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\begin{document}

\title{Uncovering the hillslope scale flow and transport dynamics in an
experimental hydrologic system}



\author[1]{Minseok Kim}%
\author[2]{Till Volkmann}%
\author[1]{Bugaj Aaron}%
\author[1]{Yadi Wang}%
\author[1]{Antônio Meira Neto}%
\author[1]{Katarena Matos}%
\author[3]{Ciaran Harman}%
\author[1]{Peter Troch}%
\affil[1]{The University of Arizona}%
\affil[2]{Accenture}%
\affil[3]{Johns Hopkins University}%


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  \date{May 22, 2024}


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\textbf{Uncovering the hillslope scale flow and transport dynamics in an
experimental hydrologic system}

Minseok Kim\textsuperscript{1}, Till H. M.
Volkmann\textsuperscript{1,2}, Aaron Bugaj\textsuperscript{1}, Yadi
Wang\textsuperscript{3}, Ant\selectlanguage{ngerman}ônio A. Meira Neto\textsuperscript{4},
Katarena Matos\textsuperscript{4}, Ciaran J.
Harman\textsuperscript{5,6}, Peter A. Troch\textsuperscript{1,4}

\textsuperscript{1}Biosphere 2, University of Arizona, Tucson, AZ,
USA,\textsuperscript{2}Applied Intelligence, Accenture, Kronberg im
Taunus, Germany, \textsuperscript{3}Department of Environmental Science,
University of Arizona, Tucson, AZ, USA, \textsuperscript{4}Department of
Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ,
USA, \textsuperscript{5}Department of Environmental Health and
Engineering, Johns Hopkins University, Baltimore, MD,
USA,\textsuperscript{6}Department of Earth and Planetary Sciences, Johns
Hopkins University, Baltimore, MD, USA

Hillslope scale water flow and transport dynamics have been extensively
studied (Burt \& McDonnell, 2015; Hewlett \& Hibbert, 1963), but
observing those internal dynamics in high spatial and temporal
resolutions remains challenging. In this study, we uncover internal
water flow and transport dynamics in an artificial hillslope in the
Landscape Evolution Observatory (LEO), Biosphere 2, University of
Arizona, Tucson, USA, using the experimental dataset collected in
December 2016. Complete information about the hillslope and experiment
can be found elsewhere (Pangle et al., 2015; Till H. M. Volkmann et al.,
2018); Here, we only summarize some relevant information.

The first part of the animation describes the experimental system and
setup (time 00:12 -- 04:14 in Animation S1). The LEO hillslope is 330
m\textsuperscript{3} (30 m long, 11 m wide, and 1 m deep) sloping soil
lysimeter\textbf{.} The hillslope is primarily made up of loamy-sand
textured basaltic tephra, and the most downslope 5.5
m\textsuperscript{3} is filled with gravel-textured basaltic tephra. A
custom irrigation system supplies reverse osmosis filtered water onto
the LEO surface. The downslope boundary is exposed to atmospheric
pressure, creating the seepage face boundary condition. The sensor
networks (including pressure transducers and volumetric water content
sensors) and the water isotope sampling locations and intervals (7 hrs
to 101 hrs) are illustrated in Animation S1 (time 02:09 -- 03:01). The
isotope composition of subsurface water is obtained from laser-based
online measurements of vapor that is extracted via custom gas probes
through equilibrium calculation (T. H.M. Volkmann \& Weiler, 2014). The
irrigation sequence of this experiment was designed to generate a
periodic steady state, which allows the application of the PERidoic
Tracer Hierarchy method (Harman \& Kim, 2014) for the observation of the
time-variable transit time distributions and the StorAge Selection
functions. Deuterium-labeled water was irrigated during the first two
irrigation events.

The second part of the animation shows the dynamics of the perched water
table and soil water content (time 04:15 -- 06:53). The extent of the
saturated zone was estimated using the pressure transducer data and
Delaunay triangulation (Delaunay, 1934)\textbf{.} The experimental data
show the saturation from below mechanisms---wetting up from the bedrock
surface into the soil profile (McDonnell, 1997)---and the saturation
from downslope to upslope. The water table profile forms a wedge-like
shape, which is a characteristic of hillslope with a high hillslope
(Peclet) number (Berne et al., 2005; Brutsaert, 1994). The hillslope
Peclet number of the LEO hillslope during the experiment is high
(\textgreater{} 10) (Kim et al., 2020)\textbf{.} Significant time delays
in the water table dynamics are observed at some upslope locations
(e.g., at 13 m upslope), which is mostly due to the delayed water supply
from the convergent upslope area. The water content data indicates that
the convergent upslope water content began to decrease around the timing
of the water table peak at 13 m upslope.

The third part of the animation shows the tracer dynamics (from time
06:43). The animated experimental data reveal two notable water
transport dynamics. First, the vertical tracer movement is faster at the
upslope. This faster movement at the upslope is, in a sense,
counter-intuitive because the upslope region is drier than the
downslope. This is due to the lateral flow in the saturated zone and the
tension saturated zone, that are thicker at the downslope. While water
velocity is higher at the downslope, the direction of velocity is not
vertical but rotated towards the downslope in those
zones\textbf{.}Second, the animated data illustrate that old water is
present only at the downslope. This observation is a characteristic of
hillslope with a high hillslope number, in which old water is
preferentially discharged (Kim et al., 2020)\textbf{.} Indeed, the
observed SAS function in this hillslope is concave (Kim et al., 2020),
indicating that the hillslope preferentially discharges old water that
is stored at the downslope.

\textbf{Acknowledgment}

This work is supported by the National Science Foundation under grants
EAR-1344552 and EAR-1417175. The authors also gratefully acknowledge
support from the Philecology Foundation of Fort Worth, Texas. Additional
funding support was provided by the Office of the Vice President of
Research at the University of Arizona and by the Technology and Research
Initiative Fund (TRIF) Water, Environmental, and Energy Solutions (WEES)
initiative at the University of Arizona (Shared Equipment Enhancement
Funds). The animation is accessible at:
http://b2.arizona.edu/LEO\_2016/LEO\_PERTH\_EXP2016.mp4\textbf{.}

\textbf{References}

Berne, A., Uijlenhoet, R., \& Troch, P. A. (2005). Similarity analysis
of subsurface flow response of hillslopes with complex
geometry.\emph{Water Resources Research} , \emph{41} , 1--10.
https://doi.org/10.1029/2004WR003629

Brutsaert, W. (1994). The unit response of groundwater outflow from a
hillslope. \emph{Water Resources Research} , \emph{30} (10), 2759--2763.
https://doi.org/10.1029/94WR01396

Burt, T. P., \& McDonnell, J. J. (2015). Whither field hydrology? The
need for discovery science and outrageous hydrological
hypotheses.\emph{Water Resources Research} , \emph{51} (8), 5919--5928.
https://doi.org/10.1002/2014WR016839

Delaunay, B. N. (1934). Sur la sphère vide. \emph{Bulletin of Academy of
Sciences of the USSR} , \emph{6} , 793--800.

Harman, C. J., \& Kim, M. (2014). An efficient tracer test for
time-variable transit time distributions in periodic hydrodynamic
systems. \emph{Geophysical Research Letters} , 1567--1575.
https://doi.org/10.1002/2013GL058980

Hewlett, J. D., \& Hibbert, A. R. (1963). Moisture and energy conditions
within a sloping soil mass during drainage. \emph{Journal of Geophysical
Research} , \emph{68} (4), 1081--1087.
https://doi.org/10.1029/JZ068i004p01081

Kim, M., Volkmann, T. H. M., Wang, Y., Harman, C. J., \& Troch, P. A.
(2020). Direct observation of hillslope scale StorAge Selection
functions in an experimental hydrologic system: Geomorphologic structure
and the preferential discharge of old water. \emph{Earth and Space
Science Open Archive} , 44. https://doi.org/10.1002/essoar.10504485.1

McDonnell, J. J. (1997). Comments on ``The changing spatial variability
of subsurface flow across a hillside'' by Ross Woods and Lindsay
Rowe.\emph{Journal of Hydrology (NZ)} , \emph{36} (1), 97--100.

Pangle, L. A., DeLong, S. B., Abramson, N., Adams, J., Barron-Gafford,
G. A., Breshears, D. D., Brooks, P. D., Chorover, J., Dietrich, W. E.,
Dontsova, K., Durcik, M., Espeleta, J., Ferre, T. P. A., Ferriere, R.,
Henderson, W., Hunt, E. a., Huxman, T. E., Millar, D., Murphy, B.,
\ldots{} Zeng, X. (2015). The Landscape Evolution Observatory: A
large-scale controllable infrastructure to study coupled Earth-surface
processes. \emph{Geomorphology} , \emph{August} .
https://doi.org/10.1016/j.geomorph.2015.01.020

Volkmann, T. H.M., \& Weiler, M. (2014). Continual in situ monitoring of
pore water stable isotopes in the subsurface. \emph{Hydrology and Earth
System Sciences} , \emph{18} (5), 1819--1833.
https://doi.org/10.5194/hess-18-1819-2014

Volkmann, Till H. M., Sengupta, A., Pangle, L. A., Dontsova, K.,
Barron-Gafford, G. A., Harman, C. J., Niu, G.-Y., Meredith, L. K.,
Abramson, N., Neto, A. A. M., Wang, Y., Adams, J. R., Breshears, D. D.,
Bugaj, A., Chorover, J., Cueva, A., DeLong, S. B., Durcik, M., Ferre, T.
P. A., \ldots{} Troch, P. A. (2018). Controlled Experiments of Hillslope
Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward
Prediction of Coupled Hydrological, Biogeochemical, and Ecological
Change. In J. Liu \& W. Gu (Eds.), \emph{Hydrology of Artificial and
Controlled Experiments} . InTech.
https://doi.org/10.5772/intechopen.72325

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