A simple experiment to trace stemflow infiltration and subsurface flow paths based on stable water isotopes

Stemflow has a relevant role in forested catchments because it affects the amount of precipitation reaching the soil, and how water infiltrates and interacts with soil particles. The role of stemflow in various subsurface processes depends on the infiltration area and its size is currently a topic of interest and debate within the ecohydrological community. Stemflow infiltration area is generally estimated based on the ratio between stemflow input rate and the mean soil infiltration capacity, whereas direct observations of stemflow infiltration areas are rare. Direct observations of stemflow infiltration areas are usually made by the application of dye tracers, which have proven to be useful for monitoring double-funneling. On the contrary, few direct observations are based on the application of isotopically-labelled water to assess stemflow infiltration area and subsurface flow paths. 

Therefore, in this study, we present a simple experiment carried out in a forested catchment in the Italian pre-Alps to simulate stemflow by using isotopically-labelled water and to quantify stemflow infiltration area and volume. The experiment was conducted during a dry period to observe better changes in the isotopic signal in the soil water. Stemflow was simulated with a rainfall depth and intensity similar to typical summer storms in the catchment, and by using water with an isotopic composition very different compared to the composition of soil water during summer months. The isotopically-labelled water was applied to a beech tree monitored by electrical resistivity tomography during different wetness conditions, as well as during this stemflow experiment.

Soil samples collection for isotopic analysis was carried out after the experiment, at different distances from the stem and at different depths (e.g., 0-15, 15-30, and 30-45 cm). Soil moisture was also measured at 0-6 and 0-12 cm depths at different distances from the stem. Preliminary results showed a rapid infiltration of stemflow along the root system of the beech tree and the usefulness of isotopically-labelled water to simulate stemflow and trace double-funneling.  

Keywords: stable water isotopes; soil water; stemflow; forested catchment.