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Mechanisms controlling DOC transport; Surface flow vs pipes
vs groundwater
Pippa Chapman,
School of Geography
University of Leeds
water@leeds
• Sources of DOC in aquatic systems– Allochthonous sources – are derived outside of
the stream from soil and vegetation within the catchment (terrestrial organic matter)
– Autochthonous sources – are derived from in-stream biological production (e.g. algae and macrophytes)
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• DOC is produced during the decomposition of organic matter in soil
• Peat soils contain most organic matter– Major source of DOC
• Production of DOC is a biological process:– Increases with temperature and
aeration– So more produced in summer
From Hope et al., 1994
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Overland flow - typically saturation-excess driven.
Micropore throughflow - typically close to the surface (top 5 cm).
Macropore/tunnel/pipe ‘bypassing’ flow
Transport of DOC in peatlands
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There is a strong relationship between soil water DOC at 10 cm depth and stream water DOC (r2 = 0.47, P<0.001) at Cottage Hill Sike, Moor House, N Pennines (Clark et al., 2008).
This is consistent with hydrological studies at this site that have shown most runoff originates from the top 5 cm (Holden and Burt, 2003).
Note: despite large fluctuations in discharge little variation in DOC
Role of throughflow
From Clark et al., 2008
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Role of overland flow: based on Cottage Hill Sike from Clark et al., 2007
•DOC concentrations decreased during autumn storms•No relationship observed between DOC and flow •Flux calculation based on weekly ECN sampling was 16% greater than flux based on 4 hourly sampling.•Excluding storm events resulted in over-estimation of DOC flux
•50% of DOC export is associated with the highest 10% of discharge values. Hinton et al (1997) reported similar findings for a catchment in central Ontario, Canada.
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North
0 2.50 km
1: 50000
Soil Type
Hill Peat
Charr Peaty Podzol
Countesswells Iron Podzol
Alluvial
Drumlasie Peaty Gley
Strathgyle Peaty Gley
Mundurno Peaty Gley
Ranker
Charr Flume
Brocky Burn
Soil map of Glendye, NE Scotland (from Dawson)
From Hinton et al., 1997. Biogeochem.
Pipeflow runoff pathway coupling
Mineral substrate
Blanket peat
Pipe network
Rapid near-surface macropore and matrix infiltration
Saturation-excess overland flow and near-surface through flow
Very limited deep matrix seepage ?
Localised ephemeral interface flow
0
10
20
30
40
50
60
DO
C a
nd
PO
C (
mg
l-1
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Flo
w (
l s-1
)
DOC
POC
Flow
Autosamplers – much more detail
water@leeds
P5 13 March 2008
Importance of pipeflow in Cottage Hill SikeInitial calculations suggest that:• 20-30 % of stream flow originates from pipes• 53 % of the DOC originates from the pipes (typically ranges
between 5-75 % depending on conditions)• Pipe probably tap deep/old carbon as well as new carbon (to be
further tested with isotopic analysis)• Very variable carbon response between pipes and between
storms• Chemistry of at least one pipe suggests that ground water feeds
pipe at low flow
water@leeds
water@leeds
Summary•DOC transported from peat to stream by a combination of overland flow, through flow and pipe flow•Response of stream water DOC to storm events dependent on number of source waters and location within the catchment with respect to soil distribution•Transport of DOC from peat and organo-mineral soils is not the same due to differences in hydrology and soil profile properties•Some streams fed by groundwater during low flow – low DOC, pH 6-7.•Largest loss of DOC (~50%) from peat and organo-mineral soils occurs in only 10% of time during high flow events