Detecting the occurrence of preferential flow in soils with stable water isotopes

Preferential flow in soils and hillslopes may transport water faster than the soil matrix. These features activate quickly during precipitation events, increase infiltration and vertical pathways can play an important role in runoff generation. However, preferential pathways are difficult to identify as common techniques (e.g. piezometer, soil moisture sensors, hillslope trenches) do not sufficiently represent the spatial scale and frequency of these features and other approaches (e.g. dye patterns) are labour intensive and heavily invasive.

Here, we present a method to derive locations of preferential flow only by using vertical stable water isotope profiles in soils. In four catchments, we each drilled 120 soil cores (1-3m) and analysed the stable isotope composition of the soil water in 10-20cm increments to derive depth profiles. Visually selecting profiles with similar isotopic seasonality patterns not influenced by preferential flow, we determined a reference isotope profile for each catchment using a LOESS regression. These represent a soil profile only influenced by vertical matrix infiltration. To account for differences in soil conductivity and porosity, the reference profiles were scaled by depths to each profile. Locations where the measured profile deviates significantly from the reference, we assume an influence of vertical or lateral preferential flow.

With this method we found evidence for preferential flowpaths in all catchments. Especially in the alpine catchment with highly heterogeneous soils many profiles showed distinct preferential flow features. There, some profiles also indicate multiple, vertically independent pathways. The depth at which these pathways occurred were highly variable, even at neighbouring profiles.

Overall our results demonstrate the feasibility to assess preferential flow only using soil water isotope profiles while also underlining the large spatial and vertical variability of preferential flowpaths at the hillslope and catchment scale.