Detecting Ecohydrological Separation

Detecting ecohydrological separation of water between soil and trees using stable isotopes

Student: Elisabeth Tschumi

Supervisors: Prof. Dr. Sonia Seneviratne, Dr. Dominik Michel, Dr. Lukas Gudmundsson

This study investigates the separation of water between soil, xylem and runo water. To do so, stable water isotopes were used as tracers. Samples of soil, xylem, precipitation, creek water and lysimeter seepage were collected at the Buel measurement site in the Rietholzbach catchment on two field days, the first on 29 March, before the photosynthetic phase, and the second on 11 May 2016, during the photosynthetic phase. The water was extracted from the samples in the laboratory using centrifugation for the soil water extraction and cryogenic extraction for the xylem water. All water samples were then analysed for their stable water isotope compositions (18O and D) using either a Picarro cavity ring-down spectrometer or a Los Gatos water-vapor isotope analyzer. The results show differences between the two field days. This was due to different weather conditions and the photosynthetic activity of the plants. This difference in photosynthesis could be seen in the different isotope values of stem and branch xylem water. There was also a difference between the two sampled soil depths, -5 cm and -30 cm, which is most likely due to different precipitation inputs. The soil water was separated into weakly bound and tightly bound water to see if the soil water is well mixed and it was found that the tightly bound water is slightly less depleted, probably because it is older and thus longer exposed to evaporation. Furthermore, the xylem water isotope compositions are much more enriched than the soil water, for which no explicit explanation was found.