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.
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J.E. thanks the Saskatchewan Innovation and Opportunity Scholarship, Global Institute for Water Security, and School of Environment and Sustainability (University of Saskatchewan) for financial support.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 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.
Extended Data Figure 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.
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.
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.
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.
Extended Data Figure 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.
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Evaristo, J., Jasechko, S. & McDonnell, J. Global separation of plant transpiration from groundwater and streamflow. Nature 525, 91–94 (2015). https://doi.org/10.1038/nature14983
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