Download - Claire S. Goulsbra and Martin G. Evans
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
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
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