Global separation of plant transpiration from groundwater and streamflow

Journal name:
Nature
Volume:
525,
Pages:
91–94
Date published:
DOI:
doi:10.1038/nature14983
Received
Accepted
Published online

Current land surface models assume that groundwater, streamflow and plant transpiration are all sourced and mediated by the same well mixed water reservoir—the soil. However, recent work in Oregon1 and Mexico2 has shown evidence of ecohydrological separation, whereby different subsurface compartmentalized pools of water supply either plant transpiration fluxes or the combined fluxes of groundwater and streamflow. These findings have not yet been widely tested. Here we use hydrogen and oxygen isotopic data (2H/1H (δ2H) and 18O/16O (δ18O)) from 47 globally distributed sites to show that ecohydrological separation is widespread across different biomes. Precipitation, stream water and groundwater from each site plot approximately along the δ2H/δ18O slope of local precipitation inputs. But soil and plant xylem waters extracted from the 47 sites all plot below the local stream water and groundwater on the meteoric water line, suggesting that plants use soil water that does not itself contribute to groundwater recharge or streamflow. Our results further show that, at 80% of the sites, the precipitation that supplies groundwater recharge and streamflow is different from the water that supplies parts of soil water recharge and plant transpiration. The ubiquity of subsurface water compartmentalization found here, and the segregation of storm types relative to hydrological and ecological fluxes, may be used to improve numerical simulations of runoff generation, stream water transit time and evaporation–transpiration partitioning. Future land surface model parameterizations should be closely examined for how vegetation, groundwater recharge and streamflow are assumed to be coupled.

At a glance

Figures

  1. [dgr]18O and [dgr]2H values of groundwater, stream water, plant xylem water and soil water at 47 globally distributed sites.
    Figure 1: δ18O and δ2H values of groundwater, stream water, plant xylem water and soil water at 47 globally distributed sites.

    The median (interquartile range) δ18O and δ2H values are: a, groundwater: −7.7 (7.4), −51.5 (62.6), n = 2,749; b, stream water: −6.2 (8.8), −37.1 (66.9), n = 336; c, plant xylem water: −5.5 (6.1), −50.6 (50.6), n = 1,460; d, soil water: −7.5 (7.4), −63.9 (52.2), n = 1,830. The inset in a shows the locations of 47 globally distributed stable isotopic data sets. The histogram borders show partitioning of the data sets at 30 identical intervals or bins. The global meteoric water line (GMWL13) is also shown. V-SMOW, Vienna-standard mean ocean water.

  2. Precipitation offset values of groundwater, stream water, plant xylem water and soil water for 47 sites grouped by biome.
    Figure 2: Precipitation offset values of groundwater, stream water, plant xylem water and soil water for 47 sites grouped by biome.

    Extents of plant xylem (white) and soil (grey) water bars show 25th and 75th percentiles. All values of groundwater (squares) are shown for visualization of data density (that is, darker regions) and dispersion (that is, lighter regions). Mean values of stream water (circles) are also shown, as are the transpiration-amount-weighted values of plant xylem water (triangles).

  3. Schematic representation of tracing the isotopic composition of source precipitation.
    Extended Data Fig. 1: Schematic representation of tracing the isotopic composition of source precipitation.

    Plant xylem water isotopic values plot on a linear regression called the evaporation line. The point on the local meteoric water line (LMWL) where the plant xylem water evaporation line intersects provide a good approximation of the mean isotopic value of plant xylem source precipitation. The same method is used in tracing the soil water δ source value.

  4. Tracing the isotopic composition of plant xylem source precipitation versus mean groundwater value.
    Extended Data Fig. 2: Tracing the isotopic composition of plant xylem source precipitation versus mean groundwater value.

    Plant xylem water (grey triangles, n = 88) plotted in δ18O–δ2H space. Shown are the mean plant xylem source precipitation value (green triangle with error bars, ±1 s.d., n = 88), mean groundwater value (blue circle with error bars, ±1 s.d., n = 271), amount-weighted average precipitation (yellow star), GMWL (solid black line) and LMWL (dashed black line). This is an example of a case in Oregon, USA (ref. 1) where mean groundwater isotope value is more positive than plant xylem source precipitation value. This is the case in 41 of 47 sites in our database.

  5. The difference between plant xylem [dgr]-source precipitation values and mean groundwater [dgr]2H values, plotted against increasing distance of groundwater locations from actual plant xylem study sites.
    Extended Data Fig. 3: The difference between plant xylem δ-source precipitation values and mean groundwater δ2H values, plotted against increasing distance of groundwater locations from actual plant xylem study sites.

    The extents of the boxes show the 25th and 75th percentiles; whiskers show the extents of outliers. Also shown are median (interquartile range) values (P > 0.90, Tukey–Kramer honest significant difference) for five (n = 7; n = 8; n = 7; n = 9; n = 11) arbitrary distance ranges.

  6. Groundwater and plant xylem source precipitation.
    Extended Data Fig. 4: Groundwater and plant xylem source precipitation.

    Plot of δ18O versus δ2H for global plant xylem water (green triangles, n = 1,460), soil water (grey circles, n = 1,830), and groundwater (blue circles, n = 2,749). Also shown are the isotopic composition of source precipitation that leads to groundwater recharge (blue circle with error bars, mean ± 1 s.d.) and precipitation that leads to plant water uptake (green triangle with error bars, mean ± 1 s.d.). The inset shows the linear regression of plant xylem water and soil water, forming distinct evaporation lines (ELs) whereby, at a site level, plant xylem water is completely bounded by soil water. Also shown are GMWL and LMWL in the main plot and inset, respectively.

  7. Comparison of plant xylem (black boxes) and soil water (grey boxes) [dgr]18O, based on water extraction techniques.
    Extended Data Fig. 5: Comparison of plant xylem (black boxes) and soil water (grey boxes) δ18O, based on water extraction techniques.

    Cryogenic vacuum (n = 2,640) and azeotropic distillation (n = 441) are significantly different from liquid–vapour equilibration methods (n = 204) (P < 0.0001, nonparametric Dunn method for joint ranking). Cryogenic vacuum and azeotropic distillation are not significantly different from each other (P = 0.35, nonparametric Dunn method for joint ranking). The extents of the boxes show the 25th and 75th percentiles; whiskers show the extents of outliers. Also shown are median (interquartile range) values for each water type and water extraction technique.

  8. Global map of plant xylem water precipitation offsets from 47 study sites.
    Extended Data Fig. 6: Global map of plant xylem water precipitation offsets from 47 study sites.

Tables

  1. Site-by-site source precipitation [dgr] values for plant xylem water, groundwater and soil water
    Extended Data Table 1: Site-by-site source precipitation δ values for plant xylem water, groundwater and soil water
  2. Site-by-site soil water precipitation offset values
    Extended Data Table 2: Site-by-site soil water precipitation offset values
  3. Biome-level soil water precipitation offset values
    Extended Data Table 3: Biome-level soil water precipitation offset values

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Author information

Affiliations

  1. Global Institute for Water Security and School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan S7N 3H5, Canada

    • Jaivime Evaristo &
    • Jeffrey J. McDonnell
  2. Department of Geography, University of Calgary, Calgary, Alberta T2N IN4, Canada

    • Scott Jasechko
  3. School of Geosciences, University of Aberdeen, Aberdeen AB34 3FX, UK

    • Jeffrey J. McDonnell
  4. Department of Forest Engineering, Resources and Management, Oregon State University, Corvallis, Oregon 97331, USA

    • Jeffrey J. McDonnell

Contributions

J.J.M. conceived the idea of testing the ecohydrological compartmentalization hypothesis with global data. J.E., S.J. and J.J.M. brainstormed on how to do this. J.E. designed the approach, compiled the data set, and conducted the statistical analyses. J.E. wrote the first paper draft. S.J. and J.J.M. edited and commented on the manuscript and contributed to the text in later iterations.

Competing financial interests

The authors declare no competing financial interests.

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Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Schematic representation of tracing the isotopic composition of source precipitation. (79 KB)

    Plant xylem water isotopic values plot on a linear regression called the evaporation line. The point on the local meteoric water line (LMWL) where the plant xylem water evaporation line intersects provide a good approximation of the mean isotopic value of plant xylem source precipitation. The same method is used in tracing the soil water δ source value.

  2. Extended Data Figure 2: Tracing the isotopic composition of plant xylem source precipitation versus mean groundwater value. (122 KB)

    Plant xylem water (grey triangles, n = 88) plotted in δ18O–δ2H space. Shown are the mean plant xylem source precipitation value (green triangle with error bars, ±1 s.d., n = 88), mean groundwater value (blue circle with error bars, ±1 s.d., n = 271), amount-weighted average precipitation (yellow star), GMWL (solid black line) and LMWL (dashed black line). This is an example of a case in Oregon, USA (ref. 1) where mean groundwater isotope value is more positive than plant xylem source precipitation value. This is the case in 41 of 47 sites in our database.

  3. Extended Data Figure 3: The difference between plant xylem δ-source precipitation values and mean groundwater δ2H values, plotted against increasing distance of groundwater locations from actual plant xylem study sites. (113 KB)

    The extents of the boxes show the 25th and 75th percentiles; whiskers show the extents of outliers. Also shown are median (interquartile range) values (P > 0.90, Tukey–Kramer honest significant difference) for five (n = 7; n = 8; n = 7; n = 9; n = 11) arbitrary distance ranges.

  4. Extended Data Figure 4: Groundwater and plant xylem source precipitation. (342 KB)

    Plot of δ18O versus δ2H for global plant xylem water (green triangles, n = 1,460), soil water (grey circles, n = 1,830), and groundwater (blue circles, n = 2,749). Also shown are the isotopic composition of source precipitation that leads to groundwater recharge (blue circle with error bars, mean ± 1 s.d.) and precipitation that leads to plant water uptake (green triangle with error bars, mean ± 1 s.d.). The inset shows the linear regression of plant xylem water and soil water, forming distinct evaporation lines (ELs) whereby, at a site level, plant xylem water is completely bounded by soil water. Also shown are GMWL and LMWL in the main plot and inset, respectively.

  5. Extended Data Figure 5: Comparison of plant xylem (black boxes) and soil water (grey boxes) δ18O, based on water extraction techniques. (77 KB)

    Cryogenic vacuum (n = 2,640) and azeotropic distillation (n = 441) are significantly different from liquid–vapour equilibration methods (n = 204) (P < 0.0001, nonparametric Dunn method for joint ranking). Cryogenic vacuum and azeotropic distillation are not significantly different from each other (P = 0.35, nonparametric Dunn method for joint ranking). The extents of the boxes show the 25th and 75th percentiles; whiskers show the extents of outliers. Also shown are median (interquartile range) values for each water type and water extraction technique.

  6. Extended Data Figure 6: Global map of plant xylem water precipitation offsets from 47 study sites. (297 KB)

Extended Data Tables

  1. Extended Data Table 1: Site-by-site source precipitation δ values for plant xylem water, groundwater and soil water (297 KB)
  2. Extended Data Table 2: Site-by-site soil water precipitation offset values (142 KB)
  3. Extended Data Table 3: Biome-level soil water precipitation offset values (68 KB)

Additional data