«J. Holden1*, P.J.Chapman1 and J.C. Labadz2 1 School of Geography, University of Leeds, Leeds, LS2 9JT, UK. 2 The Nottingham Trent University, ...»
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This is an author produced version of an article published in Progress in Physical
Holden, J. and Chapman, P.J. and Labadz, J.C. (2004) Artificial drainage of peatlands:
hydrological and hydrochemical process and wetland restoration. Progress in Physical
Geography, 28 (1). pp. 95-123.
ARTIFICIAL DRAINAGE OF PEATLANDS: HYDROLOGICAL
AND HYDROCHEMICAL PROCESS AND WETLAND
RESTORATIONJ. Holden1*, P.J.Chapman1 and J.C. Labadz2 1 School of Geography, University of Leeds, Leeds, LS2 9JT, UK.
2 The Nottingham Trent University, Brackenhurst, Southwell, Nottinghamshire, NG25 0QF, UK.
* Corresponding author: Tel 0113 343 3317, Fax 0113 343 3308, Email:
1 Abstract Peatlands have been subject to artificial drainage for centuries. This drainage has been in response to agricultural demand, forestry, horticultural and energy properties of peat and alleviation of flood risk. However, the are several environmental problems associated with drainage of peatlands. This paper describes the nature of these problems and examines the evidence for changes in hydrological and hydrochemical processes associated with these changes. Traditional black-box water balance approaches demonstrate little about wetland dynamics and therefore the science of catchment response to peat drainage is poorly understood. It is crucial that a more process-based approach be adopted within peatland ecosystems. The environmental problems associated with peat drainage have led, in part, to a recent reversal in attitudes to peatlands and we have seen a move towards wetland restoration.
However, a detailed understanding of hydrological, hydrochemical and ecological process-interactions will be fundamental if we are to adequately restore degraded peatlands, preserve those that are still intact and understand the impacts of such management actions at the catchment scale.
Keywords: peat, moorland gripping, wetland restoration, water table, blanket peat, afforestation, drainage 2 I Introduction Peat is decaying organic matter that has accumulated under saturated conditions.
Formation of peat therefore occurs in areas of positive water balance. Peatlands are more likely to form in regions with high precipitation excess, such as upland areas of the temperate and boreal zones or in lowland areas where shallow gradients, impermeable substrates or topographic convergence maintain saturation.
Classification of peatland types is generally related to two fundamental factors: source of nutrients and source of water. Bogs are ombrotrophic peatlands dependent on precipitation for water and nutrient supply, whereas minerotrophic peatlands or fens are reliant on groundwater for water and nutrient supply (Johnson and Dunham, 1963). Bogs are therefore highly acidic (pH 4) and contain low amounts of calcium and magnesuim, whereas minertrophic peats are less acidic and tend to be base rich.
In England and Wales peat is classified as a deposit of at least 30 cm depth (50 cm in Scotland) containing more than 50 % organic carbon (Johnson and Dunham, 1963).
This definition is arbitrary as there is no clear break between a highly organic mineral soil (e.g. podzol) and an almost purely organic Sphagnum peat (Clymo, 1983).
However, from this definition it is possible to say that 2.9 million ha or 13 % of Britain is covered in peat, most (2.6 million ha) of which is in Scotland (Milne and Brown, 1997). This represents less than 1 % of the 350 million ha of the northern peatlands that mainly occupy the boreal and subarctic zones (Gorham, 1991). In Britain the dominant peatland is blanket bog which occurs on the gentle slopes of upland plateaux, ridges and benches and is primarily supplied with water and nutrients in the form of precipitation. Blanket peat is usually considered to be hydrologically disconnected from the underlying mineral layer. The British blanket
1998). In some areas there are raised bogs where the peat has grown into a dome with a halo of lagg fen, overlying level mineral terrain or an infilled basin (Bragg and Tallis, 2001). However, Lindsay (1995) and Charman (2002) suggest that raised bogs and blanket bogs are simply end-pints of an ecological continuum. Britain is also covered in approximately 6.1 million ha of peaty gley and peaty podzol soils that can be classified as shallow peats (Milne and Brown, 1997). There are now few areas of lowland Britain covered by extensive peat deposits, with the exception of the Somerset Levels and Cambridgeshire Fens; drainage for agriculture and peat-cutting for fuel and horticulture have reduced their extent (Burt, 1995).
The relative position of the water table within the peat ultimately controls the balance between accumulation and decomposition and therefore its stability. Peat is therefore very sensitive to changes in hydrology that may be brought about by climate or land use change. Greater aeration above the water table increases decomposition in unsaturated conditions relative to saturated conditions below, so having fundamental implications for properties and attributes above and below the water table. Three of the main land management practices to have resulted in changes to peatland water tables in Britain and elsewhere in the world are those of moorland ditching, pumped removal of water from fens, and afforestation. However, several problems have been associated with these drainage activities; some of these problems were recognised as early as 1862 when Bailey-Denton discussed the uncertainty related to the effects of pipes and ditches on river flow. Moorland drainage is often blamed for increased flooding in UK rivers (e.g. Lane, 2001). There are also problems related to water quality, erosion, and ecosystem destruction. This paper attempts to shed light on the
hydrological and hydrochemical processes associated with drainage of peats. The paper will firstly give an overview of peat drainage practice before reviewing the literature to show that artificial drainage of peatlands is unsustainable. The paper will then discuss the future needs for wetland research and peatland restoration; our understanding of many hydrological and hydrochemical processes associated with peat drainage is still poor yet the processes may have crucial implications for global environmental change given that peatlands act as an important terrestrial carbon store.
II History and extent of drainage Many European countries have witnessed vast amounts of artificial peatland drainage including The Netherlands, Finland, Russia, Ireland and the UK. In Ireland drainage of peats and gleys has been reported since 1809 (Common, 1970; Wilcock, 1979).
Most of the Irish peat drainage was associated with the aim of reducing flooding but drainage schemes altered and accelerated after the second World War due to the need to increase livestock production in upland farms (Stephens and Symons, 1956;
Common, 1970). In Northern Ireland there are only 169 km2 of intact peat left compared with 1190 km2 of total peatland (Cooper et al., 1991). In New Zealand where peat soils cover more than 180 000 ha, peatlands were extensively drained for farmland in the 1970s with little regard to their ecological or environmental value (Bowler, 1980).
Britain is one of the most extensively drained lands in Europe (Baldock, 1984) and drainage of peatlands has played a fundamental role in the history of British farming (Williams, 1995). More than half the agricultural activity in Britain occurs on land
times and there are records of it in Domesday (Darby, 1956). In Britain drainage took off in the 17th century accompanying land tenure, enclosure and reclamation of the Anglian Fens. In the following hundred years, peat shrinkage and subsidence associated with the pumped removal of water from the fens meant that more and more water had to be removed to render the drainage works useful (Cole, 1976). Until the 20th century most drainage activity had focussed on ‘improving’ fenlands for agriculture by lowering the water table. After 1900 drainage was also directed towards flood alleviation; expansion in ditching, tile draining and channelization activity was huge. The ‘feed Britain’ post World War II era saw government grants for expansion in drainage works paid at 70 %, particularly in agriculturally marginal upland areas. It was in the 1960s and 1970s that most of the upland drainage of blanket peats took place, particularly in the English Pennines. The peak rate of drainage is estimated to be 100 000 ha yr-1 in 1970 (Green, 1973; Robinson and Armstrong, 1988). Economic incentives for upland drainage were not limited to the 20th century. In the mid 18th century Turner (1757) provided a cost-benefit analysis of moorland drainage. His essay, which also showed that the peat bogs of upland Britain were not remnants of recessional deposit left after the ‘Great Deluge’, suggested a three phase model for ‘improving moorland’ involving cutting open surface drains, adding sand and earth to the surface and the establishment of twitch grass.
The Cuthbertson plough was developed in the 1930s and has been used to create steep sided, open ditches (commonly called ‘grips’ in northern England) which are traditional for draining 1.5 million hectares of blanket peatland in upland Britain (Stewart and Lance, 1983). The drains are often contoured or in a ‘herring-bone’
ditches are sometimes used for tapping springs or other natural seepages (Stewart and Lance, 1983). Moorland draining was carried out with the purpose of lowering the water table and removing surface water to improve the vegetation for grazing and game. Partly this drainage was to improve the quality of grazing and partly to remove the hazard to stock (Ratcliffe and Oswald, 1988). However, Stewart and Lance (1983) demonstrated that there was no evidence that peatland draining fulfils the claims made for it. Grouse populations do not seem to have increased and whilst drains are the cue for increases in stocking density there is little evidence that the moors can sustain large increases. Thus Newson (1992) suggested that upland drainage was backed by very limited rationale. As such the economic benefits are very low and yet the potential environmental effects high (Newson and Robinson, 1983). In general there has been very little research into artificial drainage of hill areas. In particular hydrological monitoring and process-based measurement has been poor. This is surprising given that large sums of money have been spent on draining the slopes (and that large sums are planned to be spent on peatland restoration the future).
In addition to drainage for agricultural use, about 15 million ha of northern peatlands and wetlands have been drained for forestry, mainly in northern and eastern Europe and the British Isles (Paavilainen and Paivanen, 1995). In Britain, about 190 000 ha of deep peatland and 315 000 ha of shallow peats have been afforested with coniferous plantations since 1945 (Cannel et al., 1993). However, in order to ensure successful establishment of trees on peat soils, the water table must first be lowered. In Scandinavia, Finland, Russia, Canada, Ireland and Britain, drainage by a combination of closely spaced plough furrows and deep (usually 0.5 to 2 m) but more widely
from the hillslopes both in the short-term while the drains are active (David and Ledger, 1988; Prevost et al., 1999; Anderson et al., 2000) and in the long-term when the forest establishes. From the time of canopy closure, the increased interception of rainfall leads to greater evaporation by the trees and enhanced evapotranspiration which encourages drying of the peat and the development of shrinkage cracks. In Finland, 5.7 million ha of peatlands have been drained, so that now one quarter of the country’s forested land consists of drained peatland (Laiho et al., 1998). In Scotland 25 % of Caithness and Sutherland peatlands have been affected by differing intensities of drainage associated with afforestation (Ratcliffe and Oswald, 1988).
This area recently became the focus of major conservation protest and international condemnation (Charman, 2002).
III Impact of peat drainage on catchment hydrology Conway and Millar (1960) were the first to experimentally examine the effects of moorland drainage on the hydrological response of peatland catchments. They reported results from four small (2 ha) moorland catchments at Moor House in the English north Pennines; two had natural drainage channels and two had artificial networks of moorland drains. They concluded that runoff production in blanket peat was extremely rapid especially where hillslopes had a dense gully network, had been burned or were artificially drained giving an increased sensitivity of runoff response to storm rainfall with peak flows both higher and earlier. In contrast, relatively uneroded subcatchments exhibited a smoother storm hydrograph with greater lag times and the water balance calculations suggested that uneroded hillslopes could retain significantly more water than drained, eroded or burnt basins. This paper was
flooding downstream and reduces the water storage capacity of the hillslopes.