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Preisträger 2013: M.Sc. Benjamin Eisele für seine Masterarbeit in Hydrologie

Spatial variability of stable isotope profiles in soil and their relation to preferential flow 

Spatial and temporal changes in soil water distribution can be reconstructed by analyzing
the stable isotopic composition of soil water. The aim of this study was to examine
the spatial variability of stable isotope profiles on the plot scale with respect to the influence
of preferential flow. A sprinkling experiment with a combined tracer of Brilliant
Blue FCF added to isotopically depleted glacier water was conducted on a loess soil.
Excavation of the irrigated soil allowed for soil core sampling according to the stained
pathways of preferential water flow. The introduced isotopic signal was clearly detected.
Mechanisms of water flow were identified as mixing, piston displacement and macropore
flow. The resulting depth profiles displayed a high spatial variability, which was
in sharp contrast to the depth profiles from un-irrigated parts of the plot. Those profiles
reflecting the natural conditions showed very low spatial variability. The seasonal
precipitation isotope input was maintained in the soil, but biased towards winter precipitation,
suggesting preferential root water uptake of summer precipitation. The smooth
shape of all natural isotope profiles is in contrast to the highly variable precipitation
input, but can be explained with the strong effect of the grass cover, which effectively
transpires summer precipitation. The seasonal signal is first averaged by mixing and
then transported into the soil by piston displacement during stronger rainfall. A week
of heavy rainfall in June did most likely cause preferential flow. However the signal
did not cause variability in soil isotope values, because this precipitation was relatively
depleted and had the same isotopic signal as the soil. Preferential flow may cause high
variability in isotope profiles on a small spatial extent, but may also reduce spatial variability,
depending on the input signal. Isotopically distinct soil water pools have to be
considered, and may have an averaging effect on bulk sample measurements.