1

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

2

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

20/06/19

71

20/06/19

76

20/06/19

81

20/06/19

86

20/06/19

91

20/06/19

96

20/06/20

01

20/06/20

06

0

5

10

15

20

Tem

pera

ture

(D

egre

es C

)

Date

3

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)

4

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) **

5

Influence of woodland on energy

exchange

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lux D

iff. (M

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-1)

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lux D

iff. (M

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-1)

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Date

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lux D

iff. (M

Jm

-2d

-1)

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Date

Qb F

lux D

iff. (M

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

6

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)

7

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

2

4

6

8

10

12

Tem

pera

ture

(D

egre

es C

)

HB

OW

FW

LM

FAWSW

8

Mean Monthly Temperature

01/03/2003 01/03/2004 01/03/2005 01/03/2006

0

2

4

6

8

10

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

9

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

10

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

11

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

12

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