EGU2020-646
https://doi.org/10.5194/egusphere-egu2020-646
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Simulating preferential flow in a two water worlds framework

Jesse Radolinski1, Luke Pangle2, Julian Klaus3, Durelle Scott4, and Ryan Stewart5
Jesse Radolinski et al.
  • 1University of Innsbruck, Institute of Ecology, Austria (jesse.radolinski@uibk.ac.at)
  • 2Georgia State University, Geosciences Department, USA (lpangle@gsu.edu)
  • 3Luxembourg Institute of Science and Technology, Environmental Research and Innovation, Luxemburg (julian.klaus@list.lu)
  • 4Virginia Polytechnic Institute, Department of Biological Systems Engineering USA (dscott@vt.edu)
  • 5Virginia Polytechnic Institute, School of Plant and Environmental Sciences, USA (rds@vt.edu)

Ecohydrological separation has been observed across climates and biomes, and at a fundamental level suggests that water in mobile versus immobile domains may resist mixing over varying periods of time; however little mechanistic evidence exists to explain this separation at a process scale. Non-equilibrium flow in the vadose zone may partially account for widespread perception of distinct hydrological domains yet no studies have weighed its contribution. Using a simple isotope mixing technique, we sought to determine the amount of preferential flow necessary to maintain a two water worlds scenario (i.e., physical separation between mobile and immobile water pools). We constructed 60 cm soil columns (20 cm-ID PVC) containing low soil structure (sieved soil material), subsoil structure (intact B horizon), and soil structure without matrix exchange (tubing reinforced macropores) to simulate multiple preferential flow scenarios. Columns were subjected to 3 rain storms of varying rainfall intensity (~2.5 cm h -1, ~5 cm h -1, and ~11 cm h -1) whose stable isotope signatures oscillated around known baseline values. Isotopic analysis was performed on collected leachate and matrix water sampled via direct vapor equilibration. Preliminary estimates of matrix water indicate up to 100% mixing with infiltrating rain water under low rainfall intensity (2.5 cm h -1) in subsoil structure columns, whereas high intensity rain (11 cm h-1) produced clear separation between columns with intact or artificial soil structure and those controlled for structure (low structure treatment). This separation was confirmed by preferential flow estimates; however minimizing matrix exchange (via artificial macropores) reduced preferential flow by a factor of 4 compared to soil with intact structure. These data suggest that distinct domain separation may only be possible under extreme precipitation intensity; and that exchange with less mobile storage in the soil matrix produces more preferential flow. We intend to use these estimates of preferential flow as a benchmark to understand the prevalence, persistence, and plausibility of ecohydrological separation. As a next step, we will use this conceptual framework to define how recurrent drought, elevated CO2, and warming may alter the partitioning of mobile and immobile water in mountain grasslands.

How to cite: Radolinski, J., Pangle, L., Klaus, J., Scott, D., and Stewart, R.: Simulating preferential flow in a two water worlds framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-646, https://doi.org/10.5194/egusphere-egu2020-646, 2019

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Display material version 2 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-646, Anke Hildebrandt, 06 May 2020

    Nice follow up on the two water worlds hypothesis. I am wondering: How moist were the soil columns when you subjected them to the storm treatment? Were you able to repeat the experiment for different moisture conditions?

    • AC1: Reply to CC1, Jesse Radolinski, 06 May 2020

      Thanks for the comment. Columns were all at ~ field capacity (around 0.30 m3/m3) when each event began. We allowed draining for 72 h after/in between the events (and initial saturation).

      No, I wish we could have subjected the columns to varying mositure levels (was tossed around as a main goal). We also figured separation may be highest between domains during wet-season (osciallting around field capacity) and chose to control rainfall intensity rather than antecedent mositure, for example. 

  • CC2: Pre-treatment, fractionation 18O?, Matthias Sprenger, 07 May 2020

    Hi Jesse,

    I remember good discussion at this poster last year at the Grodon Conference. Cool experiment!

    Could you please clarify how you treated the soil prior to the irrigation experiments? Was the soil dried? If so, at what temperature and for how long?

    I ask to better understand the difference between Soil Water and Drainage as shown in your Figure 1. You explain it by potential fractionation taking place in the soil. I understand that you do not mean evaporation fractionation? I just wonder if that more enriched 18O signal could be a result from highly fractionated residual pore water after drying or after sampling of the soil and before the irrigation took place?

    Thanks,
    Matthias

    • AC2: Reply to CC2, Jesse Radolinski, 08 May 2020

      Sure Mathias, thanks for the comment.

      Soil was air dried before the experiment (I had read so many issues about potential physiochemical alterations with high heat in oven) which left around ~1% or water content at most in the soil. This was ~25-30 degC air temp. 

      Drainage is just leachate from the bottom of the column and soil water values refer to signatures taken destructively from different depths (0-5 cm, 25-30 cm, and 45-50 cm-- 3 taken per depth, per column) from direct vapor equilibration. The soil water values shown in the uploaded materials are volume-weighted from the destructive samples (one per column).

      Yes, there there should not be much evaporation fractionation here as we controlled for vapor losses at the top.

      I did a back-of-the envelope calculation asuming that the residual water was super enriched (14 delta per mil 18O--surface desert soil water) and fully mixed with 1 pore volume of saturation water. This gave a max enrichment of 0.5 delta 18O in soil water relative to saturation.  I also think that 14 delta is very high for residual water assuming only evaporation fractionation (from air drying).

      Also, this fractionation signature was apparent at all depths, in all treatments (maybe slightly more for "low" structure treatment). I think something is going at soil physiochemical level. People have suggested that oven-drying soil may alter the surface chemistry (e.g., burning off hydroxyls above 100 C) so that the propensity to enrich infiltrate (or deplete mobile water) is higher. I think we have ruled that out by air drying. I believe something is going on where maybe a thin film of soil water ( especially in more "reactive" soils and low water contents) can remain enriched relative to the bulk water. I think that it does mix with the bulk water but we cannot always detect the mixing accurately with stable isotopes because, in some scenarios, there is such a high matrix-affinity for heavier isotopologues (e.g., 18O may not be "conservative").

      Hoping that this paper will be out in HESS soonish,' but happy to discuss more.

      Jesse

  • CC3: Comment on EGU2020-646, Matthias Sprenger, 08 May 2020

    Thanks for the detailed response with the clarifications. Especially the back on the envelope calculations are very much appreciated that show that one would not get such a strong effect as I thought due to the residual enriched water.

    I am sure you are familiar with Eric Oerters (2014) work who made laboratory experiments on 18O fractionation.

    Looking forward to reading more about it soon then!

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