Study to explore the "Mechanisms of consistently disconnected soil water pools over (pore)space and time" in discussion for Hydrology and Earth System Sciences
We present in HESSD an extensive stable isotope data set gathered by the Surface Hydrology and Erosion group at IDAEA-CSIC, Barcelona. Our study sheds light on the hypothesis of “ecohydrological separation” that was stated in the seminal paper by Brooks et al. (2010) almost ten years ago. Their publication initiated many stable isotope (2H and 18O) studies in ecohydrology, tree physiology, and vadose zone hydrology. However, the mechanism of how two subsurface water pools of different isotopic compositions evolve and persist are not yet understood.
This lack of understanding is partly due to insufficient sampling frequencies of isotope data in the field. We overcome this limitation by a unique sampling design gathering isotope data of mobile and bulk soil water, rainfall, groundwater, and stream water in a fortnightly frequency over 8 months. We further extended our study by four years using long-term stable isotope and soil moisture data to reveal that the seasonal dry down of the soil and the wetting up of the soil pores with isotopically distinct rainfall can explain the disjunct isotopic compositions of water that is either stored in the soil or routed quickly to the groundwater and streams.
Our findings provide the scientific basis to refute the often-made assumption in environmental modelling that water is well mixed in the subsurface. We further provide suggestions on how to implement our findings in future modelling frame works, by accounting for pore scale variability of water transport.
Special section on "STABLE ISOTOPE APPROACHES IN VADOSE ZONE RESEARCH" in Vadose Zone Journal now complete and online
The special section on "Stable isotope approaches in vadose zone research" in Vadose Zone Journal is complete and online as open access here, including our study on "Measuring and modelling stable isotopes of mobile and bulk soil water".
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.
New paper out in Vadose Zone Journal on the differences between stable isotopic signal in mobile and bulk soil water
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.