Project 4: Evolution, structure
and function of hydrologic
subsystems in hillslopes
Paul Brooks, Jon Chorover, Ciaran
Harman, Travis Huxman, Jeff
McDonnell, Craig Rasmussen, Siva
Sivapalan, Peter Troch
Why hillslopes…?
[From a simplistic hydrologic point of view]
•
Almost all of the precipitation reaching a catchment
has passed over or through a hillslope before
evaporating, recharging, or running off (Kirkby, 1988)
•
The hydrologic response of a catchment is driven by
precipitation and solar radiation (the climate), but
controlled by the geological, topographic, hydraulic,
pedologic and ecological properties of the landscape
(hillslope, riparian zone, channel).
•
Is it possible to de-convolute the integrated output
signals present in stream flow in such a way that we
determine, upon removing climatic variations, how
catchments respond to forcing?
Why hillslopes…?
•
Linking physical flow processes/paths to landscape
characteristics (e.g., through similarity analysis), we
can make significant advances in the search for a
unifying theory of catchment hydrology (Kirchner, 2003)
by emphasizing where water goes when it rains
(McDonnell, 2003).
•
Doing this across a range of climate settings (from
humid to semi-arid landscapes) will result in a
fundamental understanding of the different dominant
controls on hydrologic response and water residence
time distributions.
Research questions
•
What are the key interactions between the soil, ecology,
geomorphology and biogeochemistry that create
hydrologic storages and flow-paths and partition
incoming water into them?
•
What role do these storages and flow-paths have in
maintaining
the
regimes
of
soil,
ecology,
geomorphology and biogeochemistry, particularly with
respect to the temporal variability imposed by the
climate?
•
Can an organizing principle be identified that could
drive the evolution of the hydrologic system in a
hillslope?
Example: Baseflow mean residence time
(cS ) cQ
  c 

 D  S    N (t )w( x)  0
t
x
x  x 
 2S
S
S 
S
f
 kpD cos i  2  ac   k sin i
 fNw
t
x 
x
 x
where:
c is concentration of inert tracer
D is dispersion coefficient
 is recharge concentration
10,000 synthetic hillslopes
Example: Baseflow mean residence time
ac L
L
Pe 
tan i 
2 pD
2
fL L
1
K 
2k 2 pD cos i
Dynamics in heterogeneous hillslopes:
multiple timescales control flow
High slope / low intensity
 log
10 K
Low slope / high intensity
1
Heterogeneous
Heterogeneous
• Heterogeneous aquifer
– Fast pathways respond to peaks in intensity
– Slow pathways create power-law recession
Ciaran Harman
Markus Weiler, UBC
Kurt Roth, University Heidelberg, Germany
Hillslope
Hydrology
Challenges
1m
0.01 m
Community Consensus
Network behavior at all scales
10,000 m
100 m
Jim Kirchner, UC Berkeley
©
Oregon
University
Weiler
andState
McDonnell,
WRR in review
Markus Weiler, UBC
Jeff McDonnell
Hillslope networks
Hillslope
Hydrology
Challenges
 Our
theory does not include them or
adequately deal with them
 These are calibrated-away in our models
We ignore them at our peril if we want to
do something more than water flow
 Our measurement technologies are not able
to describe them
 Network structures are the evolutionary
outcome of integrated climatic,
geomorphological, ecological,
pedological feedbacks
This could promote exciting research
programs between hydro-ecobiogeochemical-pedology
© Oregon State University
Jeff McDonnell
Biogeochemical, soil, vegetation processes
affecting hillslope hydrologic subsystems
Hillslope hydrology
(catena shape,
topographic fine structure,
pore structure,
flow paths, K(Ψ) distribution)
Evolution of
subsurface
connectivity
(macropores, preferential
flow, gas/solute transport,
bulk density changes )
Spatial distribution of
plants and microbes
Patterns of
biogeo-weathering
(aq. geochem. conditions,
Ω distribution, aggregation,
pore complexity,
biophysical
microenvironments,)
(Veg. structure, C fixation,
infusion of roots & C,
plant litter decay)
Jon Chorover
Biogeochemical Hot Spots and Hot Moments
•Hot spots leave a “signature” in water chemistry
•They also may leave a signature on the landscape
•These signatures should be consistent with (or can be
used to infer) hydrological flowpaths
Paul Brooks
Soil-Landscape Relationships
• (Strong) link between soil properties and landscape position needs
to be better understood to aid hydrologic controls on hillslope flow
and transport processes
Craig Rasmussen
Current Challenges in Ecohydrology
Biologically induced feedbacks
Time-depth distribution of soil water (time)
Hydraulic redistribution (space and time)
Buffering & Community Organization (dealing with variability)
Acclimation, Adaptation and Assembly
The Stoichiometry of Water
Water budget partitioning
Dry-rewetting cycle
Climatology of Size Appropriate Triggers of Biology
Are all dry years alike for all organisms (populations,
communities)?
Travis Huxman
Impact of precipitation on photoautotrophs
Rain-use Efficiency (GEP / PPT)
Grassland
4.0
Woodland
Shrubland
Grassland
3.5
3.0
Shrubland
2.5
2.0
1.5
Woodland
1.0
0.5
50
100
150
200
250
300
350
400
Growing Season PPT
Greater access to deep water keeps photosynthetic
processes high when precipitation is low in woody
plant systems
Travis Huxman
Proposed work
• Organize a series of workshops (1 each year, so 4 in total)
• Workshops are run by core group of people with different
background (hydrology, biogeochemistry, soil sciences, ecology)
• Each year, a workshop is held at different a research site
–
–
–
–
Year 1: Valles Caldera or Catalinas
Year 2: H.J. Andrews
Year 3: Panola
Year 4: Synthesis at B2-Earth Sciences
• Local scientists from different disciplines are invited to present
unsolved puzzles in their data (to provoke discussion and
possible collaboration)
• Number of participants: ~30 (+ grad students)
• Output: 2-pager that is distributed to larger hydrologic community
• Follow-up: special session at AGU meeting
• Synthesis paper submitted to WRR
• Budget: $40,000/year
Synergistic efforts
• B2 Earth Sciences’ institutional experiment: artificial
hillslopes in controlled (mass exchange and climate)
environment to study interaction between hydrology,
biogeochemistry, pedology and ecology
• Design of artificial hillslopes is the ultimate synthesis activity
(different disciplines need to agree on common design)
• B2 Institute will host SLICE-2 (Slope Intercomparison
Experiment) workshop convened by Jeff and Peter
• Main focus will be on hydrologic design, but with significant
input from other disciplines
• Similar discussion sessions will be held to arrive at final
design
Discussion
• Topics of workshops?
– Subsurface networks: evolution, structure, function?
– Organizing principle: EEMT?
• Deliverables?