut samples (skin layer). Superscript symbols are used to distinguish between the lengths of the interior and skin parts of intact aggregate samples and the length of the cut samples.

View larger version (17K): [in a new window]   Fig. 3. Relative cumulative water absorption by intact and cut soil aggregate samples of Soil Block 5B as a function of the square root of time (in min) for applied pressure head values of h0 = -1 cm and h0 = -5 cm of water. Each data series represents absorption rates obtained from a group of 21 individual soil aggregates relative to the total area-related water uptake, qin, at the end of the imbibition experiment (Table 4). Values of the sorptivity, S, indicated by solid lines, were obtained from the initial (t < 2 min) linear parts of the absorption curves.

View larger version (24K): [in a new window]   Fig. 4. Drying (solid symbols) and wetting (open symbols) water retention functions of intact and cut aggregate samples. Large symbols represent retention data obtained from the summed field-moist volumes and masses (Table 2) for each group of samples. Small symbols and errors bars indicate the arithmetic means and the 95% confidence limits obtained from individual samples. Solid lines are the fitted VG-type (van Genuchten, 1980) retention functions.

View larger version (24K): [in a new window]   Fig. 5. Water-retention functions of interior and skin parts of the intact aggregate samples derived from the arithmetic mean values of 21 aggregate samples of Soil Block 5B. Solid circles represent the difference between the water contents of intact samples and interior (Eq. [13]). The dotted line indicates the fitted VG-type water-retention function of the skin.

View larger version (20K): [in a new window]   Fig. 6. Hydraulic conductivity as a function of the matric potential head of intact and cut aggregate samples of Soil Block 5B as well as of the skin of intact samples. The symbols represent hydraulic conductivity values evaluated from data of the imbibition experiments according to Dirksen (1975) using Eq. [2] through [5] and White and Perroux (1989) using Eq. [10]. The VGM-type relative hydraulic conductivity functions were evaluated using desorption water retention and ‘matched’ at Dirksen-evaluated values.