I am happy to announce that I just started this week a two-year project on "Water age dynamics in a Mediterranean catchment and their ecohydrological implications in a changing environment" funded by the German Research Foundation (DFG).
I will work together with the team of the Surface Hydrology and Erosion Group of the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) in Barcelona on their extensive data set gathered in the Vallcebre Research Catchments (an overview of the impressive research and data sets from the IDAEA group can be found here). We will use hydrometric and stable isotope data from various hydrological compartments (precipitation, stream, groundwater, soils, plants) to assess travel times across the catchment with latest modeling approaches to improve the understanding of storage and release of water in the highly seasonal environment of the headwater catchment contributing to the water supply of the Barcelona area. If you are interested what we will be doing in the coming two years, you can follow project updates on researchgate.

I look forward to organizing a session for the AGU Fall Meeting 2018 with Natalie Orlowski, Todd Dwason, and Stephen P Good on "Stable Isotopes in the Critical Zone: Methods, Applications and Process Interpretations".
If you use stable isotopes to study the Earth's Critical Zone, we would be happy to receive your abstract before 1st of August here.

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".

We compiled stable isotope data (2H and 18O) of bulk soil water at five long-term experimental catchments across a hydro-meteorological gradient in the northern latitudes (Figure 1). The comparison of the extensive data set, covering different landscape units at each catchment, showed that vegetation, topography and elevation affect the isotopic composition over time and soil depth. Soil water beneath conifers is more enriched in heavy isotopes than beneath heather or oak vegetation. Sampling sites closer to the stream are generally less variable in their stable isotopic composition than sites at gentle hillslopes. Isotopic fractionation due to soil evaporation was observed mainly in the top 10 to 30 cm and was mainly controlled by the precipitation amount while temperature (as proxy for evaporation) was less important and soil moisture did not influence the isotopic fractionation, as revealed by multiple linear regression. 
The study will be published (here) in a Special Issue on "Water in the Critical Zone" in Hydrological Processes. The work was funded by the ERC grant VeWa and you can find a presentation by Doerthe Tetzlaff about that project here.

The final version on the manuscript on "Measuring and Modeling Stable Isotopes of Mobile and Bulk Soil Water" is now online available and can be downloaded here.

I will be working for three months on pooling ideas we gathered during a workshop on "Water Ages in the Hydrological Cycle" funded by the Wassernetzwerk Baden-Württemberg last October. My main collaborator for this short project will be Christine Stumpp and Markus Weiler.

A study on water ages and travel times in the critical zone is now out in Hydrology and Earth System Sciences Discussion. We used in this study the SWIS model that was tested at different sites in the northern latitudes in a previous investigation. We tracked the infiltrated water through the soil profiles and in the evaporation, transpiration and recharge fluxes. This way, we could derive travel times (which show how long the water takes to leave the soil via evaporation, transpiration or recharge), and median water ages (to estimate the median age of water in soil storage or the evaporation, transpiration and recharge fluxes). Our results showed for each study site, that water ages of soil storage, evaporation, transpiration and recharge were inversely related to the storage volume of the critical zone: water ages generally decreased exponentially with increasing soil water storage. These findings on the 1-D soil profile support the "inverse storage effect" as recently discussed for the catchment and hillslope scales. You can download the manuscript here.

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.

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.