One of many papers from the Coos Bay Experiments

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One of many papers from the Coos Bay Experiments

• Why read about this project?
• Innovative approach for a manipulative watershed
  study
• Massive infrastructure
• Lofty goals

CB1

• Anderson S.P. Dietrich W.E. (2001) Chemical
  weathering and runoff chemistry in a steep
  headwater catchment. Hydrological Processes 15
  1791-1815
• Anderson S.P., Dietrich W.E., Montgomery D.R.,
  Torres R., Conrad M.E. Loague K. (1997)
  Subsurface flow paths in steep, unchanneled
  catchment. Water Resources Research 33 2637-2653
• Anderson S.P., Dietrich W.E., Torres R.,
  Montgomery D.R. Loague K. (1997)
  Concentration-discharge relationships in runoff
  from a steep, unchanneled catchment. Water
  Resources Research 33 211-225

• Montgomery D.R. Dietrich W.E. (1994) A
  physically based model for the topographic
  control on shallow landsliding. Water Resources
  Research 30 1153-1171
• Montgomery D.R., Dietrich W.E., Torres R.,
  Anderson S.P., Heffner J.T. Loague K. (1997)
  Hydrologic response of a steep, unchanneled
  valley to natural and applied rainfall. Water
  Resources Research 33 91-109
• Torres R., Dietrich W.E., Montgomery D.R.,
  Anderson S.P. Loague K. (1998) Unsaturated zone
  processes and the hydrologic response of a steep,
  unchanneled catchment. Water Resources Research
  34 1865-1879

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Introduction

• Dissolved load reflects catchment hydrologic
  processes in addition to chemical processes p.
  211
• Streamflow concentration-discharge relationships
  show changes of chemistry during stormflow but
  are difficult to interpret without understanding
  hydrological processes
• Dilution of runoff during storm flow is modeled
  conceptually as resulting from a conservative
  mixing relationship between a high-solute-concentr
  ation component and a low-solute-concentration
  component
• However, if old-water dominates a hydrograph, how
  do solute concentrations dilute during events?
  Where does the low-solute component originate
  when all flowpaths are subsurface?

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Approach

• Complete watersheds that are small enough to
  characterize well and manipulate
• Measure stormflow hydrological chemical
  response from an unchannelled, zero-order
  hillslope hollow. Hydrochemistry represents
  streamflow generation and the response of
  hillslope contributing areas.
• Simulate rainfall and produce steady-state inflow
  and outflow conditions to reduce hydrological
  variability

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Measurements instrumentation

• Streamflow
• Upper weir flume with V-notch
• Lower weir several designs, measurements were
  adjusted so that the lower weir quantified water
  entering between the weirs
• Runoff chemistry base cations, Al, H, SO42-,
  Cl-, NO3-, SiO2, TDS, alkalinity
• Precipitation amount chemistry
• Sprinkling system to distribute artificial
  rainfall across the site. A de-ionization system
  for experiment 3
• Everything else (hydraulic head, soil mosture,
  soil tension, K, precip. spatial variability) as
  part of concurrent studies almost unprecedented
  characterization

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Why use artificial rainfall?

• Control rainfall variables that normally vary
• Rate
• Duration
• Chemistry
• Simulate long-duration, low intensity events
• Determine runoff chemistry responses to different
  input chemistries
• Create steady state inflows and outflows, having
  fairly constant chemistry of inputs
• Compare to natural events
• By sprinkling only on the catchment, ensures that
  all water originates within the catchment
  boundaries

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Site characteristics

• CB1 catchment within the Sullivan Creek drainage,
  near Coos Bay (Oregon Coast Range)
• Typical of Oregon Coast Range basins
• A steep (408458 m), small (860 m2) unchanneled
  valley
• Mean annual rainfall is about 2.0 m yr-1
  (Nov-May), and mean annual runoff is 1.6 1.8 m
  yr-1
• Fractured sandstone bedrock with organic-rich
  soils up to 2 m deep. Soils have a high
  hydraulic conductivity 10-3 m s-1
• Commercially clear-cut 2 years prior to
  monitoring and replanted with Douglas fir

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Mettman Ridge the CB1 catchment
Mettman Ridge the CB1 catchment
Figure 1
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Maps
CB1 subsurface saturation
Figure 2
Cross section
CB1 instrumentation

• Montgomery D.R., Dietrich W.E., Torres R.,
  Anderson S.P., Heffner J.T. Loague K. (1997)
  Hydrologic response of a steep, unchanneled
  valley to natural and applied rainfall. Water
  Resources Research 33 91-109

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Photos
CB1
CB1
Upper Weir
860 m2 0.086 ha
http//pangea.stanford.edu/hydro/research/coos_bay
/coosbay_content.htm
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Experimental design

• Intensity Exp1 ? Exp3 ? ½ Exp2
• Duration (days)
• Total rain
• Chemistry Exp1 ? Exp2 gtgt Exp3 natural
  storms

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Hydro- hyetographs
Natural storm
Natural storm
Exp.1
Exp.2
Exp.3
Winter baseflow little variability
Figure 3
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Experiments 1 2
Ca2
K
SO42-
Alk
concentration
Na
Al
SiO2
Cl-
Mg2
H
NO3-
TDS
Date
Figure 4
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Experiments 1 2
Average rain chemistry
A shift of outflow chemistry into the shaded area
would indicate a conservative mixture of rainfall
and subsurface water
concentration
Initial outflow chemistry
Ca2
Shift to lower concentrations indicates that
conservative mixing is not occurring during the
time-frame of the rainfall event
Date

• Consider calcium
• The upper weir outflow chemistry diverges from
  the expected mixture
• Obviously, the chemical signature of rainwater is
  not observed in the runoff

Figure 4
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Experiments 1 2
concentration
Ca2
Date
precipitation
Anomalous spike???
Variable chemistry during hydrograph rise
Variable chemistry during hydrograph fall
Stable chemistry during steady inflow
Figure 4
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Concentration-discharge relationships
Jan 90
Exp1-3
Feb 92
concentration
Upper weir
Figure 5
discharge
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C-Q TDS

• Both natural storms rainfall simulations
  reasonably fall along a C-Q relationship line.
• Different precip chemistries do not really
  influence the C-Q relationship
• Suggests that soils or bedrock are controlling
  the outflow chemistry more so than the initial
  precipitation concentrations (at least on the
  event time scale)

range of 0 to 50 ppm
Broader concentration range of 0 to 70 ppm.
Higher concentrations are consistent with mineral
dissolution in bedrock
Figure 7
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New water increases with storm duration
Runoff concentrations increase over time with
high rainfall inputs (i.e. moves marginally
towards new water)
Rain
Exp.1
chloride
discharge
outflow
Exp.2
outflow
Runoff concentrations decrease over time with low
rainfall inputs (i.e. moves marginally towards
new water)
Upper weir Exp.3
discharge
chloride
outflow
Rain
Date
Figure 8
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Water contributions

• Old-water still dominates the hydrograph at the
  end of rainfall. Despite the input rainwater in
  excess of the estimated soil water reservoir, new
  water does not displace all the old water. N
• New water contributions are significant, but
  never dominate the runoff chemistry.
• Bedrock reservoir fracture flow important!!
• Exchange of solutes among pore class sizes is
  important

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Their conclusions

• Despite the differences in rainwater chemistry,
  concentrations were similar.
• Soils and the fractured bedrock are important
  reservoirs that control hydrochemical response
• Conceptual model for C-Q relationships at CB1
• Runoff cannot be characterized as a simple
  dilution mixture of old-water with new water
• Runoff depends on runoff proportions from
• Soil bedrock
• Large small pores
• Soils buffer precipitation chemistry
• Bedrock water chemistry changes over time
• Rain appears to enhance the exchange of solutes
  in large small pores

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Interesting data and lots of it, but

• Given a manipulative experimental approach, a
  statement of hypotheses and ensuing tests could
  have been informative
• What are the processes causing the dilution of
  conservative solutes?
• What about geochemical controls? What are they?
  How could they influence the contribute to the
  observed chemical patterns?
• Nutrients discussion lacks substance

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Complicating factors???

• What vadose or bedrock processes contribute to
  the runoff dilution with increasing precip?
• The watershed is tiny. Does this size
  limitation constrain the utility of the findings?
• They achieved a steady state water inputs
  outflows. Does this equate to steady state
  flowpaths?
• How could the use of unfiltered water, with high
  ionic strength compared to natural rain water, be
  criticized? Should we be concerned about the
  generality of their results? Could they have
  poised the chemistry of the vadose zone by
  loading so many ions into the hillslopes? Could
  the pre-experiment system test of the sprinkler
  system explain the slightly higher concentrations
  of runoff during experiments 1 2?
• These flow mechanisms are very different than
  those described by variable source area concepts
  does the Anderson et al conceptual model make
  us rethink the VSA mixing model? That is, does
  time variant baseflow chemistry matter in VSA
  catchments?
• Have macropores been completely discounted or
  de-emphasized?