empirical evidence of the changing hydrology of upland ... hydrology 2015... · coalburn bhs2015 1...

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

mr@ceh.ac.uk

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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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

Integrated science for our changing world www.ceh.ac.uk

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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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