British Hydrological Society Birmingham University 16th October 2015
Empirical evidence of the changing
hydrology of upland forestry through a
plantation cycle
Dr Mark Robinson
Centre for Ecology and Hydrology
Coalburn BHS2015 1
Coalburn – what and why?
UK’s longest running research catchment (1967)
Aim to follow the hydrological effects of
forestry through a plantation cycle and provide
answers to questions about its impacts
• Felling began 2015 with replanting from 2018
• Canopy closure reached 1995 - 2000
• Drained and afforested 1972 - 1973
• Moorland calibration 1967 - 1971
Integrated science for our changing world www.ceh.ac.uk
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COALBURN CV
Unique collaborative study over 45 years between
Environment Agency, Forestry Commission, Newcastle
University and CEH monitoring precipitation, streamflow
and weather
PLUS many projects and process studies at key stages of
forest cycle including:
- Impacts of drains vs trees, water chemistry (Forest Research)
- Canopy interception, water balance, floods (CEH)
- Peat and boulder clay hydrology, water chemistry (NU
Geography)
- Water colour and THMs (UU)
- Catchment modelling - SHE (NU Engineering)
Integrated science for our changing world www.ceh.ac.uk
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0.5 km 0
Longmoss
A B
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300
310
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290
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Coalburn location and main
physical characteristics
Part of Kielder Forest
Precipitation ~1300 mm / yr.
1.5 km²
Sitka spruce
Peaty soils
Kielder Reservoir
COALBURN
CARLISLE
10 km
Raingauge
Flow gauge
Automatic Weather Station
KEY
Major unplanted areas
Contours m
Soil series boundary
Integrated science for our changing world www.ceh.ac.uk
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Original moorland and double boundary ditches
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Catchment changes: Forestry drainage 1972
Integrated science for our changing world www.ceh.ac.uk
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Large area of bare soil
Forest growth across the catchment 1983 2008
Growth was initially variable About 10% unplanted
(Google Earth – Europa Technologies/Tele
Atlas/Infoterra Ltd and Bluesky)
Integrated science for our changing world www.ceh.ac.uk
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Annual catchment precipitation:
increase over time
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Precipitation : Good agreement between
onsite and regional gauges and networks
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Precipitation, flow and evaporation: P increasing, PE and Q little change…
LOSS ~ PE LOSS < PE LOSS ~ PE LOSS > PE LOSS >> PE
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Impact of land cover on water use: Actual Evaporation (P – Q) minus Potential evaporation (PE)
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Average annual losses: increasing after tree establishment
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Baseflow decline since afforestation Baseflow Index (BFI):
--- Pre-drainage
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Annual max flows – no time trend
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Number of peak exceedances No time trend apparent
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Felling not yet studied by Coalburn but many
UK forests are nearing economic maturity
Observational studies can play a crucial role:
If forests peaks (as claimed) then felling MUST peaks
BUT
If felling has little impact on peaks* then forests CANNOT
claim to have a major impact on flood reduction.
* with minimal ground disturbance
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Felling of adjacent Howan Burn Jan - May 2008
Approximate boundary
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Howan Felling (Malcolm Newson, NU and Tyne Rivers Trust)
Sediment Water colour
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Howan felling: Peak flows Jan 2000 – Oct 2011
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Observations showed felling had
little impact on large floods. A
surprise and disappointment …. ?
It is very easy to uncritically
accept statements such as
‘forests reduce floods’ because
that is what we want to hear, and
not challenge these claims.
A forest canopy holds only
~ 2-5mm of rain, which is easily
swamped by a 50 mm storm.
Paper in preparation
COALBURN FINDINGS
• Low flows reduced by forest growth whilst
forest cultivation increased baseflows
• Little evidence forest growth reduces flow
peaks or felling increases flood peaks
• Water yield reduced by ~ 200 mm/yr
__i.e. 15 – 20 % (for 90% forest cover)
Integrated science for our changing world www.ceh.ac.uk
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*Caveats:
Forest plough drains increase flood response when newly cut
Forest felling may increase peak flows due to soil compaction not biomass removal
CONCLUSIONS
Tackling environmental issues needs both
- observational studies (to set constraints and
reveal key processes) AND
- models to allow for extrapolation and studies of
consequences
Avoids the twin dangers of ‘mindless monitoring’ or
model building being an ‘indoor sport’
‘It is a capital mistake to theorize before one has data’
(A Scandal in Bohemia Sherlock Holmes)
Integrated science for our changing world www.ceh.ac.uk
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