uk doc long-term trends in an international perspective

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UK DOC long-term trends in an international perspective Don Monteith CLAD 21 st October 2009

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UK DOC long-term trends in an international perspective. Don Monteith CLAD 21 st October 2009. Secchi disc depth (m). Long term trends Č ern é Lake – Bohemian Forest. Vrba et al. 2003. STOTEN, 310, 73–85. Long term trends Alster ålven - Sweden. - PowerPoint PPT Presentation

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Page 1: UK DOC long-term trends in an international perspective

UK DOC long-term trends in an international perspective

Don Monteith CLAD 21st October 2009

Page 2: UK DOC long-term trends in an international perspective

•S

ecch

i dis

c de

pth

(m)

Long term trendsČerné Lake – Bohemian Forest

Vrba et al. 2003.

STOTEN, 310, 73–85.

Page 3: UK DOC long-term trends in an international perspective

Long term trendsAlsterålven - Sweden

Fosberg, C. 1992. Hydrobiologia 229: 51-58, 1992.

Page 4: UK DOC long-term trends in an international perspective

Large trends in the chemical climate over the UK over last 2 decades (AWMN data)

Non-marine SO42- concentration Cl- concentration

Plots by Gavin Simpson UCL.

Page 5: UK DOC long-term trends in an international perspective

DOC trends over 2 decades at AWMN sites

Plots by Gavin Simpson UCL.

Page 6: UK DOC long-term trends in an international perspective

Monteith, Stoddard et al. 2007, Nature 450, 537-541

Page 7: UK DOC long-term trends in an international perspective

UK outliers

only region where model unable to explain the regional trend

Page 8: UK DOC long-term trends in an international perspective

Explanations for UK’s deviant tendency

50

55

60

65

70

-10 -5 0 5 10 15 20 25 30 35

• Sulphate deposition trend underestimated, due to early retention & later desorption of seasalt derived SO4

2-?

• Two sources of Cl in some regions (seasalt and industrial) resulting in dual effects?

• Non-linear responses to declining deposition – most outliers are for sites showing largest %ΔDOC trends?

Yellow circles proportional to Nature model outlier size

Page 9: UK DOC long-term trends in an international perspective

0

2

4

6

8

10

12

14

1988 1988 1989 1990 1992 1992 1993 1994 1996 1996 1997 1998 2000 2000 2001 2002 2004 2004 2005 2006 2008

DO

C (

mg

/l)

RLGH

NAGAR

CHON

TINKER

GRANN

SCOAT

BURMOOR

LLAGI

BLUE

-6

-4

-2

0

2

4

6

8

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

stan

dard

ised

res

idua

ls (f

rom

dep

mod

el)

RLGH_SRES

NAG_SRES

CHON_SRES

TINK_SRES1

GRANN_RESI1

SCOAT_SRES1

BURN_SRES1

LLAGI_SRES1

BLUE_SRES1

[DOC] = [DOC] = αα-(log [SO-(log [SO44]) + ]) + ββ-(log [Cl]) + c-(log [Cl]) + c

Raw data

deposition model standardised residuals

On site-by-site basis SO42- and Cl- variables are capable of completely

explaining long term DOC trends in AWMN lakes

Page 10: UK DOC long-term trends in an international perspective

Upland Water Myths• Evans et al.. (2005) have shown that decreases in SO4 deposition did not begin until the 1980s in

the UK.– Evans et al. (2005) never said this, and S deposition has been falling since the mid 1970s

• Worrall et al. (2003) have shown that increases in DOC flux started in at least the 1960s.– Time series stretch back to 1960s but no indication of inflection until the mid 1970s

• Worrall and Burt (2007) present huge reductions in DOC at all sites within one operational region only - SW England (HMS data) - to illustrate diverging trends within UK.

– Trends not supported by independent time series from same region. Data compromised by insufficiently documented changes in methodology.

Page 11: UK DOC long-term trends in an international perspective

• UK DOC trends over last two decades are consistent with other regions of northern Europe and North America that have undergone large reductions in sulphate / and or chloride.

• Relationship suggests a control on DOC solubility by either/or acidity and ionic strength. Work to address this is ongoing.

• There are few, if any, examples of long-term DOC trends in the UK, where high quality sulphate and chloride concentration data are available, where these two variables cannot entirely explain the trend.

• Important to distinguish between this long-term driver and shorter term climatic and land-use drivers. The latter will set the boundaries for DOC variability as sulphur deposition fall background levels.

• Important to determine what is happening to DOC quality.

• Where land-use is largely unaltered, DOC is returning toward pre-acidification levels – but where soil base cation saturation is depleted, will it overshoot?

• Water treatment chlorination was only implemented after DOC had been suppressed by “acid rain”? Is it sustainable?

• What are the consequences for increasing solubility of DOC for the transport of industrial pollutant legacy compounds from catchments?