Water tables in Peak District blanket peatlands

Research output: Book/ReportCommissioned report

  • External authors:
  • M.G. Evans
  • J.B. Lindsay
  • Clive Agnew
  • J.E. Freer
  • A. Jones
  • M. Parnell

Abstract

1. This report presents research carried out during the pilot project ???Hydrological Benefits of Moorland Restoration??? funded by the Environment Agency and the National Trust. The main objectives of the research were (i) to evaluate water table conditions and behaviour in blanket peat systems in the Peak District, (ii) to develop a model describing water table conditions at the landscape scale, and (iii) to provide a preliminary assessment of the impacts of moorland restoration on local peatland water tables.2. A detailed programme of water table monitoring was undertaken during 2008, involving regular measurements of water table depth in over 530 dipwells at 19 sites across the 47 km2 peatland landscape of the Kinder Scout / Bleaklow area. This included a campaign of regular, simultaneous water table measurements from clusters of dipwells at the main sites, supplemented by continuous (hourly) water table monitoring in selected dipwells. It also included studies to evaluate within-site variation in water table conditions and local water table drawdown effects associated with gully erosion.3. Analysis of within-site variation in water table depths shows that multiple randomly located dipwells (preferably >15) are required for reliable quantification of water table conditions at the site scale, where site scale is 30m x 30m.4. There is substantial between-site variation in average water table conditions across the blanket peat landscape, with median site water table depths varying from 26 to 451 mm. This variation is strongly associated with site erosion status. Water tables at intact sites with no erosion gullies at or proximate to the site are consistently close to the ground surface (median site water table depth typically <100 mm). However, sites with dense erosion gullies are associated with lower water table conditions (median site water table depths > 300 mm).5. Gully erosion causes water table drawdown through two distinct processes. The first is local water table drawdown immediately adjacent to erosion gullies. This effect is restricted to a zone within 2 m of gully edges, and water tables within the gully edge drawdown zone are approximately 200 mm lower than in the adjacent peatland. The second effect is a more general water table lowering at eroded sites, with median water table depths at heavily eroded sites up to 300 mm lower than intact sites. This site-scale effect is hypothesised to result from reduced hydrological contributing areas (drainage areas) at eroded sites, with hillslope drainage diverted into gully channels.6. Distinct patterns of temporal water table behaviour are apparent between intact and heavily eroded locations. At intact locations water tables are predominantly close to the ground surface, except during periods of dry weather when a pattern of gradual water table drawdown occurs. Water tables rise rapidly following rainfall. This behaviour is characteristic of intact blanket peats in other regions. Water table behaviour at heavily eroded locations is very different, characterised by predominantly low water table conditions with ???wet-up??? responses to rainfall, i.e. very rapid rises in water table followed immediately by rapid drain-down after the cessation of rainfall. These patterns demonstrate the very different hydrological behaviours of eroded and intact peats with clear implications for the hydrological functioning of the peatland.. 7. Evaluation of the topographic (wetness) index shows that it is a good predictor of water table conditions across the range of site types in the Peak District peatland landscape and a suitable basis for water table model development. However, the index does not effectively represent water table variation within the intact sites. In particular, it predicts much higher water table conditions at intact plateau/flat sites than at intact hillslope sites, a pattern not observed in the measured data. This finding has important implications for the use of the topographic index to represent hydrological conditions in intact blanket peat systems.8. A landscape-scale water table model has been developed based on high resolution topographic (LiDAR) data. This assumes that topography represents the key control on peat water tables at the landscape scale. The model is based on calibration of the topographic (wetness) index against measured site water table data, modified to account for the effect of local gully edge water table drawdown. It predicts median water table depth at the site scale. The model requires further development and validation, and is therefore described as ???first-order???, but initial model application provides a prediction of water table conditions across the peatland landscape. This application identifies high water table conditions in the remaining intact areas of the peatland and demonstrates the extent of water table lowering associated with gully erosion in the Bleaklow and Kinder Scout areas.9. Comparison of water tables at bare peat and restored (re-vegetated) sites indicate higher water tables at the restored sites. This suggests that water tables can be raised by re-vegetation of bare peat. If confirmed this has significant implications for moorland restoration strategies as well as for hydrological and runoff processes in bare and restored systems. However, the current analysis is based on too few sites to be statistically significant and further work is required to confirm the observations10. The final section of the report makes recommendations of further work required to more fully evaluate the hydrological functioning of the peatlands and the hydrological effects of moorland restoration. In particular we recommend:??? further model development and validation, including evaluation of model uncertainty to underpin future applications of the water table model, and terrain analysis to more fully establish the processes of water table lowering at eroded sites;??? a field study to characterise drainage and water table behaviour on intact hillslopes, in order to improve hydrological modelling at intact sites;??? a field study to confirm the observation that restoration by re-vegetation results in significant rises in peat water tables;??? a more comprehensive research programme to evaluate the hydrological functioning of these peatlands, focusing on the impacts of restoration on runoff generation and downstream flow;??? an assessment of the relationship between water table conditions and runoff water quality.11. The research has provided important baseline data on water table variation and behaviour in the Peak District blanket peats and demonstrated the viability of water table modelling at the landscape scale. It has several immediate applications, including the provision of water table data for carbon models and for targeting Sphagnum propagation work. It will also underpin further research efforts, including comprehensive evaluation of the hydrological effects of moorland restoration and the implications of predicted climate change for water table conditions in these peatland systems.

Bibliographical metadata

Original languageEnglish
Place of PublicationEdale, Derbyshire, UK.
PublisherMoors for the Future
Number of pages46
Publication statusPublished - Jul 2009

Publication series

NameMoors for the Future Reports.
PublisherMoors for the Future Partnership

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