Dynamik der Verweilzeiten von Wasser in Mediterranen Einzugsgebieten und deren ökohydrologischen Auswirkungen in einer sich wandelndem Umwelt
Zusammenfassung der Projektergebnisse
The project made use of the extensive stable isotopes of water (2H & 18O) data set covering rainfall, stream water, groundwater, soil water, and plant water in the Vallcebre research catchment, sampled by the Surface Hydrology and Erosion Group at the Institute of Environmental Assessment and Water Research (IDAEA-CSIC). Based on this isotope data and in combination with hydrometric data describing the rainfall, evapotranspiration (ET), and discharge (Q) fluxes, as well as the soil moisture and groundwater storage, we managed to derived new insights into 1.) subsurface mixing and the lack thereof between fast and slow flowing soil pore waters, 2.) dynamics of recent rainfall contributing to stream water generation at low flows and during flashy hydrograph responses to intense rainfall events, and 3.) the impact of plant water uptake of relatively old water on the age dynamics of stream water. With regard to the subsurface mixing, we could show that over a period of eight month covering dry and wet conditions, the most mobile water, sampled with suction lysimeter, were consistently isotopically different than the bulk soil water, sampled with cryogenic extraction. Thus, our data set is an example for heterogeneous subsurface water flow, where newly infiltrating water bypasses large parts of the soil matrix. As such, these observed disjunct subsurface water pools are in line with the ecohydrologic separation initially observed and defined by Brooks et al. (2010) in a Mediterranean catchment in Oregon. We connected these plot scale findings with catchment scale observations of rapid response to rainfall events and we showed that Q of the Can Vila catchment is to large parts sourced by recent rainfall, defined as young water fraction by Kirchner (2016). A high sampling frequency of rainfall and stream water, triggered during high rainfall and elevated Q, allowed to infer a relationship between stream flow and young water fractions and a strong relationship between the young water metric and hydrometeorological forcing. Thus, when comparing the young water fraction among different catchments, sampling frequency and rainfall intensities should be considered. We then combined the plot scale and catchment scale isotope dynamics using the StorAge Selection (SAS) function approach with the code provided by Bennetin & Bertuzzo (2018). While SAS functions are usually calibrated based on stream water tracer data, we included observed plant water isotopes to constrain the storage selection for the ET flux. Our results showed that limiting the SAS function calibration to stream water isotopes cannot provide well identified parameters for the age distribution of the ET. However, a multi-objective calibration approach with an objective function considering both stream and plant water stable isotopes allowed to increase parameter identifiability for the selection functions of ET and stream water contributions. This improvement came at little cost regarding the goodness of fit for the stream water isotope simulations. Additionally, estimated water ages of the transpiration flux were several months older when including the plant water isotope information, compared to simulations calibrated solely with stream water isotopes, which is in line with other ecohydrological studies that showed that plants use partly water infiltrated during the dormant season. Within the project, also theoretical aspects on water ages have been discussed. A review brought together the methods, findings and research gaps of investigations of water ages in the Critical Zone and a commentary outlined current limitations and potential avenues regarding the research on ecohydrologic separation.
Projektbezogene Publikationen (Auswahl)
- (2019): Mechanisms of consistently disconnected soil water pools over (pore)space and time, Hydrology and Earth System Sciences, 23, 2751-2762
Sprenger M, Lorens P, Cayuela C, Gallart F, Latron J
(Siehe online unter https://doi.org/10.5194/hess-23-2751-2019) - (2019): The demographics of water: A review of water ages in the critical zone, Reviews of Geophysics, 57(3), 800-834
Sprenger M, Stumpp C, Weiler M, Aeschbach W, Allen ST, Benettin P, Dubbert M, Hartmann A, Hrachowitz M, Kirchner JW, McDonnell JJ, Penna D, Orlowski N, Pfahl S, Rinderer M, Rodriguez N, Schmidt M, Werner C
(Siehe online unter https://doi.org/10.1029/2018RG000633) - (2020): Commentary: What ecohydrologic separation is and where we can go with it, Water Resources Research
Sprenger M & Allen ST
(Siehe online unter https://doi.org/10.1029/2020WR027238) - (2020): Investigating young water fractions in a small Mediterranean mountain catchment: both precipitation forcing and sampling frequency matter, Hydrological Processes
Gallart F, Valiente M, Lorens P, Cayuela C, Sprenger M, Latron J
(Siehe online unter https://doi.org/10.1002/hyp.13806)
