HYDROLOGICAL PROCESSES
Hydrol. Process. 20, 51– 66 (2006)
Published online 1 August 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5898
Afforestation and stream temperature in a temperate
maritime environment
B. W. Webb1 * and D. T. Crisp2
1
School of Geography and Archaeology, University of Exeter, Amory Building Rennes Drive, Exeter EX4 4RJ, UK
2 21A Main Street, Mochrum, Newton Stewart, Wigtownshire DG8 9LY, Scotland, UK
Abstract:
There have been few long term investigations of the effects of afforestation on stream temperatures in the UK, and the
present study uses the results of continuous monitoring of water temperatures in a forest and a moorland stream of the
Loch Grannoch area in southwest Scotland over a 4 year period to investigate the effects of planting coniferous forest
on stream thermal regime. The presence of a coniferous tree canopy resulted in a lowering of mean water temperatures
by ¾0Ð5 ° C but larger reductions in summer monthly mean maxima and diel ranges of up to 5 ° C and 4 ° C respectively.
The diel cycle in the forested stream lagged behind that of the moorland site in all months of the year, but the delay in
timing was greater for the peak than for the trough in the diel cycle. Mean water temperatures were higher in the forest
stream during the mid-winter months, reflecting higher minimum values. Contrasts in stream thermal regime between
forest and moorland showed relatively little interannual variability over the study period. Continuous monitoring of
air temperatures during 2002 revealed contrasts between the study sites that were less pronounced for air than for
water temperature, and suggested it is the shading of incoming solar radiation that has a strong effect in determining
the water temperature behaviour of the forested stream. Although the biological impact of the observed contrasts in
stream temperature between land uses is likely to be relatively modest, the presence of forest cover moderates the
occurrence of high summer temperatures inimical to the survival of some salmonid species. Copyright  2005 John
Wiley & Sons, Ltd.
KEY WORDS
stream and air temperatures; land use impacts; forestry; annual, seasonal and diel variation
INTRODUCTION
Temperature is an ecologically important parameter of lotic freshwaters and is highly sensitive to human
impacts, both directly through the discharge of heated effluents (e.g. Langford, 1990) and indirectly through
modification of catchment land use, in particular by impoundment of rivers (e.g. Petts, 1984, 1986) and
forestry practices (e.g. Beschta, 1987; Crisp et al., 2004). The effect of harvesting coniferous forest on stream
temperatures has been the subject of much research in North America, dating back more than 50 years (e.g.
Greene, 1950; Moore et al., 2005). Paired catchments and other approaches (e.g. Holtby, 1988) have been
employed to assess changes in thermal regime following tree removal (e.g. Brown and Krygier, 1970; Rishel
et al., 1982; Hostetler, 1991), the effectiveness of buffer strips (e.g. Bourque and Pomeroy, 2001; Curry
et al., 2002), downstream persistence of temperature changes (e.g. Zwieniecki and Newton 1999), the period
of recovery to pre-harvest temperatures (e.g. Johnson and Jones, 2000) and the potential impacts on aquatic
ecosystems (e.g. Macdonald et al., 2003). Studies of forest harvesting impacts on water temperature outside
North America are relatively limited, but some investigations have been carried out in New Zealand (e.g.
Graynoth, 1979; Rowe and Pearce, 1994).
* Correspondence to: B. W. Webb, Department of Geography, University of Exeter, Amory Building, Exeter, Devon EX4 4RJ, UK.
E-mail: [email protected]
Copyright  2005 John Wiley & Sons, Ltd.
Received 1 September 2004
Accepted 12 December 2004
52
B. W. WEBB AND D. T. CRISP
In the UK, studies of indirect human impacts on stream and river temperatures have tended to focus on
changes following impoundment (e.g. Lavis and Smith, 1972; Crisp, 1977, 1987, 1995; Cowx et al., 1987;
Webb and Walling, 1993, 1996, 1997; Webb, 1995). However, there has been a growing interest in the
effects of forest cover on the thermal regime of watercourses from the perspective of both deforestation and
afforestation. Planting of coniferous tree species has been recognized as the largest single land-use change
in Britain during the 20th century, and only a relatively small proportion of the timber planted has yet been
harvested (Stott and Marks, 2000).
British studies of contrasts in temperature behaviour between streams under forest and other land uses
have focused on mid-Wales, and especially on the headwaters of the River Severn (afforested with Sitka and
Norway spruce) and the neighbouring River Wye (mainly covered by semi-natural grassland and subject to
pastoral farming of relatively low intensity). Results based on weekly spot samples suggested that temperatures
in the forested Severn headwaters were 2Ð4 ° C lower in the summer and 0Ð6 ° C higher in the winter compared
with the Wye (Roberts and James, 1972). More detailed monitoring, from November 1977 to September 1979,
showed mean daily water temperatures to be lower in the Severn headwaters during summer months by up
to ¾7 ° C (Kirby et al., 1991), whereas removal of trees in 1985 from Afon Hore, a tributary of the Upper
Severn, resulted in rises in mid-morning temperatures of 4–9 ° C during the summer months compared with
the mainstem of the River Severn (Afon Hafren), where the forest cover was left intact (Neal et al., 1992). The
latter study also suggested that the rise in temperature caused by deforestation increased progressively during
the first four summers following tree removal. A more recent investigation, based on hourly monitoring of
stream temperatures in the clear-felled Afon Hore and in the mature forest of the Afon Hafren (Crisp, 1997),
has suggested that the presence of forest cover lowered annual mean water temperature by ¾0Ð4 ° C. This
impact was caused mainly by a suppression of daily minima and, more especially, daily maxima during the
summer period. A study based on hourly observations before, during and after removal of trees from 20% of
the small Nant Tanllwyth tributary in the Upper Severn during 1996 (Stott and Marks, 2000) revealed a 0Ð6 ° C
rise in mean stream temperature and increases in monthly mean maximum values of 7Ð0 and 5Ð3 ° C for July
and August respectively between the pre- and post-felling years. These increases were independent of changes
in air temperature over the study period. Comparison of water–air temperature regression relationships for
the periods before and after felling suggested little impact of tree removal on winter temperatures in the Nant
Tanllwyth.
Outside the mid-Wales area, a study of afforested and open reaches of the Kirk Burn, a tributary of the
River Tweed in southern Scotland, found that shading by conifers led to lower water temperatures in summer
but higher values in winter, and reduced seasonal and diel temperature ranges (Smith, 1980). Comparison of
detailed temperature records for several afforested and unafforested streams in the headwaters of the River
Tywi in South Wales (Weatherley and Ormerod, 1990) demonstrated that mean daily temperatures were
0Ð6–2Ð8 ° C lower in forest than in moorland between April and August, but higher in winter by up to 0Ð9 ° C.
Felling of bankside trees at one site also resulted in a fall in temperature by 0Ð7–1Ð2 ° C in January and
February, but a rise of up to 1Ð0 ° C in May and June. Clear-felling of deciduous forest in a small tributary
of the River Coquet in northern England resulted in an increase in summer temperatures by 6Ð5 ° C (Gray
and Edington, 1969), whereas a recent study of the Girnock Burn in northeast Scotland has demonstrated the
substantial impact of native riparian woodland in reducing diel variability and temperature extremes compared
with open moorland (Malcolm et al., 2004).
This study reports the impact of coniferous forest cover on stream temperatures in southwest Scotland,
a region of Britain where the effect of afforestation on thermal regime has, hitherto, not been investigated.
Furthermore, the present investigation differs from previous UK work in two respects. First, the study has
involved detailed (resolution 30 min) temperature monitoring over a 4 year period (2000–03), which is longer
than in previous investigations, and allows a perspective to be gained on the stability of land-use effects from
year to year under different conditions. Second, for one year of the study (i.e. 2002), detailed measurements
of air temperature were available at the sites and allowed the impact of this important micro-climatic variable
on stream temperature contrasts between the study sites to be assessed.
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
AFFORESTATION AND STREAM TEMPERATURE
53
STUDY AREA AND METHODS
The study catchments are located close to Loch Grannoch in the Galloway region of southwest Scotland
(Figure 1). The study streams are underlain by the coarse-grained Cairnsmore of Fleet granite, which is
Devonian (Lower Old Red Sandstone) in age. Devensian morainic deposits comprising sandy boulder clay
and outwash sands and gravels occur in the Loch Grannoch area. Details of the study streams are summarised
in Table I.
At the outlet of both study basins, water temperature was monitored at 30 min intervals using Onset Optic
StowAway loggers, which have an accuracy of at least š0Ð2 ° C. Tests in a water bath over the range of 0Ð0
to 30 ° C before deployment showed no statistically significant differences in the performance of the loggers.
In the present study, continuous data on stream water temperature were analysed for the 4 year period from 1
January 1999 to 31 December 2003. At the study sites, shielded Onset Hobo loggers were mounted on wooden
posts at a height of 2 m above the river channel to measure air temperature continuously at 30 min intervals.
These loggers had an accuracy of at least š0Ð2 ° C, and tests before deployment showed no statistically
significant differences in the temperatures recorded by the two instruments. Problems with the data shuttle
used to download the Hobo loggers resulted in a number of significant gaps in the air temperature record
Figure 1. Location of the study catchments
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
54
B. W. WEBB AND D. T. CRISP
Table I. Summary of study catchment characteristics
NGRa of outlet
Altitude of outlet (m)
Maximum elevation (m)
Catchment area (km2 )
Dominant aspect
Land cover
Soils
a
Forested
Moorland
5455 6925
240
417
0Ð73
5245 6780
330
612
1Ð09
SE-facing
82Ð1% of catchment under Sitka spruce (at least
25 years old)
W-facing
Acid Molinietum heath species with Calluna
and Myrica gale locally abundant,
low-density sheep grazing
Peats and organic soils of the Garrary
Complex, sub-alpine soils of the
Mulltaggart Series
Hill and basin peats of the Dalbeattie
Association, organic soils of the Gala, Loch
Fleet and Garrary Complexes, peaty podzols
of the Carsphairn Series
National Grid Reference.
during the study period. However, a continuous record of air temperatures was available at both sites for the
calendar year 2002; and these data were analysed in conjunction with the water temperature record for the
same period.
RESULTS
The thermal characteristics of the forested and moorland streams are compared with respect to a series
of different time scales ranging from statistics relating to the study period as a whole to consideration of
diel variability. The relationship of water and air temperature is also investigated for a single study year.
Differences between the catchments are expressed as temperature value for the forested catchment minus that
for the moorland stream, so a positive difference indicates the forest to be warmer.
Mean and extreme temperatures
The forested stream exhibited lower mean water temperatures than the moorland catchment (Table I). The
difference in the study-period mean was 0Ð5 ° C, whereas differences in the annual mean ranged from 0Ð4 ° C
in 2002 to 0Ð7 ° C in 2003. Very low water temperatures were relatively uncommon during the study period.
Temperatures of 0 ° C or below were not recorded in the forested stream, and freezing conditions occurred
for less than 0Ð9% of the time in the moorland catchment. During the study period, temperatures were below
1 ° C in the forested stream for 851Ð5 h (2Ð4% of time) compared with 1492 h (4Ð3% of time) at the moorland
monitoring site. Further evidence of the elevating effect of coniferous tree cover on minimum temperatures
is provided by duration curves (Figure 2), which show for all study years that the water temperature equalled
or exceeded for 80% of the time was higher in the forested stream.
Maximum temperatures experienced in the forested stream were significantly lower than those recorded
in the moorland catchment (Table II). Temperatures remained below 16 ° C in the forested stream, but they
exceeded 20 ° C at the moorland monitoring site in three of the study years and reached a maximum of
23Ð4 ° C in 2003. Annual temperature duration curves for the forested stream exhibited a clear flattening
beyond values equalled or exceeded 30% of the time and became increasingly divergent from those for the
moorland catchment with respect to the occurrence of high water temperatures (Figure 2).
Seasonal variation
Study-period variations in monthly mean water temperature parameters for the catchments are plotted in
Figure 3, and differences in these parameters between the forested and moorland sites are plotted in Figure 4.
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
55
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
2000
Water Temperature (°C)
Water Temperature (°C)
AFFORESTATION AND STREAM TEMPERATURE
Forest
Moorland
0.01
0.5 2
10
30 50 70
90
98 99.5
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
99.99
2001
0.01
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
2002
0.01
0.5 2
10
30 50 70
90
0.5 2
10
30 50 70
90
98 99.5
99.99
Percent Time Equalled or Exceeded
Water Temperature (°C)
Water Temperature (°C)
Percent Time Equalled or Exceeded
98 99.5
99.99
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
2003
0.01
Percent Time Equalled or Exceeded
0.5 2
10
30 50 70
90
98 99.5
99.99
Percent Time Equalled or Exceeded
Figure 2. Annual water temperature duration curves for the study catchments
Table II. Mean and extreme water temperatures (° C) at the study sites
Year
2000
2001
2002
2003
2000–3
Forest
Moorland
Min.
Mean
Max.
Min.
Mean
Max.
0Ð1
0Ð1
0Ð2
0Ð1
0Ð1
7Ð3
6Ð9
7Ð4
7Ð4
7Ð3
13Ð8
14Ð6
15Ð4
15Ð9
15Ð9
0Ð1
0Ð1
0Ð1
0Ð1
0Ð1
7Ð8
7Ð4
7Ð8
8Ð1
7Ð8
21Ð9
20Ð9
19Ð8
23Ð4
23Ð4
The annual cycle of monthly mean values of minimum, mean and maximum temperatures reached a peak in
July or August and a trough in January or February. The seasonal variation for the forested and moorland
sites was generally in phase; however, the annual cycle of mean and mean maximum temperatures peaked
earlier at the latter site in 2000, whereas the peak and trough of mean minimum values during 2001 occurred
earlier in the forested stream.
During the period from March to September, and in some years to October, mean temperatures were
lower in the forested catchment (Figures 3a and 4a). The greatest differences, of more than 2 ° C, were
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
56
B. W. WEBB AND D. T. CRISP
16
12
12
Monthly Mean Minimum
Temperature (°C)
Monthly Mean Water
Temperature (°C)
14
14
Forest
Moorland
10
8
6
4
0
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
(b)
8
6
4
0
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
18
16
6
Monthly Mean Maximum
Temperature (°C)
Monthly Mean Range in Water
Temperature (°C)
8
2
2
(a)
10
4
2
14
12
10
8
6
4
(d)
0
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
(c)
2
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
Figure 3. Variation in monthly mean water temperature parameters during the study period
recorded in May of 2000 and 2001, but monthly mean temperatures were generally at least 1 ° C lower in
the forested stream during the mid-summer period. In the winter months, mean temperatures were warmer in
the forested stream, but these differences were generally less than 0Ð5 ° C. Differences in the annual march in
mean temperatures between the forested and moorland catchment strongly reflected differences in the seasonal
variation of monthly mean maximum values (Figures 3c and 4c). From early spring (February or March) to
late autumn (October or November), mean maximum temperatures were lower in the forested stream with
differences of 3 ° C or more typical of the mid-summer period. In the mid-winter months, mean maximum
temperatures were higher in the forested catchment but the difference from the moorland stream did not
exceed 0Ð5 ° C. For most months in the study period, mean minimum temperatures were higher in the forested
stream, and largest differences from the moorland catchment of ¾0Ð8 to 1Ð1 ° C were recorded in December,
January or February (Figures 3b and 4b). In the late spring and early summer (May to July) of several years,
mean minimum temperatures were higher in the moorland stream, but differences from the forested catchment
were small (<0Ð5 ° C).
The tendency towards lower maximum and higher minimum values suppressed significantly the monthly
mean range in temperature (difference between mean values of daily maxima and minima) in the forested
stream compared with the moorland stream throughout the study period (Figures 3d and 4d). Monthly mean
range did not exceed 2 ° C in the forested catchment, but it approached 7 ° C for the moorland catchment in
May of 2001 and 2002. Although the moorland catchment exhibited a clear peak in mean range in spring
during the first 2 years of the study period, the range was equally as high or greater in mid-summer (July
and August) during the last 2 years (Figure 3d). The lowest mean ranges in temperature in the moorland
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
57
1.0
1.2
0.0
0.8
Forest - Moorland
Forest - Moorland
AFFORESTATION AND STREAM TEMPERATURE
-1.0
0.4
0.0
-2.0
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
(b)
-0.4
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
0.0
1.0
-1.0
0.0
-2.0
-3.0
-4.0
-5.0
(d)
Monthly Mean Minimum
Water Temperature (°C)
Forest - Moorland
Forest - Moorland
(a)
Monthly Mean Water
Temperature (°C)
Monthly Mean Range in
Water Temperature (°C)
-6.0
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
-1.0
-2.0
-3.0
-4.0
-5.0
(c)
Monthly Mean Maximum
Water Temperature (°C)
-6.0
Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03
Figure 4. Differences in monthly mean water temperature parameters (forest minus moorland) during the study period
catchment (<2 ° C) were recorded in the mid-winter months. The annual variation in monthly range for the
forested catchment generally followed that in the moorland stream, but the seasonal march was much more
muted and somewhat less regular (Figure 3d). The difference in range between the catchments showed a clear
seasonal pattern, whereby the largest contrasts were recorded in spring or summer and the smallest differences
in mid-winter (Figure 4d).
Daily and diel fluctuations
The extent of short-term variations in water temperature was suppressed significantly in the forested stream
compared with the moorland catchment (Figures 5 and 6). During much of the autumn and winter period,
water temperatures in the study streams tended to vary from day to day in response to the passage of different
weather systems, but temperatures in the forested stream were less responsive to spells of colder weather, as
a graph of variations during December 2001 indicates (Figure 5a). In the spring and summer months, a clear
diel cycle of temperature fluctuation occurred in both streams, but the magnitude of the diel fluctuation was
significantly less in the forested catchment, as shown by a plot of temperatures for May 2000 (Figure 5b). On
average during the study period, diel variation was greatest in both streams during May. However, the mean
daily range in water temperature for this month approached 5 ° C in the moorland catchment but it only just
exceeded 1 ° C in the forested stream (Figure 6a). The lowest daily ranges occurred in the mid-winter months;
but whereas the mean values for November to January were in excess of 0Ð5 ° C in the moorland stream, they
were ¾0Ð2 ° C at the forested monitoring station.
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
58
B. W. WEBB AND D. T. CRISP
10
Water Temperature (°C)
9
Forest
Moorland
8
7
6
5
4
3
2
1
0
-1
December 2001
Water Temperature (°C)
(a)
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
(b)
May 2000
Figure 5. Characteristic daily and diel variations in water temperature in the study catchments
The timing of daily variations in water temperature also differed between the study catchments (Figure 6b).
Maximum temperatures tended to occur around 1800 h (GMT) in the forested catchment, but at around
1500 h (GMT) in the moorland stream. There was no systematic variation in the timing of the daily maximum
throughout the year in either study stream. In contrast, minimum temperatures appeared to be sensitive to
the seasonal pattern of sunrise and occurred later in the morning during winter than in summer (Figure 6b).
The occurrence of daily minimum temperature was also delayed in the forested stream compared with the
moorland catchment, but to a lesser extent than the daily maximum. The difference in timing in the daily
minimum, on average for the study period, did not exceed 2 h in any month and was 30 min in some winter
months.
Water–air temperature relationships
Data for 2002 indicate that mean air temperatures were lower than mean water temperatures for both study
sites (Tables II and III). Mean air temperature was 0Ð2 ° C lower at the forest site than at the moorland station,
which was a smaller contrast than that between mean water temperatures at these sites. Air temperature
fluctuated over a greater range than water temperature in the study streams, and standard deviation values
indicate that air temperature was more variable in the moorland site than the forested stream site (Table III).
The minimum air temperature recorded during 2002 was the same at both sites, but the maximum air
temperature monitored at the forested station was ¾1 ° C lower than at the moorland site.
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
59
Mean Daily Range in Water Temperature (°C)
AFFORESTATION AND STREAM TEMPERATURE
6
Forest
Moorland
5
4
3
2
1
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(a)
2000 - 2003
Mean Timing in Day (hours)
24
18
12
6
Forest Maximum
Moorland Maximum
Forest Minimum
Moorland Minimum
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(b)
2000 - 2003
Figure 6. Characteristics of the diel cycle of water temperature in the study catchments
Table III. Air temperature statistics (° C) for 2002
Forest
Moorland
Minimum
Mean
Maximum
Standard
deviation
4Ð3
4Ð3
7Ð3
7Ð5
22Ð1
23Ð2
4Ð29
4Ð41
Variations in monthly mean air and water temperature parameters during 2002 are plotted in Figure 7. In
the forested catchment, mean air temperatures exceeded mean water temperatures from May to September,
but were lower in other months of the year (Figure 7a). In contrast, mean water temperatures in the moorland
stream were greater than mean air temperatures in all months except January and August (Figure 7b).
Monthly mean minimum air temperatures remained below those for water throughout 2002 in both catchments
(Figure 7c and d). The greatest contrasts between mean minimum values for air and water occurred in the
autumn and winter months and were larger for the forested stream. Mean maximum temperatures were higher
for air than for water in all months except for December at the forested site (Figure 7e). Differences in mean
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
60
B. W. WEBB AND D. T. CRISP
14
14
Monthly Mean Temperature (°C)
12
10
8
6
4
Moorland
Monthly Mean Temperature (°C)
Water
Air
Forest
6
4
12
Monthly Mean Minimum Temperature (°C)
Monthly Mean Minimum Temperature (°C)
12
Forest
10
8
6
4
2
Moorland
10
8
6
4
2
0
0
(d)
18
16
Monthly Mean Maximum Temperature (°C)
Monthly Mean Maximum Temperature (°C)
8
(b)
(c)
Forest
14
12
10
8
6
4
18
16
Moorland
14
12
10
8
6
4
2
2
(e)
(f)
8
8
Monthly Mean Range in Temperature (°C)
Monthly Mean Range in Temperature (°C)
10
2
2
(a)
Forest
6
4
2
0
Moorland
6
4
2
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(g)
12
2002
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(g)
2002
Figure 7. Seasonal patterns of air and water temperature parameters in the study catchments during 2002
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
61
AFFORESTATION AND STREAM TEMPERATURE
maximum values between air and water during spring and summer months were considerably greater for the
forest monitoring site (Figure 7e and f). The mean range in air temperature (difference between mean values
of daily maxima and minima) exceeded that for water in all months at both study sites (Figure 7g and h).
Contrasts in the mean range of air and water were greater for the forested catchment.
Differences in air and water parameters between the forested and moorland sites are plotted in Figure 8.
In general, the contrasts between the forest and moorland monitoring sites were less pronounced for air than
for water temperatures. For monthly mean values (Figure 8a), air temperatures were lower in most months
by relatively modest amounts (<0Ð5 ° C) at the forested site, whereas mean water temperatures were lower
by 1 ° C or more in the forested stream site than in the moorland catchment for most of the summer period.
Unlike mean maximum water temperatures, which were moderately higher in the forested stream site during
the mid-winter months, mean maximum air temperatures remained lower at the forest site throughout 2002.
The greatest contrast in mean maximum temperatures between the forest and moorland stations was 2 ° C in
August for air, but was 3Ð5 ° C in July for water (Figure 8c). Mean minimum water temperatures remained
higher in the forested catchment throughout 2002, but contrasts were greater in the winter than in the summer
months. Differences in mean minimum air temperatures between the forest and moorland monitoring sites
were generally modest (<0Ð5 ° C) and showed a different pattern throughout the year, whereby values were
lower at the forest station during autumn and spring (Figure 8b). The monthly mean range in temperature
1.0
1.5
Water
Air
0.5
Forest - Moorland
Forest - Moorland
1.0
0.0
-0.5
-1.0
-1.5
0.5
0.0
Monthly Mean Temperature (°C)
Monthly Mean MinimumTemperature (°C)
-0.5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2002
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(b)
2002
0.0
1.0
-1.0
0.0
Forest - Moorland
Forest - Moorland
(a)
-2.0
-3.0
-1.0
-2.0
-3.0
-4.0
Monthly Mean Range in Temperature (°C)
Monthly Mean Maximum Temperature (°C)
-4.0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(d)
2002
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(c)
2002
Figure 8. Differences in monthly mean temperature parameters (forest minus moorland) for water and air during 2002
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
62
B. W. WEBB AND D. T. CRISP
was lower in the forest stream throughout 2002 for both air and water. In the winter months, similar and
relatively small differences in mean range between the catchments were evident for both air and water. In
the mid-summer months, the difference in range between the forested and moorland sites was significantly
greater for water than for air (Figure 8d).
The characteristics of variations in air and water temperatures during 2002 at the daily time scale are
illustrated in Figure 9. The mean daily ranges in air and water temperatures were lower at the forested site
compared with the moorland station in all months (Figure 9a). The suppression of mean range in the forest
catchment was much more pronounced for water than for air. At the forested site, the mean timing of the daily
maximum in water temperature lagged behind that of air temperature in all months (Figure 9b). Although
the extent of this lag did not vary regularly throughout the year, there was a tendency for the peak in water
temperature to be delayed longer behind the peak in air temperature during the winter period. The mean timing
of the daily minimum was also later in the day for water than for air temperature in the forested catchment
throughout 2002. The extent of this lag was variable between months, but there was a tendency for both water
and air temperature minima to occur earlier in the summer months than in the winter period. Differences in
the timing of the diel cycle of air and water temperatures were less pronounced for the moorland monitoring
site (Figure 9c). Daily maxima in both air and water temperature tended to occur around 1500 h (GMT)
throughout the year, whereas daily minima were coincident for air and water in May, June, July and October.
In other months, the daily trough in water temperatures was delayed compared with that of air temperature.
Water–air temperature relationships based on daily mean values for 2002 demonstrate that air and water
temperatures are closely related in the study catchments. The relationship was more scattered and had a lower
gradient for the forest stream (Figure 10a) relative to the moorland catchment (Figure 10b).
DISCUSSION AND CONCLUSIONS
The impact of forest cover in lowering mean water temperature by ¾0Ð5 ° C observed at Loch Grannoch was
very similar to that recorded in the Plynlimon catchments of mid-Wales (Crisp, 1997). Although a clear effect
of forest cover in moderating winter water temperatures was not apparent in the Upper River Severn, the
present study found monthly mean water temperatures to be warmer in the forested catchment during the
mid-winter period in each year of the study period, which reflected mainly an elevating effect of forest cover
on mean minimum water temperatures. In common with the findings of other UK studies, the presence of
forest cover reduced significantly the summer maximum water temperatures and led to much smaller monthly
and diel ranges of water temperature compared with an unafforested situation. Relatively little information
has been available on how the timing of the diel cycle is affected by the presence of a coniferous forest cover
in UK catchments, but the present study has demonstrated that the attainment of the daily minimum and
maximum water temperatures was delayed in the forested stream compared with the moorland stream. The
delay was more marked for peak than for trough temperatures, but was less pronounced than that observed
in some forest harvesting experiments in the USA (e.g. Hewlett and Fortson, 1982).
Observations over four annual cycles suggested that the effects of land use on thermal regime were consistent
from year to year. Differences in monthly temperature parameters between the forest and moorland streams
exhibited relatively small interannual variability and had a similar annual march in all years.
Monitoring during 2002 showed that there were differences in air temperature between the forest and
moorland sites, but these differences were generally less pronounced than those recorded for water temperature.
Therefore, it is likely that pronounced differences in stream temperature observed between the study
catchments in the summer months were more strongly influenced by the effect of the coniferous canopy in
attenuating inputs of heat from shortwave solar radiation than by differences in sensible heat transfer between
the stream and the overlying air (Webb and Zhang, 1977). Similarly, differences in air temperature between the
study catchments in the mid-winter months were smaller than those recorded for water temperature. Therefore,
the occurrence of higher winter temperatures in the forest catchment is unlikely simply to reflect differences
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
AFFORESTATION AND STREAM TEMPERATURE
63
Mean Daily Range in Temperature (°C)
6
5
Forest Water
Forest Air
Moorland Water
Moorland Air
4
3
2
1
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(a)
2002
24
Mean Timing in Day (hours)
Forest
18
12
6
Water Maximum
Air Maximum
Water Minimum
Air Minimum
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(b)
2002
Mean Timing in Day (hours)
24
18
Moorland
Water Maximum
Air Maximum
Water Minimum
Air Minimum
12
6
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(c)
2002
Figure 9. Characteristics of the diel cycle of air and temperature in the study catchments during 2002. For some months in the moorland
catchment (part c), the timing of maxima or minima for air and water are identical and only the symbol for air is shown
in the temperature of the air above the study streams. Other factors, such as influence of the forest canopy
in reducing longwave energy losses from the water column, may be equally or more important (Webb and
Zhang, 2004). Daily mean water and air temperatures were strongly related in both study streams, but the
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
64
B. W. WEBB AND D. T. CRISP
Daily Mean Water Temperature (°C)
18
16
Tw = 0.73515 Ta + 2.04845
R2 = 0.93
14
12
10
8
6
4
2
0
Forest
-2
-4
-2
0
2
4
6
8
10 12 14 16 18 20
Daily Mean Air Temperature (°C)
(a)
Daily Mean Water Temperature (°C)
18
16
Tw = 0.88519 Ta + 1.2103
R2 = 0.95
14
12
10
8
6
4
2
0
Moorland
-2
-4
(b)
-2
0
2
4
6
8
10 12 14 16 18 20
Daily Mean Air Temperature (°C)
Figure 10. Water–air temperature relationships in the study catchments for 2002
less steep relationship for the forested catchment, which has been noted in other studies (e.g. Crisp, 1988),
suggests that tree cover would act to some extent to moderate potential future rises in water temperature as
a consequence of global warming (Webb and Walsh, 2004).
Under present conditions, the effect of forest cover in moderating high temperatures greatly reduces the
potential for game fish, such as trout, to become thermally stressed (Crisp and Beaumont, 1977). Even in
the uplands of southwest Scotland, temperatures in the upper critical range for Salmo trutta (½19 ° C) were
recorded in the moorland stream for more than 150 h during the study period, whereas at the forested site the
maximum temperature did not exceed 16 ° C. Previous studies have demonstrated that contrasts in temperature
regimes between forested and non-forested water courses in the UK have the potential not only to affect fish
survival, but also to have a modest influence on embryonic development and growth of fish and invertebrate
species (e.g. Weatherley and Ormerod, 1990; Crisp et al., 2004). It is likely that similar effects are present in
the moorland and forested streams of southwest Scotland.
Thermal regime is sensitive to many catchment characteristics other than land use, including channel
morphology, topographic configuration and groundwater contribution, and small differences in these conditions
over relatively short distances may have subtle effects on water temperature behaviour (e.g. Crisp, 1977;
Malcolm et al., 2004). In making a comparison between the forested and moorland environment in the present
study, it was impossible to select catchments that were identical in all respects, apart from land use. However,
the study streams were very similar in terms of geology, pedology and size, and the nature and magnitude of
Copyright  2005 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 51–66 (2006)
AFFORESTATION AND STREAM TEMPERATURE
65
the differences detected indicate a significant impact of land use on thermal regime. Further evidence on this
impact will be gathered by continuing monitoring during coming years, when there are plans to harvest the
trees in the forested catchment.
ACKNOWLEDGEMENTS
We are very grateful to the Forestry Commission Scotland, Galloway Forest District and to Scottish Natural
Heritage for allowing access and permission to work at the study sites. A small grant from the University
of Exeter to support this study is also gratefully acknowledged. Thanks are also due to Diane Crisp, who
assisted with the fieldwork, and to Sue Rouillard, who drew Figure 1.
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