Evidence for the occurrence of infiltration excess overland flow in an

eroded peatland catchment: implications for connectivity

Claire S. Goulsbra and Martin G. Evans

UPLAND ENVIRONMENTS RESEARCH UNIT

Upland Environments Research Unit, School of Environment and Development, University

of Manchester, UK

IntroductionOverland flow and Connectivity• OLF generation is a crucial processes in catchment hydrology • Different flow pathways and flow processes influence the magnitude and

timing of the delivery of water, sediment and solutes. • Need to understand the spatial and temporal distribution of different flow

processes so models can be adequately developed.

OLF in peatlands• OLF is the most important runoff pathway in peatlands • OLF in peatlands is produced almost exclusively by saturation excess

overland flow as opposed to infiltration excess overland flow. • high stream flows always occur at times of high water table• fluctuations in water table are swift with recoveries occurring more

rapidly than recessions.

Models of peatland OLF generation

Holden and Burt (2003) WWR• North Pennines (in tact)• OLF occurs most frequently on

footslopes from return flow and least frequently on steep mid-slopes

Daniels et al. (2008) JoH• South Pennines (eroded)• Water table drawdown at the gully

edge, esp within 2 m (‘erosional acrotelm’ effect)

What are the key controls on OLF generation in peatlands in space and time?

Monitoring OLF using ER sensors

> Binary flow/no-flow distinction (Blasch et al., 2002 Vadose Zone Journal)

• Temperature sensors can be converted to ER sensors

• Inexpensive – high spatial density; user-selectable sampling intervals – high temporal density

• Laboratory testing of the converted sensors revealed that they can consistently differentiate between the presence and absence of water

ER signal

Time

Co

nd

uc

tiv

ity

ER sensor

Electrodes

ER sensor

Electrodes

Flow – high conductivityNo flow – low conductivity

• Traditionally overland flow production in peatlands has been examined by the use of crest-stage tubes.

• limited temporal resolution of measurements

OLF sensor design

• Electrodes are housed in electrical conduit with a lid.

• Drainage holes and a small gap at the bottom of the lid allow the entry of surface flow.

• Minimise chances of false positives.

• Installed at the ground surface.

40 mm

Holes through which nails are driven to secure sensor to the ground Ø 6mm

Insulated wire connecting electrodes to data logger

Holes in bottom of sensor for free drainage Ø 3mm

Sensor electrodes ~3 mm long

Electrical conduit

Sensor base-plate

40 mm

Sensor ‘lid’ with plastic at either end to prevent entry of rain/sediment

16 mm

Small gap to allow surface flow to enter sensor

Upper North Grain research catchment• South Pennines, Peak District National

Park, UK• 0.38 km2

• Elevation 480 – 540 m • 1,500mm rainfall• Blanket peat cover (ombrotrophic)• Heavily eroded (Bower type II gullies) >

implications for carbon flux

• Previous and on-going Monitoring• Heavily instrumented

– Met station

– Discharge

– Dipwells

– LiDAR data (gully maps, water table models)

UNG Experimental Catchment

0 200 400100 Meters

Data Collection

598

586

595

583

606

616

585

624

608

615

581

587

594

611

600

623

621

603

589

908

604

584

592

601

613

630

626

591

597

599

628

607

590

618

612

619

617

609

610

629

605

593

614

0 4 82 Meters

Legend

sensors<all other values>

EventDB.N11

0

1

Krig_crop

Value

High : 100.99 m

Low : 98.66 m

Legend

krig

Value

High : 100.79 m

Low : 99.26 m

OLF sensorDipwell

584

603

620

606

595594

587

625

908

612623

629

588

598

617611

581

600

605

586583

604

626

599

589591

592

609

616

607593

610

630

615

628

608

601

624

596

619

D0.5 D1.5 D3 D8

0 1 2Meters

September to November 2008 • 40 sensors was located at a

gully edge site.

May to July 2008 • 43 sensors was

located at the head of an erosional gully (2 m grid)

In UNG catchment, the average distance to a gully is just 10.3 m; 13.7% of the intact peat mass lies within 2 m of a gully

Readings at 1 minute intervals > 36 days of continuous logging

OLF at the gully head

Legend

sensors

Flow (% of study period)

0.00 - 0.73

1.05 - 2.01

4.28 - 4.72

6.02 - 8.29

10.24 - 14.74

17.88 - 18.79

23.70 - 34.10

Krig_crop

Value

High : 100.993

Low : 98.6614

0 4 82 Meters

• No flow at one site out of 43• OLF <1% of the study period at 9 sites• Max 34.1%• Average 8.6%

R2 = 0.23

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12

Distance from gully edge (m)

% f

low

• The sites which experience OLF the most regularly are those at the eastern side of the plot, the furthest away from the gully.

• Positive relationship between distance from the gully edge and % flow (not found to be statistically significant).

• At sites within 2 m of the gully flow was recorded an average 5.2% of the time (n=19) compared with 11.2% at sites which are 2 m or more away from the gully (n=24) (not statistically significant).

OLF at the gully edge

Legend

Sheet1$ Events

Flow (% of study period)

0.000 - 0.145

0.735 - 1.674

3.179 - 3.480

6.928 - 8.233

11.518 - 16.342

19.651 - 20.429

26.464 - 28.038

krig

Value

High : 100.791

Low : 99.2553

0 1 20.5 Meters

• No flow at one 15 fifteen out of 40 sites• OLF <1% of the study period at 7 sites• Max 28.0%• Average 5.5%

• OLF is produced more frequently at sites closer to the gully edge than those further away.

• Weak negative relationship between distance from the gully edge and overland flow generation at each site (not statistically significant).

• At sites within 2 m of the gully flow was recorded an average 8.7% of the time (n=20) compared with 2.3% at sites which are 2 m or more away from the gully (n=20) (statistically significant at the 0.1 level).

0

5

10

15

20

25

0 2 4 6 8 10

Distance from gully edge (m)

Flo

w (

% o

f st

ud

y p

erio

d)

Temporal pattern of OLF

• Prolongation of OLF after rainfall

0

5

10

15

20

25

30

35

40

45

50

15/11/08 04:00 15/11/08 08:00 15/11/08 12:00 15/11/08 16:00 15/11/08 20:00

Date and Time

0

1

2

3

4

Pn

(m

m)

0

5

10

15

20

25

30

35

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45

50

15/11/08 04:00 15/11/08 08:00 15/11/08 12:00 15/11/08 16:00 15/11/08 20:00

OL

F (

% s

ites)

-800

-700

-600

-500

-400

-300

-200

-100

0

100

Wa

ter

Ta

ble

(m

m)

OLF

WT-D0.5

WT-D1.5

WT-D3

WT-D8

BC D

EFA

A

FE

DC

B

010

2030

405060

7080

90100

26/06/08 08:00 26/06/08 16:00 27/06/08 00:00 27/06/08 08:00 27/06/08 16:00

Date and Time

-200-180

-160-140

-120-100-80

-60-40

-200

0

2

4

6

08:00 12:00 16:00 20:00 00:00 04:00 08:00 12:00 16:00

Pn

(m

m)

010

2030

405060

7080

90100

26/06/08 08:00 26/06/08 16:00 27/06/08 00:00 27/06/08 08:00 27/06/08 16:00

OL

F (

% s

ites

)

-200-180

-160-140

-120-100-80

-60-40

-200

Wat

er T

bal

e (m

m)

OLF

Water Table

BC D

HF

E G

H

B

D E F

A

G

C

A

• OLF ceases after rainfall• Low WT at the gully edge!

Differences in OLF

• Different patterns of OLF generation at the two sites

• Gully head site shows aspects of Holden and Burt and Daniels et al. models of OLF generation

• Gully side shows the opposite pattern– WT never at the surface at gully edge – Enhanced erosional acrotelm?

SEOLFLimited OLFIEOLF

AcrotelmErosional acrotelm

Erosional acrotelm

Hydrophobic ‘crust’

Catotelm

Gully head Gully side

Spatial pattern Less OLF close to gully edges

More OLF close to gully edges

Temporal pattern

OLF maintenance after rainfall

OLF ceases after rainfall

SCA Large Small

Gully Shallow Deep

OLF following drought• Four day period from 27 to 31 May

2008. • Low water table is low following a

prolonged period with little rainfall.

0

20

40

60

80

100

27/05/08 28/05/08 29/05/08 30/05/08 31/05/08

OL

F (

% s

ite

s)

OLF

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

27/05/08 28/05/08 29/05/08 30/05/08 31/05/08

Dis

ch

arg

e (

cu

me

cs

)

Discharge

-250

-200

-150

-100

-50

0

27/05/08 28/05/08 29/05/08 30/05/08 31/05/08

WT

de

pth

(m

m)

Water table

0

1

2

3

4

5

6

7

27/05/08 28/05/08 29/05/08 30/05/08 31/05/08

Date

Rai

nfa

ll (

mm

)

1. Rainfall produces a response in 28% of the OF sensors.

– The water table rises once overland flow has subsided.

– No discharge is produced at the catchment outlet

2. Rainfall on the morning of 28 May produces a larger overland flow response

– water table levels are much closer to the surface,

– A discharge response is produced

3. At 17:00 on 28 May, high intensity rain leads to a sharp increase in both discharge and overland flow.

1 2 3

Implications

• Importance of water table variation in time and space• Climate change > summer conditions in the UK may

become warmer and ‘stormier’ • More frequent water table drawdowns may lead to an

increase in hydrophobic conditions in time and space. • Shift in the dominant OLF process from saturation

excess to infiltration excess as expanses of the peat surface become hydrophobic.

• This has implications for floodwater delivery – IEOLF can be produced rapidly following rainfall – IEOLF will result in a lower proportion of incident rainfall will

enter the peat mass, resulting in higher runoff totals

Summary• ER sensors are a viable alternative to crest stage tubes for monitoring OLF

generation. • OLF is widespread at both the gully head and the gully side

• Water table variation in time and space is key in controlling connectivity.• Both saturation and infiltration excess overland flow are observed in this

study. – IEOLF occurs at the dry gully edge site - enhanced erosional acrotelm

effect – IEOLF is also observed at the ‘wetter’ gully head site following drought.

• IEOLF may become more widespread under future climate change scenarios

• This has implications for the timing and magnitude of floodwater delivery

• The apparent importance of infiltration excess overland flow has hitherto not been widely acknowledged and as such this represents a major advancement in our current knowledge of the dominance of various runoff mechanisms in peatlands.

Thank you

UPLAND ENVIRONMENTS RESEARCH UNIT