I am happy to present the final study of my postdoctoral research within the VeWa project at EGU 2018 on "Water ages in the critical zone of northern environments: Relation between storage and travel times of transpiration and recharge fluxes". The presentation will be a talk on Thursday morning (12 April, 9:30 a.m., room 2.31) in an interesting session on controls of water storage, mixing and release dynamics. See here a list of all contributions to the session, with links to their abstracts.
The abstract describing my work in cooperation with Doerthe Tetzlaff and Chris Soulsby reads as follows: "As the northern environments undergo intense changes due to warming climatic conditions and altered land use practices, there is a need for an improved understanding of the impact of atmospheric forcing and vegetation on water storage dynamics in the critical zone. We therefore assess the travel times of recharge and transpiration fluxes in four landscape units of podzol soils in the northern latitudes: two sites in the Bruntland Burn long-term experimental catchment (Scottish Highlands) were vegetated either with Scots pine (Pinus sylvestris) or Ericacae (Calluna vulgaris), one site in Dorset, Canada was covered with White pine (Pinus strobus), and one site in Krycklan, Sweden dominated by Scots pine (Pinus sylvestris). We simulated the forward travel times by tracking individual precipitation and snowmelt events through the critical zone using the SWIS (Soil Water Isotope Simulator) model. A previous study showed that the SWIS model could simulate the hydrometric and isotopic dynamics in the upper 50 cm of the studied soils. The resulting median travel times of soil waters percolating through the 50 cm depth plane ranged from few days to >200 days at Bruntland Burn and Dorset and >300 days at the Krycklan site. These time-variant travel times of the recharge flux showed for all sites an exponential relationship to the water storage in the soil. The lower the water volume in the considered soil volume, the more likely are longer travel times. The shortest travel times of the recharge occurred accordingly in winter and early spring when the storage was highest and evapotranspiration was lowest. Our findings on the pedon scale therefore indicate similar inverse storage effects as reported for water ages of discharge at the catchment scale. These general patterns are blurred in years of intense snow accumulation and high snowmelt volumes in spring. As shown for the Krycklan site, the travel time of recharging soil waters in such years was highly dependent on the timing of the snow melt and most water was flushed during the melt period. The travel times of the transpiration ranged between few days and about 200 days depending on the time of infiltration of the traced precipitation or snowmelt. Water that infiltrated in late autumn stayed on average about 200 days in the soil before it was transpired in the following growing season. Thus, the dynamics of the transpiration water ages was mainly driven by the onset of the vegetation period. Our findings provide new insights into the mixing and transport processes of soil water in the upper layer of the critical zone, which is relevant for hydrological modeling at the plot and catchment scales as the common assumption of a well-mixed system in the subsurface does not hold for the transpiration. Additionally, the transpiration ages show that water in the plant xylem can have relatively old ages depending on the year, which is relevant for ecohydrological studies inferring root water uptake depths using stable isotopes.
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The EGU session HS10.5/BG2.1/SSS13.40 on "Stable isotopes to study water dynamics in the soil-plant-atmosphere continuum" is scheduled for Friday, 13 April 2018 with oral presentations in the morning (08:30–10:00 in Room 2.15) and the poster presentations in the afternoon (17:30–19:00). Make sure you don't miss the interesting talks and posters covering field studies, methodological developments and modeling applications in the context of stable isotope hydrology to foster process understanding in the soil-plant-atmosphere continuum.
We compare in our latest study soil water isotope data from suction cup lysimeter, that are limited to sample the mobile water (MW), with soil water isotope data sampled with the direct water-vapor analysis, that samples the bulk soil water (BW). We present for six landscape units at three VeWa sites that the BW isotopic compositions shows a kinetic fractionation, which is indicative for soil evaporation, but MW does not. We suggest that the relative volume of MW to BW is relevant for explaining these isotopic differences, since MW volumes are usually relatively low during periods of high evaporation. We additionally use the numerical 1-D flow model SWIS (Soil Water Isotope Simulator) to simulate the hydrometric and isotopic dynamics at the studied sites. The simulations accounting for a fast and slow flow supported the conceptualization of two soil pore domains (MW and BW) with isotopic exchange via vapor exchange. Please see the manuscript here.
Very interesting discussion on the topic of water ages in the hydrological cycle during last weeks workshop funded by the Wassernetzwerk Baden-Württemberg. The interdisciplinary organizing committee (Prof. Dr. Markus Weiler , Chair of Hydrology, University of Freiburg; Prof. Dr. Werner Aeschbach , Institute of Environmental Physics, Heidelberg University; Prof. Dr. Christiane Werner , Chair of Ecosystem Physiology, University of Freiburg; PD Dr. Christiane Stumpp , Helmholtz Zentrum München and University of Freiburg) brought together early career and senior scientists working on the different compartments of the hydrological cycle. This way, the workshop fostered exchange between the disciplines to get a better understanding of how the water flow in the unsaturated and saturated zone is affected by vegetation and atmosphere. The workshop ended with a field visit of a long-term experimental forest site close to Freiburg (Conventwald).
EGU Session on "Stable isotopes to study water dynamics in the soil-plant-atmosphere continuum"4/10/2017 I organize a session for EGU 2018 together with Josie Geris, Christophe Hissler, Pilar Llorens and Natalie Orlowski on "Stable isotopes to study water dynamics in the soil-plant-atmosphere continuum". It is co-organized by the sections of Ecohydrology (HS10.5), Terrestrial Biogeoscience (BG2.1) and Soil System Science (SSS). Invited speaker will be Arthur Gessler from WSL, Zürich.
More info and the session description here. Tetzlaff, Doerthe, Ala-Aho, P; Buttle, J; Carey, SK; Kohn, M; Kuppel, S; Laudon, H; McDonnell, J; McNamara, JP; Spence, C; Sprenger, M; Smith, A; Soulsby, C :"Using stable isotopes to understand vegetation‐water linkages across northern landscapes (VeWa)"
Sprenger, M; Tetzlaff, D; Ala-Aho, P; Buttle, J; Laudon, H; Mitchell, C; Snelgrove, J; Weiler, M; Soulsby, C: Keynote on "Water fluxes, transport and transit times" including recent work on "Mobile and tightly bound soil water fluxes in northern environments" See the program of the conference here. Our study on the influence of forest and shrub canopies on precipitation partitioning and isotopic signatures is out in Hydrological Processes and available on Researchgate.
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