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(a) Welsh upland stream
(b) English chalk stream
12
12
10
10
8
8
6
6
4
4
2
2
0
0
1975
1980
1985
1990
1995
2000
2005
2010
1975
1980
1985
1990
1995
2000
2005
2010
Year
Year
(c) Welsh upland stream (residuals)
(d) English chalk stream (residuals)
1.5
3
2
1
1
0.5
0
0
-1
-0.5
-2
-1
-3
-4
-1.5
1975
1980
1985 1990
1995 2000
2005
2010
1975
1980
1985
1990
1995
2000
2005
2010
Year
Year
Figure 9.1
Recent temperature trends (1981-2007) in upland Welsh streams (redrawn after Durance and Ormerod,
2007) and a lowland English chalk stream (redrawn after Durance and Ormerod, 2009) before and after accounting for
the effects of the North Atlantic Oscillation (NAO) (Ormerod, 2009).
also import heat by advection - i.e. inwards
transport - through water arriving from their
catchments. These effects vary with season,
channel morphology, valley topography, riparian
vegetation and substratum conditions; hydrological
influences on temperature from groundwater
contributions, variations in discharge and river
regulation are also important (Caissie, 2006).
The best available studies reveal that short-wave
radiation resulting from sunlight is a dominant
source of river warming in summer, contributing
80% or more of heat gain in unshaded upland
rivers (Webb and Zhang, 1997, 2004). However,
in winter, friction with the river bed becomes
the largest heat source in these watercourses
(
40-70%). Heat losses occur largely through
long-wave (infra-red) radiation (40-65%) and
evaporative cooling (10-20%) throughout the
year, but heat transfer in winter (20-50%)
and conductance into the river bed in summer
(30%) are also important. Cooling and heating
processes
example, warming of shaded woodland streams by
short-wave radiation in summer is secondary to
the effects of scattered long-wave radiation and
heat transfer (Webb and Zhang, 2004). Lowland
chalk streams are warmed dominantly by solar
radiation, but cooled dominantly by evaporation
and heat transfer, particularly when groundwater
contributions are warmer than the atmosphere
(Webb and Zhang, 1997). In Antarctic glacial melt-
water channels, radiation can be responsible for as
much as 99% of heat gain (Cozzetto
et al.
, 2006),
while in other glacial streams frictional heating
sometimes exceeds radiative heating even though
stream temperature tracks radiative heat inputs
most closely (Chikita
et al.
, 2010).
The main implications of these processes for
understanding the role of climate change in stream
warming are: (i) radiation and insolation are
likely to be the dominant warming mechanisms
in unshaded rivers, at least in summer; (ii)
relative warming or cooling by direct heat transfer
appears only to be important where temperature
vary
also
among
stream
types.
For
