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

2. Long term trends
Černé Lake – Bohemian Forest
•
Vrba et al. 2003.
STOTEN, 310, 73–85.

3. Long term trends
Alsterålven - Sweden
Fosberg, C. 1992. Hydrobiologia 229: 51-58, 1992.

4. 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.

5. DOC trends over 2 decades at AWMN sites
Plots by Gavin Simpson UCL.

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

7. UK outliers
only region where model unable to explain the regional trend

8. Explanations for UK’s deviant tendency
• Sulphate deposition trend
underestimated, due to early 70
retention & later desorption of
seasalt derived SO42-? 65
60
• Two sources of Cl in some regions
55
(seasalt and industrial) resulting in
dual effects? 50
-10 -5 0 5 10 15 20 25 30 35
Yellow circles proportional to Nature model outlier size
• Non-linear responses to declining
deposition – most outliers are for
sites showing largest %ΔDOC
trends?

9. On site-by-site basis SO42- and Cl- variables are capable of completely
explaining long term DOC trends in AWMN lakes
14
RLGH
NAGAR
CHON
12
TINKER
GRANN
SCOAT
10 BURMOOR
LLAGI
BLUE
Raw data 8
DOC (mg/l)
6
4
2
0
1988 1988 1989 1990 1992 1992 1993 1994 1996 1996 1997 1998 2000 2000 2001 2002 2004 2004 2005 2006 2008
8
RLGH_SRES
NAG_SRES
CHON_SRES
TINK_SRES1
6
GRANN_RESI1
SCOAT_SRES1
BURN_SRES1
deposition 4 LLAGI_SRES1
BLUE_SRES1
standardised residuals (from dep model)
model 2
standardised 0
residuals -2
-4
-6
[DOC] = α-(log [SO4]) + β-(log [Cl]) + c

10. 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

11. • 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?
•