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One of many papers from the Coos Bay Experiments —Anderson S.P. & Dietrich W.E. (2001) Chemical weathering and runoff chemistry in a steep headwater catchment.

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Presentation on theme: "One of many papers from the Coos Bay Experiments —Anderson S.P. & Dietrich W.E. (2001) Chemical weathering and runoff chemistry in a steep headwater catchment."— Presentation transcript:

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2 One of many papers from the Coos Bay Experiments —Anderson S.P. & Dietrich W.E. (2001) Chemical weathering and runoff chemistry in a steep headwater catchment. Hydrological Processes 15: —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: —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: Why read about this project? Innovative approach for a manipulative watershed study Massive infrastructure Lofty goals —Montgomery D.R. & Dietrich W.E. (1994) A physically based model for the topographic control on shallow landsliding. Water Resources Research 30: —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: —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: CB1

3 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-concentration 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?

4 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

5 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 +, SO 4 2-, Cl -, NO 3 -, SiO 2, 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

6 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

7 Site characteristics CB1 catchment within the Sullivan Creek drainage, near Coos Bay (Oregon Coast Range) Typical of Oregon Coast Range basins A steep (408–458 m), small (860 m 2 ) 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 m s -1 Commercially clear-cut 2 years prior to monitoring and replanted with Douglas fir

8 Mettman Ridge & the CB1 catchment Figure 1

9 Maps 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: CB1: instrumentation CB1: subsurface saturation Cross section Figure 2

10 Photos Upper Weir 860 m 2 = ha CB1 tent.htm CB1

11 Experimental design Intensity: Exp1  Exp3  ½ Exp2 Duration (days): Total rain: Chemistry:Exp1  Exp2 >> Exp3 & natural storms

12 Winter baseflow = little variability Exp.1Exp.2 Exp.3 Natural storm Natural storm Figure 3 Hydro- & hyetographs

13 Experiments 1 & 2 Date concentration Ca 2+ Na + Mg 2+ K+K+ Al H+H+ SO 4 2- NO 3 - Cl - SiO 2 Alk TDS Figure 4

14 Experiments 1 & 2 Date Ca 2+ Figure 4 Average rain chemistry Initial outflow chemistry A shift of outflow chemistry into the shaded area would indicate a conservative mixture of rainfall and subsurface water Consider calcium The upper weir outflow chemistry diverges from the expected mixture Obviously, the chemical signature of rainwater is not observed in the runoff concentration Shift to lower concentrations indicates that conservative mixing is not occurring during the time-frame of the rainfall event

15 Experiments 1 & 2 Date Ca 2+ Figure 4 concentration Stable chemistry during steady inflow Variable chemistry during hydrograph rise Variable chemistry during hydrograph fall Anomalous spike??? precipitation

16 Concentration-discharge relationships discharge concentration Upper weir Jan ‘90 Exp1-3 Feb ‘92 Figure 5

17 C-Q TDS Figure 7 Broader concentration range of 0 to 70 ppm. Higher concentrations are consistent with mineral dissolution in bedrock range of 0 to 50 ppm 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)

18 chloride New water increases with storm duration Exp.1 Exp.2 Upper weir Exp.3 Runoff concentrations increase over time with high rainfall inputs (i.e. moves marginally towards new water) Runoff concentrations decrease over time with low rainfall inputs (i.e. moves marginally towards new water) outflow outflow outflow Rain Rain chloride discharge Figure 8 Date

19 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

20 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

21 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

22 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?


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