Regional pattern of the mobile water fraction in soils as determined by disc infiltrometer experiments

In order to characterize the partitioning of solutes into the mobile and th e immobile fraction of pore water at the soil surface, field.-measurements of infiltration in unsaturated loamy sands were carried out using a disc in filtrometer in a region of about 10 km(2) at the Luneburger Heide, Germany. The aims of these measurements were to obtain the mobile water fraction an d the hydraulic properties of the soil, to relate them to other measured so il properties like the soil texture, and to assess their spatial variabilit y in the region. From the infiltration results we obtained the hydraulic co nductivity K-0 and the sorptivity S-0 using a three-dimensional infiltratio n analysis and the two-term infiltration equation, respectively. The mobile water fraction theta(m)/theta of the soil was estimated from the infiltrat ion results of a second disc infiltrometer filled with bromide solution. Th e estimated values of K-0 and S-0 were in good agreement with those reporte d in the literature as typical for loamy sands. The average value of theta( m)/theta was calculated to be 71% under the experimental conditions. Neithe r the horizontal spatial distribution of water and bromide concentration be neath the disc infiltrometer nor the change of the infiltrometer during the experiment had a notable influence on the experimental results. The spatia l variability of the data was analyzed by means of variograms. All variogra ms were spatially structured and indicated a high small scale variation of the variables. In conclusion, the technique applied is suitable for the est imation of the hydraulic and solute transport properties of soils, especial ly at larger scales.