What Ecohydrologic Separation Is and Where We Can Go With It

Abstract

The “ecohydrologic separation” hypothesis challenged assumptions of translatory flow through the rooting zone. However, studies aiming to test ecohydrologic separation have largely done so with insufficient discussion of infiltration and rooting zone recharge processes and instead have mostly focused on either isotopic differences between stream water and plant water or the presence of fractionated isotope ratios in plant water. Based on extensively observed heterogeneities in soils and watersheds, we posit that differences in isotopic compositions of water in plants, streams, and other subsurface pools are expected in most scenarios. Interpretation of those plant‐and‐stream water isotopic differences is important, but diagnosing the role of any specific process is typically confounded by the diversity of potential mechanisms contributing to those isotopic differences. Thus, we should progress from simply describing their occurrences and refocus the discussion of ecohydrologic separation on how heterogeneous infiltration and root uptake processes lead to such differences. Consequently, we outline areas where plant and soil‐water stable isotope data may be useful for advancing our understanding and representation of soil‐water transport and plant‐water recharge.

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…In some cases, these effects can lead to biases and possibly incorrect interpretation of the results even at small scales. Some of these issues have been discussed in recent works (Penna et al, 2018;Barbeta et al, 2019Barbeta et al, , 2020Beyer et al, 2020;Sprenger and Allen, 2020;Von Freyberg et al, 2020) but in this opinion paper we aim at summarizing them in a comprehensive way and stimulating community efforts to address uncertainty in isotope-base studies of vegetation water sources and uncertainty propagation across spatial scales.…”

“…f) Plant traits such as species, age, size, leaf area index, specific leaf area, and root plasticity (Dubbert and Werner, 2019). g) Soil evaporation, different size of soil pores, and related complex flow pathways (mixing processes between soil water of different layers, and between water in the unsaturated rootzone and in the saturated zone; capillary rise) resulting in non-monotonic water age and isotope soil profiles (Sprenger and Allen, 2020;Von Freyberg et al, 2020). h) Mineral-mediated fractionation in soil samples (Oerter et al, 2014;Gaj et al, 2017).…”

An improved practical approach for estimating catchment‐scale response functions through wavelet analysis

“…Whereas the Piston Flow TTD represents quick responses from short‐term storages such as overland and near surface flow, the Gamma TTD represents a combination of quick responses from short‐term storages and long‐term memory of the subsurface reservoir [Kirchner et al, 2001]. This framework is supported by recent work at MGC that used multiple tracers and end‐member mixing analysis to show that near‐surface flows and soil water storages are the dominant contributors to streamflow [Dwivedi, Meixner, et al, 2019], and thus that an incompletely mixed reservoir provides a more realistic model of subsurface water storage [Sprenger & Allen, 2020; Sprenger, Leistert, Gimbel, & Weiler, 2016; Sprenger et al, 2018; Zhao, Tang, Zhao, Wang, & Tang, 2013].…”

“…The tables in plots (a) through (c) show the best-fit model parameters for the various ETTD types where parameter 1 is the mean age (in years) and parameter 2 (not applicable for the piston flow ETTD) is the shape parameter (dimensionless) for the Exp and gamma ETTDs and the Péclet number for the ADE-1x and ADE-nx ETTDs memory of the subsurface reservoir [Kirchner et al, 2001]. This framework is supported by recent work at MGC that used multiple tracers and end-member mixing analysis to show that near-surface flows and soil water storages are the dominant contributors to streamflow , and thus that an incompletely mixed reservoir provides a more realistic model of subsurface water storage [Sprenger & Allen, 2020;Sprenger, Leistert, Gimbel, & Weiler, 2016;Sprenger et al, 2018;Zhao, Tang, Zhao, Wang, & Tang, 2013].…”

Clarifying misconceptions regarding the relationship between Hewlett and Hibbert's translatory flow process and ecohydrological separation

“…However, this contrast only holds if translatory flow results in the complete displacement of pre‐event soil water, which HH did not argue for and which their citation of the Horton and Hawkins (1965) experiment did not support. Figure 1 of Sprenger and Allen's (2020) commentary on the future of the ecohydrological separation hypothesis also suggested that translatory flow leads to complete vertical displacement of pre‐event soil water, which ignores the considerable evidence for incomplete displacement of soil water by infiltration inputs provided by Goldsmith et al (2012) and the references therein.…”