Scott Jasechko

Terrestrial water fluxes dominated by transpiration

Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000u2009km3 per year) (refs 1, 2, 3, 4, 5). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000u2009±u20098,000u2009km3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129u2009±u200932u2009gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

Global separation of plant transpiration from groundwater and streamflow

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 Oregon and Mexico 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.

The pronounced seasonality of global groundwater recharge

Groundwater recharged by meteoric water supports human life by providing two billion people with drinking water and by supplying 40% of cropland irrigation. While annual groundwater recharge rates are reported in many studies, fewer studies have explicitly quantified intra-annual (i.e., seasonal) differences in groundwater recharge. Understanding seasonal differences in the fraction of precipitation that recharges aquifers is important for predicting annual recharge groundwater rates under changing seasonal precipitation and evapotranspiration regimes in a warming climate, for accurately interpreting isotopic proxies in paleoclimate records, and for understanding linkages between ecosystem productivity and groundwater recharge. Here we determine seasonal differences in the groundwater recharge ratio, defined here as the ratio of groundwater recharge to precipitation, at 54 globally distributed locations on the basis of 18O/16O and 2H/1H ratios in precipitation and groundwater. Our analysis shows that arid and temperate climates have wintertime groundwater recharge ratios that are consistently higher than summertime groundwater recharge ratios, while tropical groundwater recharge ratios are at a maximum during the wet season. The isotope-based recharge ratio seasonality is consistent with monthly outputs from a global hydrological model (PCR-GLOBWB) for most, but not all locations. The pronounced seasonality in groundwater recharge ratios shown in this study signifies that, from the point of view of predicting future groundwater recharge rates, a unit change in winter (temperate and arid regions) or wet season (tropics) precipitation will result in a greater change to the annual groundwater recharge rate than the same unit change to summer or dry season precipitation.

Jasechko et al. reply

replying to A. M. J. Coenders-Gerrits et al. 506, http://dx.doi.org/10.1038/nature12925 (2014)In their Comment, Coenders-Gerrits et al. suggest that our conclusion that transpiration dominates the terrestrial water cycle is biased by unrepresentative input data and optimistic uncertainty ranges related to runoff, interception and the isotopic compositions of transpired and evaporated moisture. We clearly presented the uncertainties applied in our Monte-Carlo sensitivity analysis, we reported percentile ranges of results rather than standard deviations to best communicate the nonlinear nature of the isotopic evaporation model, and we highlighted that the uncertainty in our calculation remains large, particularly in humid catchments (for example, figure 2 in our paper).

Evaristo et al . reply

isotopic measurement and could therefore not be used for regression analysis. Figure 1a shows one example of the difference between the original estimate of Evaristo et al.1 (green triangle) and our revised estimate (orange triangle) of the plant water source (site ID 26). For most of the sites, our estimates differ considerably from those of Evaristo et al.1 (see Extended Data Figs 1, 2 and 3). Extended Data Figs 1, 2 and 3 also reveal that the data set is extremely heterogeneous in terms of the number of sampled points for plant and groundwater, with sometimes inconsistent data leadxad ing to unrealistic values of δ Ointersect and δ Hintersect for certain sites. Therefore, we applied two nonxadexclusive criteria to assess the consistency of the calculated intersection values: criterion 1, δ Hintersect < max(δ Hplant); and criterion 2, δ Hintersect > − 200‰. The first criterion implies that m < a, while the second criterion evaluates whether the plant water source hydrogen isotopic composition value is realistic. Eleven sites failed at least one of the two criteria (IDs 3, 7, 8, 17, 20, 21, 23, 24, 32, 36 and 44). Figure 1b shows one example of an inconsistent data set (site ID 3). Results of this analysis (summarized in Table 1) show that at 26 sites, where data were consistent, δ HGW was statistically different from δ Hintersect using the nonxadparametric Wilcoxon rank sum test (α = 0.05). In conclusion, rainfall segregation (as defined by Evaristo et al.1) could be observed for only 57% of the sites of the authors’ data set and at 74% of the sites with consistent estimates of the intercept as defined by our two criteria. Online Content Methods, along with any additional Extended Data display items and Source Data, are available in the online version of the paper; references unique to these sections appear only in the online paper.