Testing the ‘two water worlds’ hypothesis under variable preferential flow conditions

Abstract

Widespread observations of ecohydrological separation are interpreted by suggesting that water flowing through highly conductive soil pores resists mixing with matrix storage over periods of days to months (i.e., two ‘water worlds’ exist). These interpretations imply that heterogeneous flow can produce ecohydrological separation in soils, yet little mechanistic evidence exists to explain this phenomenon. We quantified the separation between mobile water moving through preferential flow paths versus less mobile water remaining in the soil matrix after free‐drainage to identify the amount of preferential flow necessary to maintain a two water world's scenario. Soil columns of varying macropore structure were subjected to simulated rainfall of increasing rainfall intensity (26 mm h−1, 60 mm h−1, and 110 mm h−1) whose stable isotope signatures oscillated around known baseline values. Prior to rainfall, soil matrix water δ2H nearly matched the known value used to initially wet the pore space whereas soil δ18O deviated from this value by up to 3.4‰, suggesting that soils may strongly fractionate 18O. All treatments had up to 100% mixing between rain and matrix water under the lowest (26 mm h−1) and medium (60 mm h−1) rainfall intensities. The highest rainfall intensity (110 mm h−1), however, reduced mixing of rain and matrix water for all treatments and produced significantly different preferential flow estimates between columns with intact soil structure compared to columns with reduced soil structure. Further, artificially limiting exchange between preferential flow paths and matrix water reduced bypass flow under the most intense rainfall. We show that (1) precipitation offset metrics such as lc‐excess and d‐excess may yield questionable interpretations when used to identify ecohydrological separation, (2) distinct domain separation may require extreme rainfall intensities and (3) domain exchange is an important component of macropore flow.

A spectrum of preferential flow alters solute mobility in soils

“…Suction cups can also have biased representation of water in larger—more “mobile”—pores 89 – 91 . We note that, though matrix and macropore waters can resist mixing during extreme rainfall 92 , complete mixing between pores can occur within days 92 – 94 . Thus, point measurements from our samplers (after 7 days of rainfall exclusion and equilibration) likely yielded representative samples of pre-event water from the matrix and labeled event water from mobile water in maropores, while capturing a wide range of stable isotope signatures.…”

Improved Lumped‐Parameter and Numerical Modeling of Unsaturated Water Flow and Stable Water Isotopes

“…Further, to better describe short time fluctuation and measured peaks we extend the traditional LPM approach by implementing a dual‐permeability system (separation of transport through subsurface matrix and along preferential flow paths) and consideration of possible contribution of immobile water. The importance of preferential flow on adequately characterizing unsaturated zone flow and transport processes has been studied intensively by Stumpp et al (2009c), Isch et al (2019), Benettin et al (2019), and Radolinski et al (2021). The combination of LPM and preferential flow has successfully been applied by Stumpp et al (2007) and Shajari et al (2020).…”

Surface‐to‐tile drain connectivity and phosphorus transport: Effect of antecedent soil moisture

“…Combining laboratory and field approaches allowed for multiple lines of evidence for assessing hydrologic processes and nutrient transport as a function of antecedent soil moisture. Future work using a combination of laboratory and field approaches could further our understanding of the processes explored herein by, for example, creating artificial soil horizons in repacked boxes or columns to mimic field soil physical and chemical properties (e.g., Radolinski et al, 2021), using undisturbed soil boxes or different soil types, and improving sampling frequency (Williams, King, Ford, Buda, & Kennedy, 2016) and using multiple tracers (e.g., dye tracers) in the field to quantify flow paths. Further, more in‐depth analysis of the Ohio edge‐of‐field network data could be explored to examine the impact of rainfall patterns on nutrient transport at a broader scale.…”

“…Although highly artificial, this approach eliminates the influence of textural and structural variations on the soil's physical behaviour and permits study of the impact of individual macropore properties (Akay et al, 2008). For example, using repacked soil columns with single (or multiple) artificial macropores has been used to examine the effect of tillage (Logsdon, 1995), macropore characteristics (Akay & Fox, 2007; Allaire‐Leung et al, 2000), and matrix‐macropore interaction (Germer & Braun, 2015; Radolinski et al, 2021). Many artificial macropore studies however are subject to a seepage face bottom boundary condition as opposed to a subsurface drained bottom.…”