Monitoring the impacts of peatland drain blocking. LIFE-Nature Active Blanket Bogs in Wales Lorraine Wilson, J. Wilson, J. Holden, A. Armstrong, I. Johnstone & M. Morris
The study siteLake Vyrnwy catchment:- 10,000 ha- approx. 4,800 ha blanket bog- upland areas were drained in 1950-70s- SAC, SPA, SSSI.- blanket bog in ‘unfavourable’ condition due todrainage & historic overgrazing & burning- Lake is the water supply to Liverpool-Upland areas are hill farmed by the Royal Societyfor the Protection of Birds (low levels, April-Oct only)
The project aims• Restore damaged blanket bog to ‘favourable’ condition – Blocking drainage ditches across a large upland catchment• Structure restoration to provide experimental conditions – 4 sub-catchments, blocked sequentially 1 per winter – Longitudinal (before/after) data per sub-catchment – Experiment/Control data between sub-catchments• Research targeted to address stakeholder concerns & project priorities. – Recovery of vegetation? – Recovery of conditions for vegetation (higher, stable water tables)? – Effects on organic carbon release & discharge water colour? – Implications for flood risk management? – Effects on hill sheep farming?
Field methods: Hydrology •Dipwell & Creststage tubes at: 0.5, 1, 5, 10, 20m from drains. •Water grab samples from drains & streams. •Automatic pressure transducers in 4 drains, 3 streams & 3 dipwells. •Nov 2007 – ongoing.
Water tables 5 0 Water table depth (cm) 0 100 -5 Unblocked -10 Blocked -15 -20 % Time water table was below given depthWater tables and surface water increased within 5m of drains, especiallydownslope.Surface water also increased much more widely.Water tables less variable after blocking, particularly during the summer.
Peak flow events 600 1Total discharge (m3 s-1) Unblocked 0.8 400 Blocked 0.6 0.4 200 0.2 0 0 Baseflow Runoff Efficiency Flashiness Based on 20-25 events from 4 drains and 3 streams. Also: peak flow rates declined & recession duration increased Response in streams less marked than in drains, but still significant. Blocking leads to less flashy peak flow events, with slower release of rainwater & more water being held in the bog.
Drought events 0 0.5m 1m 5m Unblocked Adjusted drain flow rate -2 0.E+00 BlockedWater table (cm) -4 -6 -8 -5.E-04 -10 Unblocked -12 Blocked -1.E-03 -14 Dist. to drain 1 5 9 13 Days Based on data from 17 drought events (of 5-18 days) from 4 drains, 3 streams and 78 dipwells. Blocking reduces the degree of water table drawdown during droughts, and maintains more constant flow rates in both drains and streams.
Water colour 0.1 2.37 Absorbance 400nm Flow weighted Abs400 Absorbance Units 0.05 0 Unblocked Blocked Unblocked Blocked Grip StreamModels show no significant changes in absolute Absorbance at 400nmFlow weighted absorbance declined in streams, and stayed stable in grips.
Organic carbon100.00 10.00 DOC POC Concentration 769 38.2 (mg/L) 80.00 8.00 Concentration (mg/L) Load (mg/s) Load (mg/s) 60.00 6.00 4.00 40.00 2.00 20.00 0.00 0.00 Unblocked Blocked Unblocked Blocked Unblocked Blocked Unblocked Blocked Grip Stream Grip Stream DOC levels increased in grips after blocking. DOC concentrations in streams did not change & reductions in flow led to very large reductions in load. POC concentrations were unaffected by blocking. Loads declined in both grips and streams.
Organic carbonDrought water colour and organic carbon release showed slight declines after blocking.Post drought ‘flushes’ of water colour and organic carbon were significantly reduced.
Organic carbonEstimating total annual organic carbon fluxes from the study site:• Variable POC levels during baseflow = possible overestimation• Extrapolated high resolution datasets: – Rating equations for baseflow, rising limb and falling limb flow periods per sub-catchment. R2 values = 0.30-0.67• Flux estimation using Method 5 (Walling & Webb, 1985) – Accurate but imprecise = large error margins – Per sub-catchment stream, per full year blocked/unblocked – Averaged across all sub-catchments & total peatland area of catchmentBefore blocking: 554 - 927 t/km2/yrAfter blocking: 22 - 37 t/km2/yr
Hydrology Summary1. Has drain blocking allowed water tables to recover adjacent to drains. – YES. Although recovery is still underway. – Water tables also become more stable & more ‘drought resistant’.2. Has drain blocking reduced water colour & fluvial organic carbon loss? – YES.3. Does drain blocking have the potential to mitigate downstream flood risk? – YES.
Vegetation recovery 60 0.68 Sedges & rushes 50 Sphagnum spp. Condition score 40 0.64% cover 30 20 0.6 10 0 0.56 Control Unblocked Blocked Control Unblocked Blocked Wet specialist species have increased in blocked areas & dry tolerant species have declined. Statutory ecological condition is improved by drain blocking. Implications for gaseous flux trends but further study needed to model this.
Hill sheep farmingLocal farming community initially unwilling to consider drain blocking.Research through meetings, talks, open & demonstration days identified main areas of concern:1. Decrease in grazing quality.2. Decreased access to blocked areas.3. Increased stock entrapment in blocked drains.4. Increased prevalence of parasite-borne diseases.Vegetation survey data & targeted basic field surveys carried out to attempt to address each point.
Hill sheep farming 100 Control Unblocked 75 Blocked 50 25 0 Grass Sedge Dwarf shrub SheepPreferred fodder species (grasses) showed no change after blocking.Sheep accessed wet areas more widely after blocking, probably due to increased availability of crossing points.
Hill sheep farming 0.2 Ticks /km 0.15 Carcasses /km/yr 0.1 0.05 0 Control Unblocked BlockedParasite surveys hampered by very low local abundances, but suggest that ticks decline after drain blocking.Stock loss in blocked drains significantly less than prior to blocking.
Vegetation & farming summary1. Has drain blocking allowed vegetation condition to recover? – YES. – Change is still ongoing, current communities are not the end point.2. Does drain blocking reduce sheep farming viability? – NO. It may even be a positive management tool. – Grazing is unchanged and access to wet grazing increases. – Stock entrapment in drains declines. – Parasite abundances may decline.3. Regular communication of these studies has led to the LIFE project & CCW entering into 12 private landowner agreements to carry out drain blocking on 2500ha of blanket bog.
Conclusions• Drain blocking at Lake Vyrnwy appears to be delivering for several key ‘ecosystem services’, ranging from habitat conservation to drinking water quality.• Fluvial organic carbon fluxes, and changes to vegetation communities will have profound impacts on overall carbon balances from restored peatlands. More research is needed to model the role of vegetation change in gaseous fluxes.• Peatland restoration is at least a neutral agricultural management tool & thus has potential to be spread beyond protected sites with appropriate advocacy and research.Ongoing research:• Study of vegetation responses to tree removal and experimental grazing.• Study of vole and passerine trends on blanket bogs, testing for responses to drain blocking, using RSPB survey data.
AcknowledgementsM. Morris, F. Walker & J. Lane for help with fieldwork. The study site is owned by Severn Trent Water.LIFE-Nature fund & project partners: