Influence of hydroclimaticforcing on stream temperatures –role … · 2018-01-19 · 1 Influence...
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Influence of hydroclimatic forcing on stream
temperatures – role of riparian management on
ecosystem response
“North-Watch” Workshop III: Hydroecological responses to climate change in northern catchments
Iain Malcolm
Outline
• Why are we interested in stream temperature?
• Evidence of changing stream temperatures in Scotland
• Processes controlling stream temperature
• Physical controls on stream temperature
• Riparian woodland and stream temperature
• Riparian woodland: Understanding implications for fish
• Practical value, what are we asked, what can we say
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Why are we interested?
• Influences physical, chemical and biological processes
• In fish influences growth and survival at various life stages, ova, juvenile, adult
• Also has potential to influence population demographics and production
• Potentially important for assessment (including tool development)
• Increasing interest under climate change
• Implications for fish populations and mitigation / adaptation?
Evidence for changing stream temperature
• Not allot of data
• Very few records > 2
decades
• Especially sites independent
of significant landuse
change
• Data from Girnock Burn -
First 30 years published by
Langan et al., 2001
• 1966-2006 Ca. 0.6 degree
increase in mean T20
/06/19
66
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0
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10
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Tem
pera
ture
(D
egre
es C
)
Date
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Seasonal variability in mean monthly temperature trends
(1966-2005)
JanApril
OctJuly
Gurney et al., 2009
-0.76 Deg. C1.04 Deg. C
1.46 Deg. C-0.57 Deg. C
Processes controlling stream temperature
• Stream temperature is the net outcome of range of energy exchange processes
• Boundaries at stream – streambed and stream-atmosphere
• Thermal exchange processes include:– Radiation (shortwave, longwave)
– Sensible heat (heat transfer not involving change of state)
– Latent heat (transfer involving a change in state e.g. evaporation, condensation)
– Bed heat flux (conduction, friction)
– Advected heat (horizontal transfer e.g. tributaries, Groundwater, inflow, outflow)
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Heat added
•incident short-wave radiation
•long-wave (down) radiation
•condensation
•friction at bed and banks
•chem and bio processes
Heat lost
•reflected short-wave radiation
•long-wave (skyward) radn
•evaporation
Advection:
•Channel (inflow, outflow)
•Precipitation
•Tributary inflow
•Groundwater (gaining, losing)
Hannah et al., 2008
Controls on stream temperature
• Often affect multiple processes
• Include
– Altitude
– Topography
– Channel geometry (width, depth)
– Channel orientation
– Channel incision
– Groundwater – surface water exchange
– Thermal capacity of stream
– Riparian tree cover (subject to management) **
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Influence of woodland on energy
exchange
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Date
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lux D
iff. (M
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lux D
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Qb F
lux D
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Jm
-2d
-1)
Moorland minus forested differences: (a) net radiation,
(b) sensible heat, (c) latent heat and (d) streambed heat flux
positive = moor>forest negative = moor<forest
Influence of riparian woodland
on stream temperature
• Typically two field based methods for assessing
influence of woodland on stream temperature
1. Spatial differences between open and forested sites
2. Temporal changes in stream temperature pre- / post-
felling or planting
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Spatial variation in stream temperature & role
of riparian tree cover
Inverness
Dundee
N
AberdeenGirnock
FW (0.75km), LM (1.5
km), FAWSW (2.0km)
Riparian woodland sites
HB and OW open sites
0.01 1 10 40 70 95 99.5
0.1
1
10
Dis
charg
e (cum
ecs)
% Time exceeded
200304
200405
200506
•Data collected over 3 annual cycles
•2003-04 driest year
•2004-05 wettest year
•2005-06 variable (intermediate)
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Maximum Monthly Temperature
01/03/2003 01/03/2004 01/03/2005 01/03/2006
5
10
15
20
25
30M
ax T
em
p (D
egre
es C
)
HB
OW
FW
LM
FAWSW
Minimum Monthly Temperature
01/03/2003 01/03/2004 01/03/2005 01/03/2006
-2
0
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12
Tem
pera
ture
(D
egre
es C
)
HB
OW
FW
LM
FAWSW
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Mean Monthly Temperature
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0
2
4
6
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12
14
16
18
Tem
pera
ture
(D
egre
es C
) HB
OW
FW
LM
FAWSW
(Some) examples of potential implications of riparian
woodland for fish populations
• Riparian woodland affects physical habitat, temperature, food
availability. Assessing effects complicated (Phil to discuss
further Tuesday afternoon)
• Potential to reduce maximum temperatures (under climate
change): implications for fish mortality
• Reduced temperature variability: implications for performance
of juvenile fish
• Overall effect of woodland on fish performance
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Potential for riparian woodland to mitigate
increased maximum temperatures
• Difficulties in assessing
future temperature
changes
• Air T – Water T
relationships potentially
offer insights
• Assumption relationship
will remain constant under
climate change
Hrachowitz et al. 2010
Regression model for mean weekly maximum
temperatures based on monitoring of Dee catchment
Hrachowitz et al. 2010
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Regression based predictions of climate change
temperatures and effects of land management
•Riparian tree cover strong control
on summer temperatures
• Regression model suggests
average reduction in mean maximum
weekly T of 1.4 degrees C
•Targeted tree planting could
mitigate against temperature
extremes
Temperature variability and fish
performance
•Constant regime
•Variable regime
•Same mean
Imholt et al. in press
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Temperature variability and fish
performance
We
igh
t(g
)L
en
gth
(mm
)
Condition
01
02
03
04
0
*b
Weight
80
10
01
20
14
0 a
Length
•Fish measured fortnightly
•Replicate tank treatments
•High and low ration
•Effect of daily T range small
•2.6 % (length), 8% (weight)
•Mean T adequate for
assessing performance
•T effects of forestry, likely to
be limited
High rations
Low rations
Variable
Constant
Field assessment of forest effects on juvenile
salmon
01/Dec/02 00:00 01/Jun/03 00:00 01/Dec/03 00:00 01/Jun/04 00:00 01/Dec/04 00:00 01/Jun/05 00:00 01/Dec/05 00:00
30
40
50
60
70
80
90
100
110
120
Fork
length
(m
m)
Moorland
Forest
01/12/2002 01/06/2003 01/12/2003 01/06/2004 01/12/2004 01/06/2005 01/12/2005
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Fry
density p
er m
2
• Separating forest or T
effects from density
effects difficult
• Further discussion
PJB Tues afternoon
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Conclusions• Limited data to assess long-term changes in stream T
• Available data suggests seasonally variable long-term changes in stream T
• Riparian woodland affects a wide range of exchnage processes
• Net outcome of forestry is reduced T variability, reduced max T, increased min T, slight increase in mean T
• T affects fish populations, from changes in performance to mortality (see PJB Tuesday)
• Experimental work suggests changes to T variability likely to have limited effect
• Field based studies and modelling suggest riparian land management could mitigate against T extremes
• Hard to assess overall influence of stream temperature and riparian management in field based studies due to complexity of processes involved
• Hard to assess likely impact of climate change on fish given unknowns such as changes in food availability