Artificial hillslopes to study critical zone processes in climate controlled experiments
Twenty scientists1 representing a wide variety of Earth science disciplines (hydrology,
ecology, pedology, geomorphology, biogeochemistry, atmospheric sciences) gathered on
November 18-19, 2007 at Biosphere 2 to discuss the experimental design of artificial
hillslopes to be constructed in the Experimental Biome of the B2 EarthScience program
of the University of Arizona. B2 EarthScience conducts research in the earth system and
environmental sciences that examines soil-water-atmosphere-plant interactions.
Hillslopes offer a useful elementary scale to understand the role of water in the
landscape, and how life modifies the interaction of water with the landscape. Hillslopes
exhibit enormous complexity and heterogeneity, much of which is not easily observable.
The complexity is driven by the interactions between water, biogeochemistry, ecology
and soils, within the constraints set by the climate and the geologic history of the system.
These interactions create complex, non-random patterns and structures in space-time.
Using artificial hillslopes in a climate-controlled environment offers tremendous research
opportunities to advance our understanding of critical zone processes2.
The group discussed several topics related to the experimental design of artificial
hillslopes (see agenda in appendix A for more details). The aim of the meeting was to
reach community consensus about the different design parameters of such hillslope
experiments (geometry (shape, slope), lithology, biology, climate forcings), and to
discuss ways to use numerical modeling to test some of the proposed hillslope designs.
Constraints of the design are physical dimensions of the three bays in the Experimental
Biome (approx. 33m x 18m), radiation spectrum due to glass construction (discussed by
Joost van Haren, see agenda), the expected duration of the experiments (10 years), and
the focus of B2 EarthScience on semi-arid environments.
In summary, the group reached the following conclusions:
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1
The hillslopes should have a common lithology (e.g. crushed basalt);
Assuming there will be three replica, biology should be the varying factor
(e.g. bare with bacterial activity, grassland, grass and woody species);
Climate should vary along the time axis, with a constant climate the first few
years (representative of Ponderosa pine habitat at 2500m elevation in the
Catalinas), followed with climate change experiments (increased temperature,
changing precipitation regime, etc.), and ended with some extreme event
scenarios (multi-year drought);
The possibility of having variable bedrock slope should be investigated (this
could be realized by constructing hydraulic lifts);
The option of changing the permeability of the lower boundary (bedrock)
from impermeable to semi-permeable should be investigated;
See participation list and meeting agenda in appendix A
The critical zone is defined as that portion of the Earth’s surface that extends from the outer boundary of
the vegetation canopy to the lowest limit of groundwater. The term critical reflects the essential role of this
zone in sustaining environmental and human systems.
2
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Different hillslope shapes should be considered: zero-order basin (see Figure
1) versus planar slopes with variable bedrock topography (Figure 2).
It was decided that the coming months (January to March) some of these suggestions will
be investigated through numerical modeling. Three-dimensional Richards’ equation
based modeling (e.g. HYDRUS 3D or CATHY (see Paniconi and Putti, 2006) can be
used to study hydrologic regimes for different hillslope geometry configurations, and
should be compared to low-dimensional modeling (e.g. Harman, 2007) to generalize the
results through dimensional analyses. Off-line coupling with geochemistry models (e.g.
CRUNCH) will provide useful information about the link between hydrologic regime and
weathering processes, and on how weathering can lead to changes in subsurface structure
through deposition altering pore size distributions.
A follow-up workshop will be organized to discuss progress on numerical modeling and
how this informs the experimental design. Target date is end of March 2008.
References
C. Paniconi and M. Putti, “Atmosphere controls and soil moisture inputs for a coupled model of
surface–subsurface interactions,” IAHR International Conference on Groundwater
Hydraulics in Complex Environments, Toulouse, 2006.
Harman, C.J., 2007. Effects of heterogeneity on subsurface flow in hillslopes and approaches to
closure, MSc thesis, University of Illinois at Urbana-Champaign.
Appendix A: Meeting agenda and Participants list
Sunday, November 18th
12:30 PM:
13:30 PM:
13:45 PM:
14:15 PM:
14:45 PM:
15:15 PM:
15:45 PM:
17:15 PM:
18:15 PM:
19:00 PM:
Lunch
Welcome and quick introduction (Peter Troch)
Research focus at B2 Earthscience (Travis Huxman)
Columbia University days at the Biosphere (Joost van Haren)
The St. Anthony Falls Laboratory Perspective on Designing Long-term
Experiments (Anne Lightbody)
Coffee break
Hillslope Research Proposals (All)
- Exciting research topic (Title Nature/Science paper)?
- What hypotheses would you like to test?
- How do these hillslopes have to behave hydrologically to allow
testing of these hypotheses?
- What would you like to measure?
From Hypotheses to Experiments (All)
- Time scales (how long does it take to observe)?
- Spatial scales (patterns and complexity)?
- Drivers, initial and boundary conditions?
Walk-through the Biosphere2
Dinner
Monday, November 19th
07:30 AM:
08:30 AM:
09:00 AM:
10:15 AM:
10:45 AM:
12:00 AM:
13:00 PM:
15:00 PM:
Breakfast
Summary of yesterday’s discussion
Continue discussion on research proposals
Coffee break
Numerical modeling – Proof of concept
- Processes that need to be simulated?
- What data sets are readily available for testing?
- Which models can be used?
- Time line?
Lunch
Continue discussion on numerical modeling – Proof of concept
- Processes that need to be simulated?
- What data sets are readily available for testing?
- Which models can be used?
- Time line?
Return to Tucson
Participants List
David Benson
Eric Betterton
Dave Breshears
Paul Brooks
Jon Chorover
Ciaran Harman
Travis Huxman
Anne Lightbody
Kathleen Lohse
Jeff McDonnell
Claudio Paniconi
Jon Pelletier
Craig Rasmussen
Matt Reeves
Scott Saleska
Murugesu Sivapalan
Peter Troch
Joost van Haren
Xubin Zeng
Chris Zou