Hydrological Sciences -Journal - des Sciences Hydrologiques, 30, 4,12/1985
Temporal variation of river water temperatures
in a Devon river system
B. W. WEBB & D, E. WALLING
Department of Geography, University
Exeter EX4 4RJ, Devon, UK
of
Exeter,
ABSTRACT
Despite several studies of spatial and temporal
variation in water temperature characteristics, few
investigations of longer-term water temperature behaviour
in Britain rivers have been undertaken. This paper
reports the results from a 10-year study of river water
temperatures at three monitoring stations on the River
Exe, Devon, UK. Data concerning annual statistics,
seasonal regime, diurnal variation, duration characteristics and accumulated temperature are analysed and reveal
essentially stable water temperature behaviour over the
decade 1974-1983. Contrasts between monitoring stations
are also evident and reflect the effects of regional and
more local controls as well as the influence of hydrological factors. In spite of these differences, water
temperature behaviour is largely synchronous across this
river system.
La variation
temporelle
de température
de l'eau dans une
rivière
du Devon
RESUME
En dépit de différents études concernant les
variations temporelles et spatiales des températures de
l'eau, il y a peu d'études au sujet des changements de
température à long terme dans les rivières britanniques.
Cet article expose les résultats d'une étude qui s'étend
10 ans en trois endroits sur la longueur de la rivière Exe,
Devon, Royaume-Uni. Les données sur les statistiques
annuelles des régimes saisonniers, les variations diurnes,
les caractéristiques de durée et de la température cumulées
sont ici analysées et indiquent un comportement stable
des températures de l'eau pour la période 1974-1983,
L'étude montre aussi quelques différences entre les trois
endroits et il nous indique également l'importance des
contrôles régionaux et locaux ainsi que l'importance des
facteurs hydrologiques. Malgré ces différences,il est
évident que les températures se ressemblent largement d'un
bout à l'autre de cette rivière.
INTRODUCTION
Water temperature behaviour has considerable ecological and economic
importance (Smith, 1972). For example, it is a very significant
influence on fish growth and behaviour, it exercises a fundamental
control on dissolved oxygen levels, it is an important consideration
449
450 B.W. Webb & D.E. Walling
in water abstraction and use for industrial purposes, and disruption
of the natural thermal regime represents a major type of river
pollution. The need for water temperature monitoring has therefore
been stressed by several authors (e.g. Herschy, 1965) and this facet
of river water quality has attracted the attention of hydrologists
and other scientists in many countries (Blakey, 1966; Nishizawa,
1967; Wundt, 1967; Mosley, 1983). In Britain, a number of studies
have documented spatial variation in water temperature behaviour at
national (Walling & Webb, 1981), regional (Walling, 1980) and more
local (Crisp & Le Cren, 1970) scales, and have described contrasts
in temperature behaviour between streams and rivers (Smith, 1979)
and between sites within major river systems (Smith, 1975; Boon &
Shires, 1976). Attention has also been directed to the study of
temporal variation in water temperatures, and diurnal (Macan, 1958),
day-to-day (Smith & Lavis, 1975), seasonal (Edington, 1966) and
annual (Smith, 1981) fluctuations have been analysed.
Many research investigations have, however, been conducted over
relatively short time periods, and little analysis and interpretation
of longer-term water temperature data have been published for British
rivers, despite the initiation of systematic monitoring of river
water temperatures in Britain nearly 50 years ago (Herschy, 1965).
Studies of longer-term water temperature behaviour are required not
only to assess the stability and representativeness of temperature
responses recorded in the short term, but also to identify trends in
temperature levels through time. In the latter case, changes in the
natural thermal regime of rivers may be generated both by changing
climatic and hydrological conditions and by the impact of man through
activities such as land-use modification (Gray & Edington, 1969), the
discharge of heated effluents (Gameson et al.,
1957; Langford &
Daffern, 1975) and reservoir construction and river regulation (Lavis
& Smith, 1972; Crisp, 1977).
The purpose of the present paper is to report the findings of a
10-year investigation of river water temperatures at three monitoring
stations in a major river system in Devon, England. Continuous
water temperature records are available for the study period and the
authors believe this to be one of the most detailed longer-term
water temperature data-sets available in Britain. These data are
used to investigate temporal variation in water temperature behaviour
from the perspectives of annual statistics, seasonal regime, diurnal
variation, duration characteristics and accumulated temperature.
THE STUDY BASINS
The study basins comprise three sub-catchments of the River Exe
system in Devon (Fig.l), which vary in topographic, geological and
hydrometeorological character. The River Barle drains 128 km of
upland moorland and hill farming country in the northwest of the Exe
basin and is underlain by massive sandstones, siltstones and slates
of Devonian age. Mean annual precipitation and runoff totals vary
from 1800 and 1500 mm respectively in the headwaters of the subcatchment to <1300 and <1000 mm respectively at the outlet. The
monitoring station at Brushford lies at an elevation of 117 m and
has a long-term mean discharge of 4.6 m 3 s _ 1 .
Temporal variation of river water temperatures
^
Fig. 1
451
Study Area
The study basin.
The River Creedy is a larger tributary (262 km2) of the Exe and
drains a predominantly pastureland area of relatively low relief
which is underlain by arenaceous and argillaceous rock sequences of
Upper Carboniferous age in the northern and southern parts of the
sub-catchment, and by a variety of Permian lithologies in the centre.
The latter are preserved in the Crediton trough and contain aquifers
which yield substantial contributions of groundwater outflow (Davey,
1982). Mean annual precipitation and runoff do not exceed 1200 and
750 mm respectively, and the long-term mean discharge at the
monitoring station, which lies at an elevation of 14.2 m, is
3,0 m 3 s _ 1 .
The Middle and Upper Exe basin comprises the third sub-catchment
and drains an area of 601 km2 above the monitoring site at Thorverton. The basin is relatively diverse in terms of its topography,
geology and hydrometeorology. Rock types vary from Devonian strata
in the north to Permian sediments in the south and east, and mean
annual precipitation and runoff totals vary from 1800 and 1500 mm
respectively in the headwaters on Exmoor to <890 and <350 mm
respectively at the outlet. The monitoring station, which lies at
an elevation of 25.3 m and has a long-term mean discharge of 16.3
m 3 s - 1 , is representative of mainstream conditions in the study area.
All of the study basins in the present investigation are free
from significant sources of thermal pollution, but the mainstream of
the Exe is now influenced by regulation releases from the Wimbleball
Reservoir in the northeast of the study area (Fig.l). Construction
of this reservoir began in November 1974 and was substantially
completed by December 1978 (Battersby et al.,
1979), so that
temperature levels at Thorverton were potentially influenced by this
factor during the second half of a 10-year study period which ran
from 1 January 1974 to 31 December 1983.
Water temperatures at the three study sites were monitored using
452 B.W. Webb & D.E. Walling
continuously recording, purpose-built, thermistor sensors linked to
strip-chart recorders. Temperature readings were checked on a weekly
or more frequent basis against standard calibrated thermometers, and
the data collected in this study are considered to be accurate to
within ±0.1°C. Furthermore, investigation of cross-sectional
variation in water temperature at the monitoring sites on a number of
occasions revealed an essentially homogeneous distribution and
suggested measurements of water temperature at the channel margin
are completely representative of conditions throughout the cross
section.
Abstraction of hourly values of water temperature from
continuous records was found entirely adequate to define the temperature fluctuations in the study area, and the 10-year run of hourly
information for the three monitoring sites forms the data-base for
the analyses described in this paper.
ANNUAL STATISTICS
Annual river water temperature statistics for the three monitoring
stations are listed in Table 1 and reveal that the variability of
the annual values over the study period for a given site was
generally greater than the contrasts between stations for a given
year. Annual mean values differed over the decade by 1.2°C at
Brushford and Cowley and by 1.6°C at Thorverton, and the coefficient
of variation for the annual mean lies between 3 and 5% at the three
sites. Annual mean river water temperatures did not exhibit a
significant upward or downward trend during the period 1974-1983
and, similarly, no simple temporal trend was present for annual
maximum river water temperatures. Annual maxima were, however,
somewhat more variable through time than the annual mean values
(Table 1 ) . The annual maximum varied by 4.5, 4.6 and 5.8°C at
Brushford, Cowley and Thorverton respectively, and the coefficient
of variation for the annual maxima lies in the range 8-9% for the
three monitoring stations. Water temperatures in the study catchments were greatest at the height of the 1976 drought (Walling &
Carter, 1980) but were almost as high in the 1983 summer period.
Coefficient of variation values are much higher for annual
minimum water temperatures and lie in the range 75-90%. The coldest
spells in the study period occurred during February 1978 and January
1979 and caused river water temperatures to fall to freezing point
or just below. Annual minima tended to be higher in the first half
of the study period, and minimum temperatures of 3.0°C or greater
contributed to the low annual range of temperature recorded at the
monitoring sites in 1974 (Table 1 ) . The extent of the annual range
is, however, more closely related to the magnitude of the annual
maximum and was therefore largest in 1976 and in 1983. The annual
range varied by 6.0°C or more at the three monitoring stations over
the study period and was associated with coefficient of variation
values in the range 10-11%.
Contrasts in the annual temperature statistics between
the
individual monitoring stations are generally maintained from year
to year, and annual mean river temperatures, for example, consistently rank in the order Thorverton, Cowley, Brushford. Average
temperatures for the study period were 0.9°C greater at the down-
Temporal variation of river water temperatures
453
stream station of Thorverton than at the higher elevation site of
Brushford on an upstream tributary, and this difference represents
a lapse rate of water temperature with increasing elevation of
approximately 1°C per 100 m, which is similar to that reported for
the mainstream of the River Wear in northeast England by Smith (1968).
In contrast to the findings from several rivers in northern England
(Smith, 1975, 1979), annual water temperature maxima were always
higher at the downstream stations of Thorverton than at the upstream
site of Brushford. The average difference in maximum temperatures
between these two stations was 1.4°C but this rose to nearer 2°C in
the 1976 drought when the annual maximum at Thorverton was 25.2°C.
It appears, therefore, that the moderating effects of a downstream
increase in flow volume and in thermal capacity on summer peak water
temperatures, which have been noted in other British rivers (Smith,
1968; Boon & Shires, 1976), are offset in the Exe system by other
factors. The latter may include changes in channel character and
greater flow residence times in the downstream direction. Annual
maximum values were consistently lowest at Cowley (Table 1) and only
just exceeded 22°C in 1976. Shading by riparian vegetation and the
influence of groundwater outflow into the River Creedy for several
kilometres upstream of the monitoring site are considered to be
responsible for the relatively low peak temperatures recorded at
Cowley.
Contrasts in annual minimum water temperatures between upstream
and downstream sites were less strongly developed in the study basin
than in some northern rivers (e.g. Smith, 1968). However, in common
with the findings from other investigations (e.g. Boon & Shires,
1976), annual minima moderated with increasing catchment size in most
years and this trend probably reflects the increased thermal capacity
Table 1
Annual river water temperature statistics
BRUSHFORD
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
Study Period
Average (°C)
Standard
Deviation (°C)
Coefficient of
Variation (%)
Mn Mx
Mi
9-9
19 9
106
22-3
10-5
10-0
9-8
19-1
COWLEY
R
Mn Mx
3-0
16-9
10-6
2-1
20-2
10-9
23-6
0-7
22-9
11-0
21-5
1-7
19-8
10-4
0-0
19-1
THORVERTON
Mi
R
Mn Mx
Mi
R
17-7
3-6
14-1
10-8
20-8
3-3
17-5
203
2-2
18-1
11-7
23-6
2-5
21-1
22-1
1-5
20-6
11-8
25-5
1-7
23-8
18-6
2-2
16-4
10-8
21-6
2-5
19-1
10-3
17-5 -0-1
17-6
110
20-9 -0-1
21-0
9-4
19-2
0-3
18-9
100
18-7
0-0
18-7
10 2
21-7
-0-1
21-8
9-8
19-2
0-8
18-4
103
17-7
0-7
17-0
10-6
19-7
1-2
18-5
18-8
9-9
191
2-1
170
10-5
18-4
2-0
16-4
10-7
21-3
2-5
10-5
21-0
01
20-9
11-2
19-4
10
18-4
11-5
22-2
0-1
22-1
10-4
23-6
0-8
22-8
10-9
21-4
1-5
19-9
11-2
25-2
1-3
23-9
10-1
20-9
1-2
19-7
10-6
19-2
1-5
17-7
110
22-3
1-5
20-8
0-4
1-8
10
2-1
0-4
1-6
1-1
1-9
0-5
1-9
1-2
2-2
3-97
8-78 87-07 10-61
3-58
8-34 76-71 10-61
4-62
8-63 81-88 10-69
Annual Mean (Mn), Maximum (Mx), Minimum (Mi) and Range (R) in Temperature (°C)
454
B.W. Webb & D . E . Walling
of downstream sites with greater flow volumes. This pattern,
however, was not maintained in the most severe winters of the study
period. Since the annual range of river water temperature is more
strongly influenced by the magnitude of the annual maximum rather
than the minimum, highest annual ranges were generally monitored at
Thorverton whilst the lowest were recorded at Cowley (Table 1).
SEASONAL REGIME
In addition to fluctuations from year to year, the study rivers
exhibit strong seasonal changes in water temperature. It has been
argued in previous studies that the form of the annual cycle in
water temperature may be closely approximated by sine-generated and
related curves (e.g. Ward, 1963; Johnson, 1971; Smith, 1981), and
the general nature of the seasonal regime in water temperature at
the monitoring stations of the present investigation has been
objectively isolated using similar Fourier analysis techniques.
Second-order harmonic curves of the form:
„
2TTD
D
.
Tw = a 0 + en cos j ^ ^ + 3! s i n
2TTD
I S 5 7 ?
4TTD
+ a 2 cos j
^
.
^ + B2
.
4TTD
^ J ^ ^
where:
Tw = daily maximum, mean or minimum river water temperature (°C),
n = day number
365
ao = average temperature level,
cti, a 2 , $i, 3 2 = coefficients,
were fitted to 3652 daily values of maximum, mean and minimum river
water temperatures at each of the three monitoring stations. Secondorder harmonic functions were selected because they yielded asymmetrical curves which closely fitted the ranges of daily values
recorded over the study period, and were associated with relatively
high levels of statistical explanation (Tig.2). Coefficient of
determination (r ) values exceeded 80% for the three temperature
parameters at the three monitoring sites, but the results for the
upstream stations of Brushford and for minimum temperatures at all
of the sites exhibited slightly greater variation of the daily
values around the seasonal trend predicted by Fourier analysis
(Fig.2).
The form of the seasonal regime represented by the harmonic
functions is similar for maximum, mean and minimum temperatures and
also for the different monitoring sites. Water temperatures are low
and change little during the first two months of the year and reach
a minimum either in late January or in early February. A steep rise
in river water temperature begins in March and temperatures continue
to increase until a peak is reached during July. Thereafter,
temperature declines more gradually towards low values which become
established again in December. Some variations in the nature of
this annual cycle are, however, apparent between the monitoring
stations and between the temperature parameters.
The magnitude of the seasonal variation in daily maximum, mean
Temporal variation of river water temperatures
COWLEY
BRUSHFORD
•2
455
THORVERTON
£
J F M A M J J A S O N D
Fig. 2
J FM A M J J A S O N D
JFMAMJJASOND
The seasonal regime of river water temperature modelled by Fourier analysis.
and minimum river water temperatures was greatest at the downstream
station of Thorverton and became progressively less for daily mean
and minimum values at Cowley and at Brushford as drainage area
decreases. Seasonal contrasts in daily maximum temperatures were
rather less pronounced at Cowley than at the other two stations
because of the local factors which influence this site. Contrasts
in the seasonal temperature regime between stations, however, are
mainly a function of water temperature characteristics during the
summer months, since significant differences between sites do not
become established until April and then increase to a maximum in
July or August. Thereafter, the difference in water temperature
between sites declines steadily until it disappears during November.
The timing of the seasonal regime modelled by the harmonic curves
also varies between monitoring stations. Daily maximum, mean and
minimum water temperatures peak 4-6 days earlier but trough 10-16
days later at Brushford in comparison with Cowley. Lagging of peak
summer temperatures has been attributed by Edington (1966) to the
influence of shading by vegetation and this factor may also be
responsible for the lag recorded in the River Creedy where temperatures also tend to decline more slowly over the autumn period in
comparison with the other two sub-catchments. Differences in the
timing of the annual cycle of water temperature between Brushford and
Thorverton are probably related to the smaller flow volumes and
lower thermal capacity associated with the former site. At all the
monitoring stations, the annual cycle of minimum values tends to lag
456 B.W. Webb &D.E. Walling
a few days behind the variation in mean, and, in turn, maximum
water temperatures.
A perspective on the year-to-year variation in the seasonal
regime of river water temperature is provided by plotting monthly
average maximum, mean and minimum temperatures over the study period
at the three monitoring stations (Fig.3). These plots reveal that
Fig. 3
Variation in the annual cycle of river water temperature.
the nature of the annual cycle varied considerably from year to year
in the study period being more peaked in some years, such as 1983
and 1976, and showing evidence of a bimodal distribution in others,
such as 1980. The month associated with highest average maximum
water temperatures also varied from June to August in different
years and the month exhibiting lowest average minimum water temperature ranged from December to March. Irregular fluctuations in
monthly temperature statistics also characterize the winter period
in several years (Fig.3). The range in water temperature between
monthly average maximum and minimum values was generally greatest
during the spring and summer period and least in the autumn months,
and this range was consistently highest at Brushford and lowest at
Cowley. Despite differences in the range and absolute magnitude of
river water temperatures recorded, Fig.3 reveals that the seasonal
regime in temperature was broadly in phase at the three monitoring
sites.
DIURNAL VARIATION
River water temperatures in the study area also exhibit diurnal
fluctuations which are superimposed on the annual cycle of variation.
In contrast to the pattern displayed by the seasonal regime, the
Temporal variation of river water temperatures
457
daily range of temperature tended to be greater in the upstream
tributary of the River Barle than in the larger water courses of the
Rivers Creedy and Exe, and the maximum daily variation recorded
during the 10-year study period was 6.1, 4.8 and 4.3°C at Brushford,
Thorverton and Cowley respectively. Relatively small flow volumes
in the River Barle make this tributary more sensitive to diurnal
changes in air temperature and in the components of the energy
budget governing river temperature, whereas local shading and
groundwater seepage limit the magnitude of daily temperature
fluctuations in the River Creedy. The daily range of river water
temperature at the monitoring sites was not constant from year to
year. The maximum daily range, for example, varied from 6.1°C in
1974 to 4.4°C in 1979 at Brushford, and from 4.3°C in 1981 to 2.8°C
in 1974 at Cowley. This lack of synchronous behaviour across the
study area suggests that diurnal water temperature fluctuations are
sensitive to local contrasts in weather conditions and to the nature
of the channel in terms of its aspect, exposure and morphology (Boon
& Shires, 1976) at, and for several kilometres upstream of, the
monitoring site.
Plotting of maximum, mean and minimum values of monthly average
daily range in river temperature during the study period (Fig.4)
b4-
BRUSHFORD
3J
I
2~
1 J
COWLEY
1
ï Ï
ï
T Maximum * .
in
| • Mean
Per,od
11 Minimum S1874-1983
ï i l
Fig. 4
Variation in the daily range of river water temperature.
reveals that the magnitude of diurnal temperature fluctuation varied
throughout the year at the monitoring stations. Low average daily
ranges characterized the winter months from October to March whereas
larger diurnal fluctuations were typical of the remainder of the
year. This contrast reflects the fact that, in the winter period,
due to a combination of lower solar heating and larger flow volumes
with greater thermal capacity, water temperatures tend to change
gradually from day to day in response to changing weather conditions.
During winter months, the daily range in temperature was typically
458
B.W. Webb & D.E. Walling
of the order of 1°C but on some occasions a diurnal fluctuation of
only 0.1°C was recorded at Cowley and at Thorverton, and no change
in temperature throughout the day was monitored at Brushford. In
contrast to the winter season, the spring and summer months were
marked by the occurrence of clear diurnal cycles in water temperature
which became established during March and persisted until October.
Daily ranges in temperature were greatest during the spring and
early summer, and average values for the whole study period were
highest in June at Brushford and at Thorverton and in May at Cowley.
In common with findings from other studies (Macan, 1958; Langford,
1970), the daily range in temperature tended to be higher in the
spring than in the autumn months, despite similar conditions of
insolation. Edington (1966) has argued that river temperatures in
the autumn period are buffered as the result of progressive heating
of the ground during the summer, and this explanation may also apply
in the present study area. Figure 4 indicates that the variability
in the magnitude of the average daily range from year to year is low
during the winter months at all three sites but increases in the
spring and summer period. The latter trend was least pronounced for
the River Creedy where daily ranges exhibit only small differences
both within and between years.
Variability of the daily range in river water temperature was
apparent not only in terms of the magnitude of the diurnal fluctuation but also with respect to its timing. Maximum, mean and minimum
values of monthly average timing for daily maximum and minimum river
water temperatures are presented in Fig.5 and reveal contrasts
throughout the year in the timing of the daily extremes. In the
period January to July, the daily maxima occurred progressively
later in the day, but thereafter a reverse trend was evident until
November and December when the timing of the maximum was at its
earliest. On average, the daily maximum occurred before 1200 h GMT
in mid-winter but nearer to 1500 h GMT in mid-summer. Although the
variation is less pronounced and less regular, a reverse pattern is
apparent for the timing of daily minimum river water temperatures.
In winter months, daily minima were recorded on average after 1000 h
GMT but in early summer they occurred between 0700 and 0800 h GMT.
The trend for minimum values may be related to the annual cycle in
the timing of sunrise which is at its earliest in mid-summer and at
its latest during mid-winter. Minimum air temperatures are generally
attained a short time after sunrise and this timing would broadly
account for the trend apparent in Fig.5 if a lag of 2-3 h between
the occurrence of air and water temperature minima is taken into
account. The annual trend in the timing of daily maxima of water
temperature is less readily explained but is possibly related to
seasonal contrasts in the factors governing the attainment of
maximum air and, in turn, water temperatures. During the summer
months, insolation is an important control giving rise to the
occurrence of maxima several hours after midday, whereas maximum air
and water temperatures in the winter months are more strongly
influenced by the passage of different air mass types which may
cause peak temperatures at different times in the day (Fig.5).
Considerable variability, however, is evident in the timing of
daily maximum and minimum river water temperatures from year to year
(Fig.5) and this may reflect the complexity of controls which affect
Temporal variation of river water temperatures
459
BRUSHFORD
, It tI 1
Î *i ' i !
i " *
!
n^;H-:<
Daily Maxima
24-
<D
>
<
Daily Minima
COWLEY
T Maximum i n period
i Mean
1974-19f«
J- Minimum ) a * ! 3 0 J |
20181B1412100806040200-
h
24- THORVERTON
2220181614Ï
: 1 i
- r
12- - Î 1
10- 1
;
0806040200J F M A M J J A S O N D J
XT
T T T r l i ï
- 11 H 11 -
*I i
Fig. 5
T
il.
jïiïï
T T
; i I
F M A M J J A S O N D
Variation in the timing of daily extremes in river water temperature.
the attainment of air and water temperature extremes and include
cloudiness, air turbulence and evaporation in addition to daily
variations in insolation and air mass type. The variability in
timing between years tended to obscure contrasts in the timing of
temperature extremes between the monitoring stations. However,
there was a tendency on summer days with strong diurnal variations
in water temperature for the daily cycle at the downstream site of
Thorverton to lag behind that at the upstream station of Brushford.
Similar trends have been observed in other river systems (Smith,
1979) and are attributed to a downstream increase in discharge and
thermal capacity which makes the water course less sensitive to
change in atmospheric heating throughout the day. The daily maximum
at Cowley exhibited an earlier timing than at the other two stations
during the autumn months and this contrast is probably related to
local factors such as vegetation shading and valley orientation and
exposure.
DURATION CHARACTERISTICS
Little information has been published on the duration characteristics
of water temperatures in British rivers but detailed records
collected over 10 years in the present study permitted the construe-
460 B.W. Webb &D.E. Walling
tion of duration curves for river temperatures at the three monitoring stations (Fig.6). Curves based on the complete study period
indicate that low water temperatures (<1°C) occurred in the study
rivers for only <0.5% of the time and that a temperature of 5°C was
equalled or exceeded approximately 90% of the time at all the
monitoring sites. A water temperature of 10°C was equalled or
exceeded 50% of the time at Thorverton and Cowley but 46% of the time
at Brushford. It is apparent from the duration curves that relatively high water temperatures were attained more frequently at the
downstream site of Thorverton than at the other monitoring stations,
and a temperature of 20°C was equalled or exceeded only approximately
0.5% of the time at Brushford and Cowley but 3% of the time at
Thorverton.
H
1
1
1
0-01 0-1 0 5 2
1
10
1
1
1
30 50 70
1
90
i
! — i
(-••'»
98 99-599-9 99-99
ï
i
1
1
001 01 0-6 2
1
10
1
1
1
30 50 70
1
90
1
1—i
f-
98 99-5 99-9 99-99
Percentage of Time
Percentage of Time
Fig. 6
Duration characteristics of river water temperatures.
Contrasts between sites in the form of the duration curves are
largely a function of the water temperatures which are equalled or
exceeded less than 90% of the time since it is beyond this duration
that the curves begin to diverge and lower temperatures characterize
the higher altitude station at Brushford (Fig.6). The curves for
Cowley and Thorverton are very similar for temperatures equalled or
exceeded more than 30% of the time, but considerably higher temperatures occur at the latter station for durations in the range 0.01-30%
of the time. Temperatures equalled or exceeded for 2% of the study
period or less were higher at Brushford than at Cowley and reflect
local influences on peak temperatures at these two stations. The
presence of a shallow flow depth and a rock floor in the River Barle
promotes relatively high maximum water temperatures at Brushford,
Temporal variation of river water temperatures
461
whereas shading by riparian vegetation and the influence of groundwater outflow in the River Creedy moderates peak water temperatures
at Cowley.
Figure 6 also indicates considerable diversity in the form of
the duration curve from year to year at the three monitoring stations
and suggests that any interpretation of water temperature duration
characteristics based on one or two years of record is likely to be
misleading. Steepest curves were associated with the very hot
summers of 1976 and 1983, whereas more gently sloping duration
curves were typical of years with mild winters, such as 1974, or
years with cooler summers, such as 1980. Variability between years
was greater with respect to the duration of high rather than low
water temperatures, and at Thorverton, for example, temperatures
exceeded or equalled 0.5% of the time ranged by >6°C from 18.4°C in
1980 to 24.6°C in 1976, whereas temperatures exceeded or equalled
99.5% of the time varied by <3.5°C from 0.3°C in 1978 to 3.7°C in
1974. Curves for individual years show most variation for durations
in the range 0.01-2% of the time at the downstream site of Thorverton, but most variability for durations in the range 98-99.99% of
the time at Cowley on the River Creedy.
ACCUMULATED TEMPERATURE
Data concerning the sequence in which a water course is heated
through time may be provided by calculating the accumulation of
degree-hours above certain threshold temperatures such as 0 C
(Macan, 1958; Smith & Lavis, 1975). Figure 7 plots the average
BRUSHFORD
Fig. 7
COWLEY
THORVERTON
Accumulated river water temperature data.
trend in accumulated degree-hours above 0°C throughout a calendar
year as a cumulative percentage for the three monitoring stations
based on the decade of observations, and also indicates, by means of
envelope curves, the variation which occurred in the accumulated
temperature data from year to year in the study period. The average
trend is very similar for the three sites and indicates that
approximately 15% of the total degree-hours above 0°C are accumulated
in the study rivers in the first three months of the year. A further
28-30% of the total degree-hours above 0°C are accumulated in the
spring and early summer months from April to June, but the greatest
thermal concentration occurs in the period from July to September
at the end of which the cumulative percentage is approximately 80%
462
B.W. Webb & D.E. Walling
4
at the three sites. The largest increment in the cumulative curve
is associated with the month of July which accounts for an average
of 14% of the accumulated degree-hours above 0°C at the monitoring
stations. The envelope curves in Fig.7 also reveal that there was
little variation in the accumulation of degree-hours above 0°C in
individual years of the study period and that by the end of June,
for example, the cumulative percentage varied by not more than 4%
from the value of 43% associated with the average trends at the
three sites. Similarly, attainment of 50% of the total degree-hours
above 0 C was always associated with a 15-day period in mid-July
and accumulated temperature data indicate the generally stable
behaviour of river water temperature over the longer term in the
study area.
CONCLUSIONS
Ten years of detailed monitoring in the Exe basin have provided
considerable insight into longer-term river water temperature
behaviour in a humid temperate environment. Annual water temperature
statistics revealed no significant upward or downward trends over
the study period but a strong seasonal regime in water temperature
was evident and could be successfully modelled by second-order
harmonic curves. Diurnal variations were superimposed on the annual
cycle of water temperature but the nature, magnitude and timing of
daily fluctuations exhibited strong contrasts between winter and
summer seasons and also considerable variability from year to year.
Duration characteristics of river water temperatures also varied
substantially between individual years of the study decade but
accumulated temperature data showed much less variability in this
respect.
Contrasts in longer-term water temperature behaviour were also
evident between the monitoring stations included in the present
study. Differences between sites reflect both controls that operate
on a regional scale, such as the influence of elevation in promoting
lower mean temperatures at higher altitudes, and more local factors,
which include the effects of vegetation shading and groundwater
outflow in limiting water temperature ranges. Hydrological controls
are of considerable significance to water temperature behaviour and
volume of flow exerts an important effect through its influence on
thermal capacity. However, other hydrological factors such as
residence time of flow in the channel are considered to have a
strong control on water temperature since contrasts monitored
between upstream and downstream sites are less simple than those
recorded in other river systems.
Despite differences between the three monitoring stations, the
data collected in the present study suggest that temperature
behaviour was synchronous at the different stations and also was
essentially stable over the decade 1974-1983. In the latter context,
it appears that the construction of the Wimbleball Reservoir and
the inception of flow augmentation has had little effect on water
temperature behaviour in the main stream of the Exe at the downstream
site of Thorverton.
The results from the present study also have implications for
Temporal variation of river water temperatures
463
water temperature investigations beyond the Exe basin. Although 10
years of monitoring have highlighted the essential stability of river
water temperature behaviour, the study has also revealed significant
variability in certain water temperature characteristics from year
to year and suggests it would be unwise to regard results collected
in short-term field monitoring programmes (1-2 years) as being
representative of the extent of temporal variation over the longer
term.
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Received
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accepted
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