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

2

2cos sinc

S S S Sf kpD i a k i fNwt x x x

10,000 synthetic hillslopes

( )( ) ( ) 0

cS cQ cD S N t w x

t x x x

where: c is concentration of inert tracerD is dispersion coefficient is recharge concentration

Size d

oesn

’t mat

ter!

Example: Baseflow mean residence time

Pe tan2 2

ca LLi

pD

1

2 2 cosK

fL L

k pD i

Dynamics in heterogeneous hillslopes: multiple timescales control flow

High slope / low intensity Low slope / high intensity

• Heterogeneous aquifer– Fast pathways respond to peaks in intensity– Slow pathways create power-law recession

10log 1K

HeterogeneousHeterogeneous

Ciaran Harman

© Oregon State University

HillslopeHydrologyChallenges

Kurt R

oth, University H

eidelberg, Germ

any

0.01 m 1 m

Markus W

eiler, UB

C

100 m

10,000 m

Jim Kirchner, UC Berkeley

Markus Weiler, UBCWeiler and McDonnell, WRR in review

Community ConsensusNetwork behavior at all scales

Jeff McDonnell

© Oregon State University

HillslopeHydrologyChallengesHillslope networks

Our theory does not include them or adequately deal with them

These are calibrated-away in our modelsWe ignore them at our peril if we want to

do something more than water flowOur measurement technologies are not able

to describe themNetwork structures are the evolutionary

outcome of integrated climatic, geomorphological, ecological, pedological feedbacksThis could promote exciting research

programs between hydro-eco-biogeochemical-pedology

Jeff McDonnell

Biogeochemical, soil, vegetation processes affecting hillslope

hydrologic subsystems

Hillslope hydrology(catena shape,

topographic fine structure,

pore structure,flow paths, K(Ψ)

distribution) Spatial distribution of plants and

microbes(Veg. structure, C fixation,

infusion of roots & C, plant litter decay)

Patterns of biogeo-weathering

(aq. geochem. conditions, Ω distribution, aggregation, pore complexity,

biophysical microenvironments,)

Evolution of subsurface connectivity

(macropores, preferential flow,

gas/solute transport, bulk density changes )

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 RelationshipsSoil-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

Growing Season PPT

50 100 150 200 250 300 350 400

Rai

n-u

se E

ffici

ency

(G

EP

/ P

PT

)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

WoodlandShrublandGrassland

Impact of precipitation on photoautotrophs

Greater access to deep water keeps photosynthetic processes high when precipitation is low in woody plant systems

Grassland

Shrubland

Woodland

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?