Blanket Peat Catchments Research Articles

Eroding blanket peat catchments: Global and local implications of upland organic sediment budgets

Globally, peatlands account for circa 50% of terrestrial carbon storage containing as much carbon as is present in the atmosphere. The uplands of the UK have an extensive cover of blanket peat but much of it is actively eroding. This paper presents a detailed organic sediment budget for a blanket peat catchment in the north Pennines and comparative data from a catchment in the southern Pennines. The catchments have total sediment yields (organic and mineral) of 44 and 267 t km − 2 a − 1 and organic sediment yields 31 and 195 t km − 2 a − 1 , respectively. They represent two extremes of a spectrum of eroded peat catchments. It is demonstrated that the lower sediment yields in the north Pennines are associated with extensive natural revegetation of the catchment and consequent reductions in slope-channel linkage. Construction of a carbon budget for the north Pennine catchment demonstrates that particulate carbon losses associated with the fluvial suspended sediment load are the largest single carbon loss from the system. The system is currently close to carbon neutral but much higher carbon losses associated with actively eroding systems such as the south Pennine site would make these systems a major carbon source. The possibility that enhanced summer temperatures and winter storminess will accelerate erosion of upland mires means there is a risk that physical degradation of peatlands could become a significant positive feedback on global warming. Mitigation of these potential global effects will depend on local management informed by a clear understanding of peatland sediment dynamics. The sediment budget data here suggest that in gullied peatlands revegetation of gully floors is an effective control on sediment flux so that techniques such as gully blocking are likely to be effective approaches to erosion control.

Piping and woody plants in peatlands: Cause or effect?

This paper presents, for the first time, evidence to show that Calluna species are one causative factor of piping in blanket peat catchments. Ground‐penetrating radar survey on 960 plots illustrated that piping was prevalent throughout blanket peats. However, soil pipe occurrence was significantly higher where bare peat (149 pipes/km) or Calluna (87 pipes/km) were present compared to other species (67 pipes/km). A case study catchment where there was an altitudinal limit to Calluna provided some control over potential factors that may lead to an association between piping and Calluna. Under the controlled conditions of topographic index, peat depth, and water table, piping was greater under the Calluna‐covered peat than under other vegetation covers. Laboratory experiments demonstrated that 10 years worth of rainfall was enough to almost double the proportion of macropore flow occurring in recently colonized Calluna peatlands. This suggests that given enough water and time, the woody Calluna plants result in water being preferentially channeled through the upper peat. Improvements are therefore required in our understanding of the relationships between peatland plant nutrient and water supply and the feedbacks between ecosystem functioning and landform development. These results are also important given the propensity to encourage Calluna growth for game bird enhancement in many northern peatlands.

Heavy metal release by peat erosion in the Peak District, southern Pennines, UK

AbstractUpper North Grain (UNG) is a heavily eroding blanket peat catchment in the Peak District, southern Pennines, UK. Concentrations of lead in the near‐surface peat layer at UNG are in excess of 1000 mg kg−1. For peatland environments, these lead concentrations are some of the highest globally. High concentrations of industrially derived, atmospherically transported magnetic spherules are also stored in the near‐surface peat layer. Samples of suspended sediment taken during a storm event that occurred on 1 November 2002 at UNG, and of the potential catchment sources for suspended sediments, were analysed for lead content and the environmental magnetic properties of anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM). At the beginning of the storm event, there is a peak in both suspended sediment and associated lead concentration. SIRM/ARM values for suspended sediment samples throughout the storm reveal that the initial ‘lead flush’ is associated with a specific sediment source, namely that of organic sediment eroded from the upper peat layer. Using the magnetic ‘fingerprinting’ approach to discrimination of sediment sources, this study reveals that erosion of the upper peat layer at UNG is releasing high concentrations of industrially derived lead (and, by inference, other toxic heavy metals associated with industrial particulates) into the fluvial systems of the southern Pennines. Climate‐change scenarios for the UK, involving higher summer temperatures and stormier winters, may result in an increased flux both of sediment‐associated and dissolved heavy metals from eroding peatland catchments in the southern Pennines, adversely affecting the quality of sediment and water entering reservoirs of the region. Copyright © 2005 John Wiley & Sons, Ltd.

Controls of soil pipe frequency in upland blanket peat

Soil pipes were surveyed in 160 British blanket peat catchments using consistent application of ground‐penetrating radar. Soil pipes were found in all catchments. The mean frequency of piping was 69 per kilometer of surveyed transect. Land management (moorland gripping) appears to exert the most important control on hillslope pipe frequency in blanket peats. Management practice in peatlands may therefore induce more rapid subsurface erosion, carbon loss, and landform denudation via enhanced preferential flow. Topographic position is also important, with topslopes having greater pipe frequencies than footslopes, followed by midslopes with lowest frequencies. Slope gradient, however, is not a significant factor in controlling blanket peat pipe frequency. I propose that peat structural properties inherited from the way a blanket peat develops on a hillslope strongly control pipe network development. This is manifested in the way slope position appears to control pipe frequency. Aspect appears not to influence pipe frequency in blanket peats, except that it does play a weak role in catchments with annual precipitation <1500 mm. Here southwesterly facing slopes tend to have more frequent piping.

Hydrological connectivity of soil pipes determined by ground‐penetrating radar tracer detection

AbstractSoil pipes are common and important features of many catchments, particularly in semi‐arid and humid areas, and can contribute a large proportion of runoff to river systems. They may also significantly influence catchment sediment and solute yield. However, there are often problems in finding and defining soil pipe networks which are located deep below the surface. Ground‐penetrating radar (GPR) has been used for non‐destructive identification and mapping of soil pipes in blanket peat catchments. While GPR can identify subsurface cavities, it cannot alone determine hydrological connectivity between one cavity and another. This paper presents results from an experiment to test the ability of GPR to establish hydrological connectivity between pipes through use of a tracer solution. Sodium chloride was injected into pipe cavities previously detected by the radar. The GPR was placed downslope of the injection points and positioned on the ground directly above detected soil pipes. The resultant radargrams showed significant changes in reflectance from some cavities and no change from others. Pipe waters were sampled in order to check the radar results. Changes in electrical conductivity of the pipe water could be detected by the GPR, without data post‐processing, when background levels were increased by more than approximately twofold. It was thus possible to rapidly determine hydrological connectivity of soil pipes within dense pipe networks across hillslopes without ground disturbance. It was also possible to remotely measure travel times through pipe systems; the passing of the salt wave below the GPR produced an easily detectable signal on the radargram which required no post‐processing. The technique should allow remote sensing of water sources and sinks for soil pipes below the surface. The improved understanding of flowpath connectivity will be important for understanding water delivery, solutional and particulate denudation, and hydrological and geomorphological model development. Copyright © 2004 John Wiley & Sons, Ltd.

Runoff production in blanket peat covered catchments

Blanket peat catchments exhibit flashy regimes, but little is known about the exact nature of runoff production processes within these catchments. Catchment, hillslope, and plot‐scale monitoring results are presented from the blanket peats of the northern Pennines, United Kingdom. Catchment efficiency for three study catchments with areas of 11.4 km2, 0.83 km2, and 0.44 km2 were 72%, 77%, and 82%, respectively. Mean rainfall peak to discharge peak lag times were 2.7 hours, 2.1 hours, and 3.2 hours, respectively. Runoff from hillslopes in the catchments was gauged using runoff troughs. Mean hillslope lag times ranged from 0.6 to 2.2 hours, and despite large differences in the scale of study, there was little difference in response time. This indicates that slopes and channels are strongly hydrologically coupled in these peat systems. Saturation excess overland flow dominates hillslope runoff, particularly on more gentle slopes, and on footslopes where overland flow occurs most frequently. Overall, 81.5% of the total overland flow and matrix through flow collected by runoff troughs occurred at the peat surface, with 17.7% between the surface and 5 cm depth, 0.7% between 5 and 10 cm depth, and less than 0.1% from below 10 cm depth, despite the thickness of the peat deposit. Most storm flow is produced as saturation excess overland flow, whereas most low flow is produced by through flow from 1 to 5 cm depth. Topography and preferential flow paths are important controls on the spatial pattern of runoff even on low‐gradient peat. On steeper midslope regions, more flow occurs within the upper 10 cm of peat rather than at the surface. Preferential flow paths are identified at both the hillslope and plot scale. Discharge deep within the peat is only produced from small macropore and pipe outlets that are well connected to the peat surface. Response to rainfall from these macropore networks is rapid, and they generate around 10% of streamflow.

Hydrological studies on blanket peat: the significance of the acrotelm‐catotelm model

Summary Runoff production in blanket peat catchments of the northern Pennine hills, UK was measured through monitoring and experimentation at the plot, hillslope and catchment scale. Water flow from soil pipes was measured in one of the study catchments and overland flow, throughflow and water table were measured in runoff plots; rainfall simulation and tension‐infiltrometry provided information on infiltration characteristics of the peat. Saturation‐excess overland flow was found to dominate the flashy flow regime; acrotelm stormflow, subsurface pipeflow and macropore flow were also found to be important components of the ecohydrological system. Surface cover, topography and preferential flowpaths were found to be important factors in controlling infiltration and runoff production. Streamflow generation processes that are consistent with the acrotelm‐catotelm model are shown to occur in blanket peat with and without Sphagnum cover, but in one of the catchments studied an estimated 10% of the discharge bypassed this route and discharged via pipes. The spatial and temporal variation in hillslope‐scale runoff production was demonstrated in the study catchments. This variability in runoff production will be important for hydroecological understanding in peatlands but is often neglected because of over‐simplification of processes provided by the traditional two‐dimensional acrotelm‐catotelm model.

Laboratory experiments on drought and runoff in blanket peat

SummaryGlobal warming might change the hydrology of upland blanket peats in Britain. We have therefore studied in laboratory experiments the impact of drought on peat from the North Pennines of the UK. Runoff was dominated by surface and near‐surface flow; flow decreased rapidly with depth and differed from one type of cover to another. Infiltration depended on the intensity of rain, and runoff responded rapidly to rain, with around 50% of rainwater emerging as overland flow. Drought changed the structure of the peat and the subsequent behaviour of the peat in response to rain. Surface runoff was reduced, infiltration increased and flow increased within the deeper peat layers. Old and new water produced from the peat during simulated storms was identified by bromide tracing; the amount of old mobile water flushed out of the top few centimetres was small and there was less from deeper peat layers. No significant difference in the old and new water mixing processes could be identified between the control plots and the drought treatment plots. Lissamine staining showed preferential bypass flow through macropores in the peat, though only in the top 5 cm. Following drought, however, macroporosity increased within the upper peat layers, and preferential flow extended deeper than in controls. Peat structure recovered somewhat after drought, but the effects of the drought were long‐lasting. If these effects extend to the field during drier summers then we can expect changes to the hydrology and associated chemistry of blanket peat catchments in the British uplands.

Infiltration, runoff and sediment production in blanket peat catchments: implications of field rainfall simulation experiments

AbstractBlanket peat covers the headwaters of many major European rivers. Runoff production in upland blanket peat catchments is flashy with large flood peaks and short lag times; there is minimal baseflow. Little is known about the exact processes of infiltration and runoff generation within these upland headwaters. This paper presents results from a set of rainfall simulation experiments performed on the blanket peat moorland of the North Pennines, UK. Rainfall was simulated at low intensities (3–12 mm h−1), typical of natural rainfall, on bare and vegetated peat surfaces. Runoff response shows that infiltration rate increases with rainfall intensity; the use of low‐intensity rainfall therefore allows a more realistic evaluation of infiltration rates and flow processes than previous studies. Overland flow is shown to be common on both vegetated and bare peat surfaces although surface cover does exert some control. Most runoff is produced within the top few centimetres of the peat and runoff response decreases rapidly with depth. Little vertical percolation takes place to depths greater than 10 cm owing to the saturation of the peat mass. This study provides evidence that the quickflow response of upland blanket peat catchments is a result of saturation‐excess overland flow generation. Rainfall–runoff response from small plots varies with season. Following warm, dry weather, rainfall tends to infiltrate more readily into blanket peat, not just initially but to the extent that steady‐state surface runoff rates are reduced and more flow takes place within the peat, albeit at shallow depth. Sediment erosion from bare peat plots tends to be supply limited. Seasonal weather conditions may affect this in that after a warm, dry spell, surface desiccation allows sediment erosion to become transport limited. Copyright © 2002 John Wiley & Sons, Ltd.

Application of ground‐penetrating radar to the identification of subsurface piping in blanket peat

AbstractNatural soil pipes are common and significant in upland blanket peat catchments yet there are major problems in finding and defining the subsurface pipe networks. This is particularly important because pipeflow can contribute a large proportion of runoff to the river systems in these upland environments and may significantly influence catchment sediment and solute yields. Traditional methods such as digging soil pits are destructive and time‐consuming (particularly in deep peat) and only provide single point sources of information. This paper presents results from an experiment to assess the use of ground‐penetrating radar (GPR) to remotely sense pipes in blanket peat. The technique is shown to be successful in identifying most of the pipes tested in the pilot catchment. Comparison of data on pipes identified by GPR and verified by manual measurement suggests that pipes can be located in the soil profile with a depth accuracy of 20 to 30 cm. GPR‐identified pipes were found throughout the soil profile; however, those within 10–20 cm of the surface could not be identified using the 100 or 200 MHz antennae due to multiple surface reflections. Generally pipes smaller than 10 cm in diameter could not be identified using the technique although modifications are suggested that will allow enhanced resolution. Future work would benefit from the development of dual‐frequency antennae that will allow the combination of high‐resolution data with the depth of penetration required in a wetland environment. The GPR experiment shows that pipe network densities were much greater than could be detected from surface observation alone. Thus, GPR provides a non‐destructive, fast technique which can produce continuous profiles of peat depth and indicate pipe locations across survey transects. Copyright © 2002 John Wiley & Sons, Ltd.

Piping and pipeflow in a deep peat catchment

Natural pipes are common in many upland blanket peats, yet little is known about pipe network morphology or pipeflow processes. Most information on soil piping comes from the shallow peaty podzols of the Welsh uplands, where monitoring suggests that pipes may be important contributors to streamflow. This paper presents information on piping and pipeflow from a deep upland blanket peat catchment in the Pennine Hills of Northern England. Pipe outlets are found throughout the soil profile ranging from the underlying substrate at ∼3-m depth to pipes which are within a few centimetres of the surface. Mean pipe diameters range from 3 to 70 cm; some pipes are over 150 m long. Slopes in the catchment are less steep than those usually associated with soil piping. Continuous flow records were obtained from 15 gauging sites on 8 separate pipes. The pipeflow response from deep blanket peat was found to be different to that reported in the shallow peaty podzols of the Welsh uplands; the distinction between ‘ephemeral’ and ‘perennial’ pipe types does not appear to be useful within the deep Pennine blanket peat. Response times from all of the pipes are short, even from pipes deep within the peat. At the same time pipes have a prolonged recession limb such that they maintain low flow for longer periods than most other runoff production processes within the catchment. Pipeflow contributes around 10% of the streamflow volume, but can at times contribute up to 30%. Soil pipes may, therefore, be far more important in some upland peat catchments than previous work has hitherto suggested.

The dynamics of dissolved nitrogen in a blanket peat dominated catchment

Precipitation, soil solution and drainage water were collected from a blanket peat catchment at Moor House National Nature Reserve in the Northern Pennine Uplands, UK, an area of moderately high N deposition. Two tributaries of the main stream were also sampled. Between 1993 and 1995 samples were analysed for NH 4 + and NO 3 − and for part of the period for organic N. Inputs of N in precipitation exceeded outputs in stream water. Organic N represented a small proportion of N inputs while inputs of inorganic N averaged 10.2 kg ha −1 a −1. Soil solution from 10 cm depth in the peat was dominated by organic N whereas at 50 cm NH 4 + slightly exceeded organic N. NO 3 − was rarely detected at either depth except during a period of exceptionally warm and dry weather in 1995. Output fluxes in stream water of organic N (5.7 to 6.5 kg ha −1 a −1) were much greater than those of inorganic N (0.6 to 2.2 kg ha −1 a −1). Inorganic N in streams was predominantly NO 3 − except in the smallest stream which had the largest concentrations of NH 4 +. This suggests that N transformations, particularly nitrification, may be taking place in the mineral soils adjacent to the streams or within the stream channel of the larger catchment.