The changing nature of peatland hydrology: implications for DOC and

flood runoff

David Gilvear

University of Stirling

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

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)

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)

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

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

Holden, 2005)

Holden 2005

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

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.

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

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

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

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)

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.

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

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

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

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

• 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

Significance of advances in peatland hydrology to DOC

• 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

Significance of advances in peatland hydrology to flooding

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

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

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

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?

Days of heavy rain (>10mm)

Total precipitation

Total precipitation

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

• 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.

Complex hydrology

Instantaneous overland flow

Throughflow via micro and macro pores

Upward and downward water movement

Re-emerging subsurface water

Implications in terms of peatland hydrology

• 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.

Courtesy of James Curran, SEPA

User needs

Time horizon

Strategic

TacticalShort-term Next 10 years

Operational needs

Strategic planning needs

Pre-operational needs

Policy domain

Research domain

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

• 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.

• Should not expect simple relationships between land management activities and flood generation

• This complexity does not lend itself to practicality of peatland management

Small-scale processes important in terms of water movement and at

hillslope-scale variety of flow pathways and residence times

• • 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.