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Project Highlights:

  • Forested headwaters supply drinking water to billions of people worldwide.
  • Over the past decade, the forest hydrology community has found that stored water and the time water spends travelling through a catchment to the stream network can play a major role in landuse change outcomes for water quality and quality.
  • But, new isotope analysis of water in trees, soils, streams and groundwater in the past 5 years has shown that storage is compartmentalized in ways we are only just beginning to understand.
  • Therefore, this NERC proposal will go after this compartmentalization of water stores and a set of key questions on how forested catchments store, mix and release water.
  • We will leverage the BIFoR site and infrastructure to ask:
    • Where do trees at BIFoR source their soil water and does the depth change throughout the hydrologic year?
    • How do above and belowground physiological traits of trees affect water source depth distribution?
    • How do different soil water extraction techniques affect our interpretation of the flowpaths, flow sources and water transit times at BIFoR?
    • How does elevated CO2 affect tree water source apportionment?


This NERC CENTA PhD studentship proposal is an ambitious research program to advance the understanding of how forested headwater catchments store, mix and release water. Forested headwaters supply drinking water to billions of people worldwide (McDonnell et al., 2018). Beyond supplying drinking water, understanding how forested headwater catchments store, mix and release water underpins our ability to predict the water resource outcomes of climate change in the UK. A significant finding in the field of catchment hydrology in the past 10 years is the importance and recognition of watershed ‘storage’ in the generation of streamflow and its associated transit times during and between rainfall and snowmelt events (McNamara et al., 2011). There is often a large contrast in the age of stored water and that of streamflow water (Berghuijs and Kirchner, 2017). For instance, Jasechko et al. (2017) showed that most groundwater below 250 m is older than 10,000 years. New work on the age of water in plants (Zhang et al., 2017 has shown transpired water can be multi-decadal in age (when we would have otherwise predicted water ages of days to weeks).

These studies hint at a terrestrial water cycle that is much more compartmentalized and poorly mixed than previously thought; and at timescales well beyond the now-standard annual catchment water balance calculation.


Water transit time (the time water spends travelling through a catchment to the stream network) is key for understanding the links between stream water hydrology and water quality (Hrachowitz et al., 2016). By examining isotope signatures in trees, soils, streams and groundwater, several studies have now shown (e.g. Evaristo et al., 2015; Good et al., 2015; Sprenger et al., 2018) have shown that water transit times in relation to compartmentalized water storage—a research frontier beneath our feet (Grant and Dietrich, 2017)—and its mixing and release (to streams) is the crucial research barrier for progress in predicting change in the headwaters. And to date, no studies have yet examined (a) trees compartmentalization of water storage in the context of climate warming and (b) the effects of elevated CO2 on tree water source apportionment.

OBJECTIVES: The goal of this research work is to understand how vegetation controls storage, mixing and release of water now and in a Greenhouse World.


Rationale & Background: The recent work on vegetation controls on storage, mixing and release has shown evidence of ecohydrological separation, or the ‘two water worlds’ hypothesis (McDonnell, 2014). The figure opposite shows a cartoon of this where mobile water in figure (a) follows the meteoric water line (the relation between 18O on the x-axis and 2H on the y-axis) and the more tightly bound soil water and plant water that hug the soil water evaporation line in figure (b). Several community commentaries (Berry et al., 2017; Brantley et al., 2017) have pointed to two key issues that must be tackled for further advances in understanding how plants affect, and are affected by, belowground water storage, mixing and release: (1) the theoretical biophysical feasibility for two distinct water pools to exist and (2) plant and soil processes that could explain the different isotopic composition between the two water pools. We will tackle these issues by leveraging data from BIFoR where we will measure temporally (bi-weekly) the stable isotopes of plant and soil water over an entire year.

Research Question (1): Where do trees at BIFoR source their soil water and does the depth change throughout the hydrologic year? We will test the null hypothesis that plant water use (of easily accessible “mobile” soil water and more tightly bound “low mobility” soil water) and depth apportionment are unchanging through the hydrologic year. We seek to reject this null hypothesis by sampling at BIFoR. We will leverage ongoing measurements of carbon, water, and energy cycles at the on-site flux towers in each of the dominant tree species (oak, ash, hazel). We will instrument 3 trees per tree type within the tower footprint using dendrometers (Ecomatik, Ltd Germany) and sap flux sensors (Granier-type heat pulse sensors that we make ourselves). We will collect bi-weekly soil and xylem samples for 12 full months for stable isotopic analysis. This will be the first time such a high-resolution isotope record through an entire hydrologic year has ever been constructed. We will use the cryogenic extraction method for soil and plant water sample processing (Orlowski et al., 2016). The McDonnell lab has in-house facilities for this, including four LWIA-45EP Los Gatos Research (LGR) laser spectrometers for extracted soil water and an Isoprime isotope ratio mass spectrometer for extracted plant water (necessary due to co-extracted volatile substances). With this high frequency temporal record we expect to see new dynamics of mobile to low-mobility soil water use and uptake as plants shift from hibernation to active growing and transpiration through senescence and hardening.

Research Question (2): How does elevated CO2 affect source apportionment? We will leverage the CO2 560 ppm environment at BIFoR FACE to answer this question. This will be the first ever test of CO2 enrichment on source apportionment using stable isotopes as tracers. We will repeat the sampling of (1) for a matched set of trees within the CO2 footprint.

Replicates of all soil and plant material will be taken and analyzed. We will use a Bayesian mixing model following Parnell et al. (2013) and as tested recently in our work at the Christchurch Botanical Garden (Evaristo et al., 2017) to quantitatively link plant xylem water isotope composition to water uptake patterns and depths. With this comparison of ambient vs elevated vs elevated CO2 we expect to see increases in grow, photosynthesis and increased transpiration from the 560 ppm CO2 stands with deeper and more varied soil water sourcing through the profile. We also expect to see new behaviors of plants in their water uptake depth distributions, where the stand maximizes its collective water use via individual tree use of different water depths.


Training and Skills

At the beginning of the first year, the PhD student take the Catchment Science Summer School during the first week of September. Late in the first year (in May), the PhD student will travel to the Global Institute for Water Security (GIWS) at the University of Saskatchewan in Canada to do on-site training as part of the National Isotope Tracers in Catchment Hydrology course. In Year 2, the PhD student will return to the GIWS to all of their soil- and plant water extractions in the McDonnell lab and run their isotope samples.


Year 1: Develop proposal and begin field sampling and monitoring

Year 2: Complete all extractions and isotope analysis at the Global Inst for Water Security

Year 3: Prepare journal articles and write thesis

The project will require weekly trips to BIFoR in Year 1

Partners and collaboration (including CASE)

PhD project benefits from supervision by some of the world’s leading research groups.

The UoB research team has been leading research into ecohydrological and biogeochemical processing in hyporheic and riparian zones, with a strong focus on the development of novel experimental technologies and modelling techniques to quantify the interplay of physical, biogeochemical and ecological processes under the impact of global environmental change.

Beside the standard NERC PhD funding, the project is supported by the HiFreq H2020 RISE project, providing unique training and international exchange opportunities.