Project 4: Evolution, structure and function of hydrologic subsystems in hillslopes

1. Project 4:Evolution, structure and function ofhydrologic subsystems in hillslopes Paul Brooks, Jon Chorover, Ciaran Harman, Travis Huxman, Jeff McDonnell, Craig Rasmussen, Siva Sivapalan, Peter Troch

2. 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?

3. 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.

4. 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?

5. Example: Baseflow mean residence time where: c is concentration of inert tracer D is dispersion coefficient  is recharge concentration Size doesn’t matter! 10,000 synthetic hillslopes

6. Example: Baseflow mean residence time

7. Dynamicsin 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 Heterogeneous Heterogeneous Ciaran Harman

8. Kurt Roth, University Heidelberg, Germany Markus Weiler, UBC 1 m 0.01 m Community Consensus Network behavior at all scales 10,000 m 100 m Jim Kirchner, UC Berkeley Jeff McDonnell Weiler and McDonnell, WRR in review Markus Weiler, UBC

9. Hillslope networks • 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-eco-biogeochemical-pedology Jeff McDonnell

10. Biogeochemical, soil, vegetation processesaffecting hillslopehydrologic 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 (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,) Jon Chorover

11. 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

12. 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

13. 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

14. Grassland Shrubland Woodland Impact of precipitation on photoautotrophs Greater access to deep water keeps photosynthetic processes high when precipitation is low in woody plant systems Travis Huxman

15. 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

16. 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

17. Discussion • Topics of workshops? • Subsurface networks: evolution, structure, function? • Organizing principle: EEMT? • Deliverables?