Using isotopes to incorporate tree water storage and mixing dynamics into a distributed ecohydrologic modelling framework

Knighton, James; Kuppel, Sylvain; Smith, Aaron; Soulsby, Chris; Sprenger, Matthias; Tetzlaff, Doerthe

doi:10.1002/eco.2201

Cited by 67 publications

(113 citation statements)

References 87 publications

(189 reference statements)

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“…Relative contributions of the upper and lower zones to streamflow varied widely among calibration trials; thus the correct separation remains indeterminate, even with the addition of isotopes. Simulated flow partitioning may be verifiable with higher temporal resolutions of isotope data (to distinguish local flow velocities) or by sampling isotope concentrations in soil water and/or xylem water (Knightly et al, 2020). It is notable that, even with a coarse resolution and irregularly sampled isotope data set, the isotope‐informed calibrations produce results that are distinct from the flow‐only calibrations and which agree with the findings from previous modeling studies in much smaller, more intensively monitored basins (e.g., Dehaspe et al, 2018; Tunaley et al, 2017).…”

Section: Discussionmentioning

confidence: 99%

Regional Calibration With Isotope Tracers Using a Spatially Distributed Model: A Comparison of Methods

Holmes

Stadnyk

Kim

et al. 2020

Water Resources Research

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Accurate representation of flow sources in process-based hydrologic models remains challenging for remote, data-scarce regions. This study applies stable isotope tracers (18 O and 2 H) in water as auxiliary data for the calibration of the isoWATFLOOD ™ model. The most efficient method of those evaluated for introducing isotope data into model calibration was the PA-DDS multiobjective search algorithm. The compromise solutions incorporating isotope data performed slightly inferior in terms of streamflow simulation compared to the calibrated solution using streamflow data only. However, the former solution outperformed the latter one in terms of isotope simulation. Approximation of the model parameter uncertainty into internal flow path partitioning was explored. Inclusion of isotope error facilitated a broader examination of the total parameter space, resulting in significant differences in internal storage and flow paths, most significantly for soil storage and evapotranspiration loss. Isotope-optimized calibration reduced evaporation rates and increased soil moisture content within the model, impacting soil water velocity but not streamflow celerity. Flow-only calibration resulted in artificially narrow model prediction bounds, significantly underestimating the propagation of parameter uncertainty, while isotope-informed calibrations yielded more reliable and robust bound on model predictions. Our findings demonstrate that the accuracy of a complex, spatially distributed, and process-based model cannot be judged from one summative flow-based model performance evaluation metric alone. Plain Language Summary To predict how water moves through watersheds and adequately anticipate hydrologic response to a changing climate, we depend on numerical models to estimate transport and storage of water on and below Earth's surface. These models are only as good as the observation data we have to test them against, which can be insufficient for many remote parts of the world. Our study introduces a new source of data (i.e., isotope tracers, a fingerprint of water's origin) into a model to improve parameter estimation and overall reliability. The largest benefit of adding these data is that it nudges the model into a more realistic distribution of water storage and flow paths to the rivers, particularly soil storage and evaporation loss. This is important for improving the reliability of model projections under changing climates and to better understand how the distribution of water on Earth is changing.

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Section: Discussionmentioning

confidence: 99%

Regional Calibration With Isotope Tracers Using a Spatially Distributed Model: A Comparison of Methods

Holmes

Stadnyk

Kim

et al. 2020

Water Resources Research

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“…The way in which processes and interactions relate to plant demand highlights the importance of the temporal integration of root uptake and water transport into the main plant stems. The non-stationary travel times from uptake to transpiration may average many months (Brinkmann et al, 2018), with tailing in the travel time distribution potentially a result of plant-stored water contributing to transpiration under dry conditions and possible mixing of xylem water with other plant water (Knighton et al, 2020).…”

Section: Seasonality Of Potential Soil Water Sourcesmentioning

confidence: 99%

Stable isotopes of water reveal differences in plant – soil water relationships across northern environments

Tetzlaff

Buttle

Carey

et al. 2021

Hydrological Processes

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We compared stable isotopes of water in plant stem (xylem) water and soil collected over a complete growing season from five well-known long-term study sites in northern/cold regions. These spanned a decreasing temperature gradient from Bruntland Burn (Scotland), Dorset (Canadian Shield), Dry Creek (USA), Krycklan (Sweden), to Wolf Creek (northern Canada). Xylem water was isotopically depleted compared to soil waters, most notably for deuterium. The degree to which potential soil water sources could explain the isotopic composition of xylem water was assessed quantitatively using overlapping polygons to enclose respective data sets when plotted in dual isotope space. At most sites isotopes in xylem water from angiosperms showed a strong overlap with soil water; this was not the case for gymnosperms. In most cases, xylem water composition on a given sampling day could be better explained if soil water composition was considered over longer antecedent periods spanning many months. Xylem water at most sites was usually most dissimilar to soil water in drier summer months, although sites differed in the sequence of change. Open questions remain on why a significant proportion of isotopically depleted water in plant xylem cannot be explained by soil water sources, particularly for gymnosperms. It is recommended that future research focuses on the potential for fractionation to affect water uptake at the soil-root interface, both through effects of exchange between the vapour and liquid phases of soil water and the effects of mycorrhizal interactions. Additionally, in cold regions, evaporation and diffusion of xylem water in winter may be an important process.

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“…With the introduction of IRIS, rapid progress in terms of continuous measurements and field-deployable systems began. Koehler and Wassenaar (2011) were the first to show that unattended, continuous measurements of the water isotope composition of natural water samples (lakes, rivers and groundwater) based on isotopic equilibration between liquid and vapor phases are possible by using a gaspermeable membrane contactor connected to a laser spectroscope. A similar gas-permeable membrane system was tested by Munksgaard et al (2011).…”

Section: In Situ Soil Water Isotope Depth Profilesmentioning

confidence: 99%

In situ measurements of soil and plant water isotopes: a review of approaches, practical considerations and a vision for the future

Beyer

Kühnhammer

Dubbert

2020

Hydrol. Earth Syst. Sci.

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Abstract. The number of ecohydrological studies involving water stable isotope measurements has been increasing steadily due to technological (e.g., field-deployable laser spectroscopy and cheaper instruments) and methodological (i.e., tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water-isotope-based methods into their studies. Several isotope effects are currently not fully understood but might be essential when investigating root water uptake depths of vegetation and separating isotope processes in the soil–vegetation–atmosphere continuum. Different viewpoints exist on (i) extraction methods for soil and plant water and methodological artifacts potentially introduced by them, (ii) the pools of water (mobile vs. immobile) measured with those methods, and (iii) spatial variability and temporal dynamics of the water isotope composition of different compartments in terrestrial ecosystems. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil–plant–atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight the potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review in order to provide a guideline for researchers with limited previous experience with in situ methods. We also include a section on opportunities for incorporating data obtained with described in situ methods into existing isotope-enabled ecohydrological models and provide examples illustrating potential benefits of doing so. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes in situ when carrying out studies at the soil–vegetation–atmosphere continuum. Several authors have shown that reliable data can be generated in the field using in situ methods for measuring the soil water isotope composition. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been paid to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty and existing methodological uncertainties. Future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision and accuracy.

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Using isotopes to incorporate tree water storage and mixing dynamics into a distributed ecohydrologic modelling framework

Cited by 67 publications

References 87 publications

Regional Calibration With Isotope Tracers Using a Spatially Distributed Model: A Comparison of Methods

Regional Calibration With Isotope Tracers Using a Spatially Distributed Model: A Comparison of Methods

Stable isotopes of water reveal differences in plant – soil water relationships across northern environments

In situ measurements of soil and plant water isotopes: a review of approaches, practical considerations and a vision for the future

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