1. The changing nature of peatland
hydrology: implications for DOC and
flood runoff
David Gilvear
University of Stirling

2. Presentation prompted by:
(i) Recent comment to me by an employee of RSPB with
extensive field experience that “the peat bog up there acts as
a sponge
(ii) Frustration in 2009 while undertaking a project on natural
flood management. Difference of opinions between policy
makers and scientists could not be reconciled
(iii) Not having the “bottle” to resist Susan Waldron’s
suggestion I give a paper after having opened my big trap

3. Aims:
(i) to explore level of agreement between the views of the scientific
community and the traditional and “stakeholders” perception of
peatlands and hydrology.
(ii) to review chronology of peatland hydrological research in the UK
and how our understanding has developed and the significance for
flood generation and DOC export.
(iii) raise awareness of how changing precipitation patterns may be
critical in terms of peatland hydrology and runoff in the future (as
well as land use)

4. The “stakeholder view”
• Rain falling in the hills, or melting in the spring, should under natural conditions,
filter slowly through bog, heath ……. (SNH, “Ill Fares the Land”)
• Rainfall which used to be absorbed by peat bogs, rushes through these moorland
drains into streams and lowland rivers .….. Restoration of these peat bogs will not
only benefit precious wildlife habitat, but also reduce run-off (Natural England)
• ““the peat bog up there acts as a sponge” (RSPB pers comm)
• What are the hydrological processes that results in peat bogs attenuating flood
hydrographs ? “they just do” (Scottish Government pers comm)
• “Lewis peatlands would have been devastated by the construction of 181 giant
turbines and 88 miles of roads … disrupted the hydrology of the peat bog, releasing
thousands of tons of CO2 into the atmosphere”. (Struan Stevenson, MEP, 2008)

5. The “scientists view - runoff ”
• “The idea that peatlands soak up water is a myth in many areas
because the water table in an intact wetland will be close to the
surface and with additional inputs of water, surface runoff may be
readily generated” (Black and Gilvear, 2009).
• A study by Jo Holden and Tim Burt on the Moor House catchment in
the Pennines suggests that 80% of runoff is via overland flow.
• Hydrological regime of Trout Beck, a rare example of a peatland
catchment with flow gauging data ,has been shown to be flashy

6. Holden 2005
The “scientists view”
• The idea that peatlands soak up water is probably a myth in many
areas because the water table in an intact wetland will be close to the
surface and with additional inputs of water, surface runoff may be
readily generated. A study by Jo Holden and Tim Burt on the Moor
House catchment in the Pennines suggests that 80% of runoff is via
overland flow.
• Hydrological regime of Trout Beck, a rare example of a peatland
catchment with flow gauging data ,has been shown to be flashy
5)

7. View across Loch Fyne; October 2010 within 30 minutes of start of rainfall event

8. The “scientists view – runoff and drainage”
• Peat soils of Plynlimon were deemed WRAP 5 - “very low infiltration capacity with a
flashy flood response” (Farquharson (1978). I of H (1991) classified the soils as
HOST 14 (water table depth > 2 m). Chappel and Trenan (1992) used this to infer
that mapping peatland hydrology was “over generalised and inconsistent”.
• Institute of Hydrology (1972) “in the short-term drained peat may be a better sponge
than an intact mire”
• Coalburn catchment research showed that flood peaks were increased by 30% as a
result of drainage ditching (Robinson, 1986).
• Robinson (1985,98) showed drainage of peat increased runoff response under wet
conditions due to reduction in transit times but larger antecedent soil moisture was
then apparent
• Gripping – There is ongoing scientific debate as to whether reduced travel time or
increased storage effect dominates (e.g. Robinson et al., 1991; Gunn and Walker,
2000). Lane (2003) concluded that the effect on flood runoff depends upon local
topography and nature of the network. Thus the effect of blocking will be catchment
specific.

9. Presentation aims:
(i) to explore level of discrepancy between
scientific community and the traditional and
“stakeholder” perception of peatland runoff.
(ii) to review how are knowledge of peatland
hydrological processes has changed and
significance for flooding and DOC export.
(iii) consider the significance of changing
precipitation patterns on peatland runoff

10. Hydrological theory
• The physical understanding of the rates with which water
moves through the environment has long-distant origins.
In 1856, Henri Darcy put forward his theory to predict the
rate at which water moves below the ground surface
through natural materials – the so-called Darcy’s Law.
This states that the rate of flow is accelerated by the
pressure gradient, but regulated by the hydraulic
conductivity of the materials through which it is flowing.
• Above the ground surface, the so-called Manning
equation was first proposed in 1867 and later developed
by the Irish engineer Robert Manning. A key component
is Mannings ‘n’ an estimate of surface roughness
• Peat is 90-98% water by mass and water table for
majority of time is within 30 cm of surface

11. Massive progress in hillslope hydrology and runoff (1970s>)

13. Key variables influencing runoff identified
• Surface roughness (controls rate of overland and
channelised flow)
• Slope and topography (hollows/concave slopes)
• Infiltration capacity (not really important under natural
conditions)
• Hydraulic conductivity (micropores and macropores)
• Antecedent soil moisture (drives Saturated overland
flow)
• Precipitation intensity ( affects rate of runoff)
• Drainage density (related to flood peaks)
• Drainage pattern (flood peak conveyance)

14. Drainage density
Wishart and Warburton (2001) by comparing
1951 and 1983 aerial photographs of the
Cheviot Hills identified that footpath erosion
has created preferential flow paths that in
effect increase the natural drainage density.

15. Despite widespread (8%)
distribution of peatlands
hillslope UK hydrology
generally avoided peatlands.
Hydrological models
developed solely on raised
bogs.
Had to look to Russia and
Scandenavia for knowledge
Knowledge in UK by-product
of upland forestry and water
yield studies by Institute of
Hydrology

16. Chronology of research on UK peat hydrology
70s/80s – Acrotelm/catotelm model; Soil hydrology
classification; water table drawdown around drainage ditches.
Drainage and flooding including some peatlands.
1990s – Raised bog hydrology; affects of drainage on
nutrients initiated; Studies of hydraulic conductivity in peat
2000s – Explosion in interest but geographically focussed;
DOC loss and runoff studies on intact, drained and burned
catchments; Use of new techniques – GPR; LiDAR, hyper-
spectral remote sensing; High temporal resolution monitoring
of DOC

17. Early peatland hydrology centred around water
table elevations and impacts of diching on
vegetation – precursor to field of hydro-ecology
Based on 2 layer acrotelm-catotelm model
Burke(1975) 1.83 metres 15-20cm lowering (4 metre spacing) - Peat
Knight et al. (3 metre spacing 10 cm lowering) Blanket Bog,
Scotland
Rayment and Cooper (1968); 11 metres sedge peat

18. Recent Advances
• Much of the time peatland water table at surface at base of slopes and
in hollows within intact peatlands (dynamic contributing areas and
saturated overland; non-linear response thus likely)
• Macropores now shown to be highly prevalent in peatlands. (Observed
in all 160 peatlands examined by Joe Holden (2004); Andy Baird (1997)
estimated 30% of runoff). Use of GPR.
• Small-scale processes now seen important in terms of water
movement and at hillslope-scale variety of flow pathways and residence
times. Multiple flow pathways (upward and downward; macro-pore flow;
re-emerging sub-surface water) and complex 3D hydrology

21. Significance of advances in peatland
hydrology to DOC
• Peat pipes which when full with water will convey flow at
high velocities and when not full possibility of oxidation
Thus may be an important source of DOC and POC
• Presence of rapid runoff pathways may exert an
important control on DOC and POC export
• Drained peat results has lower water tables and hence
space over which aerobic processes can operate. On re-
wetting macropores may still exists that can act as a
source and mode of water and nutrient export. However
contrasting result as to effect of drainage on DOC

22. Significance of advances in peatland
hydrology to flooding
• Saturated overland flow and peat pipes are hydrological
processes that create the flashy regimes observed
• Flood runoff may be generated from specific zones within
peatlands (sensitivity to drainage)
• A non-linear response between rainfall and flood runoff is to
be expected according to antecedent conditions
• Drained peat may create additional rapid flow pathways that
are irreversible; two competing impacts on flood generation –
storage capacity increased but accelerated flow pathways
• Pipeflow can bypass blocking of grips

23. More general significance of advances in
peatland hydrology
• Spatial and temporal complexity is an inherent trait of
peatland hydrology. Need to capture this at landscape scale.
• Peatlands should not be considered as solution to flooding
problem but sound land management is an issue.
• Location of peatlands and human modifications in the
catchment context and upscaling is important and a complex.
Contributes to source of debate about gripping and drainage
• Small scale hydrological processes drive carbon
sequestration and release from peatlands and thus sound
knowledge is critical

24. Further hydrological research needs
• DOC and POC sampling still does not fully account for flashy nature
of peatland catchments
• There is needs to improve our ability to predict the distribution of
water table depths, runoff pathways; scope via LiDAR (coverage
limited in Scotland), thermal remote sensing and modelling.
Hydromorphic mapping as well as vegetation mapping can assist
mapping “runoff vulnerable zones”.
• Further hillslope hydrological studies coupled to modelling would
improve situation further. Need to integrate with hydrochemistry and
ecology

25. Presentation aims:
(i) to explore level of discrepancy between
scientific community and the traditional and
“stakeholder” perception of peatland runoff.
(ii) to review how are knowledge of peatland
hydrological processes has changed and
significance for flooding and DOC export.
(iii) consider the significance of changing
precipitation patterns on peatland runoff

26. Hydroclimatic variability changing
•Wetter winters forecast – Increased likelihood of
saturation
•Summer droughts periods predicted - drop in summer
water tables likely and possibility of associated vegetation
community change
•Autumnal runoff roughly the same
•Likely to exacerbate flood and DOC potential?

27. Total precipitation
Days of heavy rain (>10mm)
Total precipitation

30. Concluding remarks - 3Cs
•COMMUNICATION: The scientific community working on
peatland hydrology needs to be better at communicating our
science and reasons for uncertainty
•COMPLEXITY: The advances made in understanding of
peatland hydrology at the local scale need to be up-scaled and
integrated in to other area of carbon landscape science. However
it is apparent that peatland hydrology is a complex multi-scalar 3D
problem and peatland hydrology is still in it infancy.
•CHANGE Need to try and determine future changes in peatland
hydrology given hydrological change scenarios and evaluate
significance for flooding and DOC export

32. • Effect of flood magnitude – many NFM measures may be most effective in small floods, and become less effective,
and less noticeable in terms of flood attenuation, in larger floods, e.g. due to the water-slowing effects becoming
‘drowned out’ by rising water levels.
•
• Effects of local climate and preceding weather conditions – the general wetness of an area, as illustrated by its
mean annual rainfall, and the specific pattern of rainfall/snowmelt in the days or weeks leading up to a flood
event, will affect soil wetness conditions and so affect the scope for water to be stored below the ground surface
and lead to delayed runoff. The direction a storm tracks across the catchment can also massively affect the flood
hydrograph shape.
•
• Physical characteristics of the upstream catchment – soils types and depths, vegetation cover, the permeability of
the underlying bedrock and the steepness of catchment slopes will all control the rates at which water in a
catchment will travel downstream. Principally, the distinction is made between ‘quick flow’ travelling either over
the ground surface or at shallow depths below the surface, and ‘slow flow’ which involves travel by deeper or
poorly-defined pathways. The same characteristics also control the amount of water which can be stored below
the ground surface, which in turn controls the timing and intensity of quick flow, and so also flooding.
•
• Type, size, shape and location of NFM features – these also control the flood-attenuating effectiveness of NFM
features. Location is particularly important: in some cases, it might be that because of runoff from a larger
adjacent catchment area, the interests of flood management may best be served by not attenuating runoff, e.g.
because of an existing pond, loch or wetland in the adjacent catchment (note that in planning NFM
implementation, it will be vital to define which downstream location is the target of protection). The type of
measure(s) deployed is also very important: there are many types of catchment intervention from which to
choose.

33. Implications in terms of peatland hydrology
Complex hydrology
Instantaneous overland flow
Throughflow via micro and macro pores
Upward and downward water movement
Re-emerging subsurface water

34. • DRAINAGE DITCH (GRIP) BLOCKING
•
• Process impact - There is still inadequate scientific understanding of peat hydrology to fully
understand the effect of drainage ditches on flood generation and hence the flood response to
blocking in all situations.
•
• Gripping – There is ongoing scientific debate as to whether reduced travel time or increased
storage effect dominates (Hudson et al., 1997; Robinson et al., 1991; Gunn and Walker, 2000). Lane
(2003) using a SCIMAP toll and LiDAR derived topographic data concludes that the effect on flood
runoff depends upon local topography and nature of the network. Thus the effect of blocking with
be catchment specific – in the specific study drainage was shown to reduce time to peaks.
•
• Weight of evidence – (COMPLEX) still continued scientific debate but possibly in more cases than
not reduced travel times dominates (e.g. Gunn and Walker, 2000) and thus a flashier response is
produced. Therefore drainage blocking may be an appropriate technique. Holden (pers comm.)
however emphasises that drainage ditch blocking should be targeted according to topography and
ditch location. He also states that in England too much money has been spent on blocking grips
spent without assessment of priority zones for blocking. Also shown that intensive continuous
maintenance is needed.
•
• On-going – work at Wharfedale by Holden (2007) on effect of grip blocking on flood peaks at the
catchment scale. Work by Labadz on the River Ashop in the Derbyshore High Peak.

35. Courtesy of James Curran, SEPA

36. Strategic
Research domain
Strategic planning
needs
User
needs Pre-operational
needs
Policy
Operational domain
needs
Tactical
Short-term Time horizon Next 10 years

37. The “politics of science, policy and action”
• Scientific uncertainty is perceived as unhelpful to policy makers and
land managers
• Science can “get in the way” of decision-making where multiple
drivers. Decision makers often need considerable ammunition to get
action.
• Statistical testing is recognised as providing rigour in the analysis of
observations, and provides results with a quantified level of
confidence. It is recognised practice in many areas of science that
statistical testing of quantitative data will be undertaken at a ‘5%
significance level’. Does not equate with knowledge exchange
requirement

38. • DRAINAGE DITCH (GRIP) BLOCKING
•
• Process impact - There is still inadequate scientific understanding of peat hydrology to fully
understand the effect of drainage ditches on flood generation and hence the flood response to
blocking in all situations.
•
• Gripping – There is ongoing scientific debate as to whether reduced travel time or increased
storage effect dominates (Hudson et al., 1997; Robinson et al., 1991; Gunn and Walker, 2000). Lane
(2003) using a SCIMAP toll and LiDAR derived topographic data concludes that the effect on flood
runoff depends upon local topography and nature of the network. Thus the effect of blocking with
be catchment specific – in the specific study drainage was shown to reduce time to peaks.
•
• Weight of evidence – (COMPLEX) still continued scientific debate but possibly in more cases than
not reduced travel times dominates (e.g. Gunn and Walker, 2000) and thus a flashier response is
produced. Therefore drainage blocking may be an appropriate technique. Holden (pers comm.)
however emphasises that drainage ditch blocking should be targeted according to topography and
ditch location. He also states that in England too much money has been spent on blocking grips
spent without assessment of priority zones for blocking. Also shown that intensive continuous
maintenance is needed.
•
• On-going – work at Wharfedale by Holden (2007) on effect of grip blocking on flood peaks at the
catchment scale. Work by Labadz on the River Ashop in the Derbyshore High Peak.

39. • Should not expect simple relationships
between land management activities and
flood generation
• This complexity does not lend itself to
practicality of peatland management

40. Small-scale processes important in
terms of water movement and at
hillslope-scale variety of flow
pathways and residence times

41. •
• CONTROL OF HEATHER BURNS
•
• Process impact - There is a poor scientific understanding of how heather burning might affect flood generation but
high intensity fires may reduce infiltration by capping the soil. More known on sediment loss impacts
•
• Hydrological response – No known information to date.
•
• Weight of evidence – (NONE) No weight of evidence
•
• On-going – PhD studentship advertised for autumn 2009 at the University of Leeds. Some work being undertaken
by Labadz at Nottingham Trent University on the Derbyshire High Peak.
•
• Policy relevance – the activity is covered by primary legislation and the Muirburn Code (Scottish Government,
2008), which provides requirements to be met under the Single Farm Payment regime. The Code does address fire
intensity indirectly, through requiring avoidance of unduly dry or windy conditions.
•
• Research Priority – HIGH due to considerable uncertainty over whether control of heather burns has any significant
impact and it being a widespread activity. In part this priority area is starting to be addressed by studies but these
alone will not give high levels of confidence. Need for more field monitoring at the sub-catchment level.