Mechanisms controlling DOC transport; Surface flow vs pipes

vs groundwater

Pippa Chapman,

School of Geography

University of Leeds

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• 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

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Role of pipeflow

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

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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

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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

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• Acknowledgments– Jo Clark, Joe Holden, Richard Smart, Andy Baird,

Mike Billett, Kerry Dinsmore – NERC- funded Jo Clark’s PhD (2000-2004)– NERC - funded Pipe project (2007-2010)– ECN – use of data– Natural England – use of Moor House NNR