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Geographia Technica, Vol. 12, Issue 1, 2017, pp 9 to 22
TEMPORAL AND SPATIAL VARIATIONS IN HEAT CONTENT
OF A FRENCH POND
Quentin CHOFFEL1, Laurent TOUCHART1, Pascal BARTOUT1,
Mohammad AL DOMANY1
DOI: 10.21163/GT_2017.121.02
ABSTRACT:
Ponds are regarded as small waterbodies with a depth of less than 6m and an area of less
than 100 ha. In the Limousine region (France), they usually bar small incised valleys,
giving them a complex morphology. This morphology may separate the pond in several
basins, as for the pond of La Ramade (45°47’04.3”N; 2°25’30.7’’E), theoretically suitable
for a spatial variation of temperature. The pond has been implemented with 4 moorings
chains composed of automatic thermistors, measuring the temperature every hour from the
surface to the bottom. A meteorological station has been deployed close to the pond, in
order to gather very precise meteorological information, in addition to the map of fetch and
bathymetry. The working hypothesis is that even small waterbodies present a spatial
heterogeneity of water temperature, so that the classic measurements only at one point give
approximate values. The aim is to get a 3D overview of the thermal dynamic of a small
waterbody for the purpose of a better understanding of its temperature dynamic, with heat
content’s approach. In polymictic waterbody, the temporal but as well spatial variations of
the heat content are together numerous and varied. The first results bring us the autonomous
function of the bays and sheltering places, while the open water has a predictable function.
Some parameters such as the fetch, the sheltering by vegetation or the bathymetry plays a
more or less important role in the particularity of each basin. The heat content’s
methodology shall be improved by consideration of the spatial variation of the pond’s
temperature.
Key-words: Heat content, Water temperature, Pond, Spatial variation, Water Framework
Directive.
1. INTRODUCTION
The heat content of lakes and thermal properties is well studied since the beginning of
the 20th century (Birge, 1909; Wedderburn, 1910; Birge & Juday, 1914; Saur & Anderson,
1956; Gorham, 1964). Except for the biological survey (Williams et al, 2003; Downing,
2010; Céréghino et al, 2013), ponds are studied less than lakes, probably because of their
small size and artificial creation. They are frequently regarded as a homogeneous
epilimnion or “mixed-layer” (Imberger, 1985) of a lake. Some studies carried on the
thermal properties of ponds, especially on a vertical scale (Touchart, 2002; Folkard et al,
2007; Touchart, 2016), revealed the presence of a possible strong thermocline on several
days. French ponds are located in several places, depending on their variety. Some ponds
are located in important wetlands, without any rivers, or even in the flood plain of biggest
rivers. They are the most famous and well known. Others ponds are located in the
headwater of small stream with a temporary flow during thermic summer.
1Université d'Orléans, Département de Géographie, 45065 Orléans, France,
quentin.choffel@gmail.com, Laurent.touchart@univ-orleans.fr, pascal.bartout@univ-orleans.fr,
mohammad-aldomany@hotmail.com.
10
In that case, ponds are manmade with the aim of stocking water used for fishing,
agricultural, industrial, and even recreational reasons (Bartout, 2015). These ponds are our
main focus and La Ramade pond is one of these manmade ponds located in headwater
stream.
The issue of the stream temperature increase in headwater is frequently awarded to
small waterbodies baring a stream, i.e. ponds. In the context of the sustainable management
of water resources set by the Water Framework Directive (WFD) of 23 October 2000, the
French Water and Aquatic environmental Law of 30 December 2006, ponds with an area
greater than 50 ha can be concerned. In that case, temperature measurements are carried
out, usually on one single point. Most of the studies, especially unpublished papers of the
French authorities, are focused on the temperature continuum stream-pond-stream, without
knowing the heat content behaviour inside the pond. In lake’s study, the horizontal scale
getting more important and some studies took spatial measurements about great lakes (Xing
et al, 2012; Wang et al, 2014), the embayment of lakes, especially about lake Ontario
(Rueda & Cowen, 2005; Murphy et al, 2012) or also about a big mire system (Ramos,
2012), without taking into consideration the ponds, where the application should be the
most achievable. It is a very important point regarding the ponds, where it is possible to
choose the depth of the outflow (monk, weir…) or to change the localization of the outfall
in order to decrease the temperature’s impact on downstreams.
Small waterbodies are more numerous than lakes, at France’s scale (Bartout &
Touchart, 2013) but also on a global scale (Downing, 2010; Verpoorter et al, 2014; Bartout
et al, 2015). The thermic operation of a lake is quite different, focusing on the surface’s
temperature and the depth of the thermocline (Liu et al, 2014). It is more complex in
polymictic waterbodies, where diurnal and temporary thermoclines are varying very often.
Lakes and ponds are sensible to the global warming, felt in their heat budget, sensible to the
climatic variations, especially for shallow waterbodies (i.e. ponds). The increase in the
maximal air temperature should cause an increase in the heat content of the waterbodies
and therefore, may be an issue to the fish population. The wind plays a major role in the
heat budget and to spatial heterogeneity of the waterbody. Furthermore, heat budget is a
mean indictor of the influence of waterbodies on the local climate. With more than 550 000
waterbodies in France (Bartout & Touchart, 2013), their heat budget is important to be
known.
This research focuses on the thermal dynamic and heat budget of a pond, at different
spatial and temporal scales. The pond has been instrumented with 4 thermistor chains in
different basins, delimited by the map of fetch and bathymetry. The aim is to analyse the
spatial variability of the heat content and to know if it is suitable to take temperature
measurements only at a single point. Two heat budgets have been calculated according to
the traditional method (BT1) and to the new method from the 4 delimited basins (BT2).
2. STUDY SITE
The pond of “la Ramade” is located in the Limousine region of France, in the “plateau
de Millevaches”. It is one of the greatest ponds in Limousin with an area of 62 ha. The
mean depth is about 1m and the maximal depth is rarely up to 3m. This site has been
chosen because of its very complex morphology, with an important length of the shoreline
(Shoreline development index of 6.01). The turbidity of the pond is high, with Secchi disc
depth maximum of 0.75m in May and 0.55m in August. It is located on a substrate of
plutonic rocks, in the old mountain of Massif Central, at an altitude of 736m. According to
Quentin CHOFFEL, Laurent TOUCHART, Pascal BARTOUT and Mohammad AL DOMANY / … 11
Joly’s climatic typology (Joly et al, 2010), La Ramade pond is at the border between
“mountain climate” and “semi continental climate and marge of the mountains”. The mean
precipitations are around 1000 to 1100 mm/year according to a local MeteoFrance
meteorological station. The pond of la Ramade bars two streams of order 2 in the Strahler
classification: the stream of Manaly in the Western part and the stream of Chavanon to the
North. The mean retention time of the pond has been estimated at 21 days (Humbert, 2014).
The long and narrow morphology of La Ramade pond is very interesting to be
thermally studied. Elongated in the South-North direction, a bend in the central part breaks
the fetch that, to a certain extent, divides the pond in two main distinguished, although
permanently linked, basins (Fig.1).
3. METHODOLOGY
The methodology of vertical thermal analysis of a pond is widely used by Touchart
(2002, 2016) in France and Folkard et al (2007) in England. The present methodology is
enhanced by the addition of the spatial scale, with the implementation of four thermal
chains instrumented with 10 to 4 thermometers, measuring every hour the temperature of
the water, from the surface to the depth. With the hypothesis that the bathymetry and the
run of the wind (i.e. fetch) on the waterbody play a major role (Birge, 1909; Birge, 1916;
Imberger & Parker, 1985; Markfort et al, 2010), four basins have been delimited according
to the bathymetry map (Humbert, 2014 changed) and the Mean Potential Fetch (MPF) map
Fig. 1 Bathymetry map and localisation of the thermal chain (Humbert, 2014, changed). Coordinates
in RGF93.
12
(Papon, 2007). Due to the remoteness of the closest meteorological station and the possible
particularity of the local climate due to the topography, a meteorological station Vintage
pro 2 have been disposed close to the lake.
3.1 Study measurements
A meteorological station Vintage pro 2 is located a few meters away from the pond, on
the roof of a habitation (around 3m high). The console allows the instant display of outdoor
and indoor temperatures, dew point temperature, the wind chill temperature, degree of
indoor and outdoor humidity, daily and cumulative rainfall, and speed and wind direction.
The station is linked with the computer by an external console. Everything is easily
managed by the WeatherLink software.
The research is based on the water temperature, main factor in the aquatic life (Wetzel,
1983), with the implementation of Tinytag Data Loggers (range of -40 to 85°C) and Hobo
Water Temp Pro V2 (range of -20 to 50°C), measuring the temperature every hour on every
chain around the pond. The precision is respectively about 0.01 and 0.02 °C and the water
resistance is from 15 to 150m depth. As a precaution, every chain is instrumented with only
one type of thermistor in order to reduce the potential precision differences. The data are
directly gathered from the boat once a month. During the study period (03/03/2016 to
02/08/2016), on the pond La Ramade, 98 469 temperature data have been implemented and
analyzed.
Fig. 2 Map of the mean potential fetch on la Ramade pond.
Quentin CHOFFEL, Laurent TOUCHART, Pascal BARTOUT and Mohammad AL DOMANY / … 13
3.2 Spatial distribution of wind: map of fetch
The early work of Birge, at the beginning of the 19th century, has demonstrated the
major role of the external forces into the thermal dynamic. His studies focused on the role
of the wind, so far disregarded (Birge, 1909; Birge & Juday, 1914). As developed by Papon
(2007) in his thesis, the map of fetch is an important element of the thermal property of a
waterbody. This map consists in drawing the maximum run of the wind at every point of
the lake (resolution of 10m). This operation can be done automatically acc ording to tools on
the GIS software, but when the shore is complex, a few manipulations are necessary
(Papon, 2007). On La Ramade pond, the treatment has been done manually, due to the very
steep morphology. An extrapolation of the point derived from the lines, where the MPF is
calculated taking the mean fetch over the pond, after extrapolation. The mean potential
fetch map is an indicator of the wind heterogeneity over a waterbody (Fig.2).
The wind blow is heterogeneous on the pond, with some parts sheltered by the wind
and others very well exposed in spite of their small size and consequently the very short
fetch. The MPF map can be compared with the thermic dynamic of the pond (Wedderburn
Number, Brunt-Väisälä Frequency, Stability of smith…). It will be the study case of a
further study.
3.3 Methods for calculating the heat budget
The heat budget, introduced by Forel (1892), is the main part of the lake temperature’s
study over the last century. The methodology for calculating the heat budget has been
widely developed by Birge & Juday (1914), enhanced by Ragotzkie (1978), Hutchinson
(1957), Stewart (1973) on lakes and Touchart (2002, 2016) on ponds. The volume of each
layer is multiplied by the represented temperature, every hour. Thermistors are located
every 25cm until 1.5m, and then every 50cm from the depth to the bottom. The volume of
the pond has been taken from the bathymetry map (Humbert, 2014 changed), made by a
depth sounder in spring 2013. The vertical component, from the bathymetry, and the
horizontal component of the thermal chain give a three dimension overview of the thermal
component of the pond. The heat content of every layer is calculated every hour, with the
possibility of several heat budgets’ calculation, cyclic or acyclic, as daily or weekly heat
budget in addition to the traditional heat budgets (Forel, 1892; Birge, 1915; Hutchinson,
1957). The operations have been done with ArcGis10 software for the bathymetry, the
basins separation and Surfer11 software for the calculations, giving the volume of the
concerned layer. For this study, the heat budget is calculated from the thermal chain
localised few meters before the dam (B1), at the deepest point of the waterbody, following
the traditional method. This heat budget is named BT1.
Table 1.
Comparison of the four basins.
Basins
area (cm²)
Volume (m3)
max
depth
(m)
mean
depth (m)
Mean fetch (m)
Digue
798010000
126652
3
1,38
192
Central
1637830000
236060
2,48
1,27
276
Manaly
1005120669
80514
1,3
0,7
187
Nord
1941500694
91928
1,05
0,41
233
14
3.4 Meteorological data
The site is widely windy, with a weak but almost permanent wind blowing over the
pond. The wind’s dominant directions are from the West, and more precisely from the
South-West quart (29%) and the North-West quart (35%). It is an oceanic wind influence,
from the Atlantic Ocean located more than 250km away. The precipitations were quite
frequent and durable, but not very intense during this period. During the study period, the
climate was particular with a rainy and cold springtime and a very warm and dry month of
July (Fig. 3). This particularity shows very different climatic stages, which are found into
the heat content of the pond. From 31/05 to 21/06 a technical failure did not enable the use
of the meteorological data. 1872 hourly data are available for this study, from 26/04 to
02/08.
Fig. 3. A, B, C, D: meteorological conditions from the Vantage Pro 2 station. 26/04/2016 to
02/08/2016. Below: wind rose from the Vantage Pro 2 station; data from 26/04/2016 to 31/05/2016
and 21/06/2016 to 02/08/2016.
Quentin CHOFFEL, Laurent TOUCHART, Pascal BARTOUT and Mohammad AL DOMANY / … 15
4. RESULTS
4.1 Thermal behavior
The thermal behaviour of the pond is determined by the four thermal chains disposed
all around the pond. The mean monthly or even daily temperature at the several locations is
quite the same. The main differences are measured with hourly data, where some external
or internal factors can modify the temperature within a few hours. The temperature in a
polymictic lake is changing fast as shown on the Fig. 4.
Fig. 4. Thermal behaviour of the La Ramade pond on the four thermal chains. 30/04/2016 to
02/08/2016.
16
The deepest part of the pond (B1 & B2) is weakly stratified during spring, with the
appearance of diurnal stratification during shiny and calm weather. During summer, the
stratification is stronger and can remain for several weeks (3 weeks maximum here). The
onset of a strong wind and a decrease in the temperature lead to a break of the thermal
stratification. In the two other basins, the stratification is weaker during one day or a few
days for the B3. B3 is more strongly stratified and warmer than in the basin 4, yet a bit
shallower.
The four basins have different thermal behaviour, making a difference in the heat
content of the pond between the traditional and the newly integrated methods of
calculating. The temperature is directly linked to the heat content. A difference in
temperature between several places in the pond induces to a difference in heat content.
From 30th of April to 2nd of August, the differences in temperature within the pond can be
important. Some places as B1 and B3 have reached the temperature of 30°C or more, while
it never happened in the central basin. The bottom temperature is changing manly due to
the bathymetry and exposure to the wind, with the phenomenon of small upwelling.
4.2 Heat content
4.2.1 Study period (03/03/2016 to 02/08/2016)
The heat content is the amount of temperature on the whole waterbody, with a three
dimensional scale. The annual heat budget of La Ramade pond, i.e. “the total of amount of
heat that enters the lake between the time of its lowest and its highest heat content”
(Hutchinson, 1957, p. 493), is 2462 cal/cm² (The study period do not include the whole
year. The annual heat budget can be different.). This annual heat budget is very close to that
one of a close pond of Les Oussines: 2485 (Touchart, 2016, data from 2002).
During hypothermia or small and quick stratifications, the heat budget increases
slightly. It can also decrease during a few days, according to the meteorological conditions.
During the stage of thermic stratifications, the heat budget increases considerably during
several days to several weeks and decreases during temporal homothermy (Fig. 5).
Fig. 5. Heat content of La Ramade pond from 03/03/2016 to 02/08/2016.
Quentin CHOFFEL, Laurent TOUCHART, Pascal BARTOUT and Mohammad AL DOMANY / … 17
4.2.2. Percentage of difference
From the end of May, at the beginning of the summer, the differences between the two
heat budgets took another shape. The values of the differences are less sharp, with a
positive maximum of 10.7% and negative maximum of -8%. The mean gap is 2.16%,
inferior to the first part of the study period (Fig.6). The curve is smoother with longer
periods of positive differences and shorter periods of negative maximum.
The differences can be punctually or temporally high. The difference in annual heat
budgets is weak, with BT1 about 2462 cal/cm² and 2329 cal/cm² from BT2. However, the
minimum occurred on 09/03/2016 for BT1 while it is one day later (10/03/2016) for BT2.
The maxima occurred at the same time (02/08/2016, 10:00) but are limited by the period of
study, stopping at this moment.
4.2.3 Study period: 17 to 21 July
The focus is on a short period from the 17th to 21st of July, where the positive
maximum (18th of July) and the negative maximum (19th of July) from the second part of
the study period, occurred. In only 24h, the negative maximum and the positive maximum
were reached, during a stable period of thermal stratification.
From the 17/7 to the 21/7, the outdoor temperature was quite similar, from 28°C (17th
and 21st of July) to 34°C (19th of July). The wind is the only main factor varying
considerably during this time. The variation of the wind speed went from a maximum of
0.4m/s the 18th of July and 1.3m/s the 17th and the 21st of July. The wind direction is
notably changing, with three main directions: 25° (North-East), on the 17th, 360° (North) on
the 18th, 205° (SouthWest) from the 19th to the 21st and 305° (North-West) at the end of the
21st of July.
Fig. 6. Percentage of difference between BT1 and BT2 during the whole study period.
18
The positive part of the difference between the two heat budgets occurs during a
blowing of the wind from the North and North-East. The change of the wind direction
(South-West), appeared on the 19th of July, involves a drop in the percent of difference
reaching -8%.
The Fig.7 shows the percentage of differences between BT1 and BT2 and the
differences between the heat content of B1 (basin of the damn) and B2 (central basin), the
two main volume basins. The two curves are related, especially from the middle of the 19th
of July. The relation of the curves shows coordination among the difference in heat budget
and a difference in the heat content of the two deep basins. It works for the two maxima,
especially when the curve reaches -8% and a few hours later. The trend is marked by the
two other basins during the fall of the end of the 18th of July. The wind plays a major role in
accentuating or diminishing the percentage of difference between the two heat budgets. The
speed but especially the directions of the wind are the main factors of differences on a few
days’ scale.
Fig. 7 Top: percentage of difference between BT1 and BT2 (Black line) and difference between B1
and B2 (dashed line). Bottom: wind speed (line) and direction (stars). From 17/07/2016 to
21/07/2016.
Quentin CHOFFEL, Laurent TOUCHART, Pascal BARTOUT and Mohammad AL DOMANY / … 19
4.2.4 Daily study
On a daily scale, the variation of the heat budget is consequent. The day of the 18th of
July is one of the days where the heat content’s amplitude was the highest (Fig. 8). The heat
content’s amplitude reached 521 cal/cm² for BT1 and 360 cal/cm² for BT2. The minimum
and the maximum occurs at the same time for both heat contents, with a minimum at 8:00
and a maximum at the end of the day at 19:00. This amplitude can be named “the daily heat
budget” of the pond. The daily heat budget reached its maximum in summer time, when the
accumulation of solar radiation and the outside temperature are the highest. This budget
changes very fast from one day to another, heavily influenced by the meteorological
conditions.
The curve of the BT2 is smoother than BT1, with a similar minimum (2110 and 2106
cal/cm² respectively), and a strictly higher maximum for BT1 (2470 and 2627 cal/cm²
respectively). This smoothing is partially due to the integration of all the basins of the pond
in BT2. Hence, it limits the specificity of the measurements in one location, sheltered by
wind or in the direction of the wind.
5. DISCUSSION
5.1. Vulnerability of ponds from meteorological conditions
The results show a high dependence of the heat budget to the meteorological
conditions. A rainy and cold spring leads to weak heat content at this time, while it is very
high in July, where the weather was warm and dry. The heat content can rise and drop
quickly, especially in these medium altitude mountains, where the air temperature
amplitude between the day and the night can be high.
Ponds are an indicator of the last climate conditions, widely studied over the last
decade. Meteorological conditions are also an important indicator on the thermal dynamic
Fig. 8. Daily variation of the heat content from BT1 (dashed line) and BT2 (dark line).
20
of the pond. The wind plays a major role in the thermal spatial heterogeneity over the water
body, especially the ones with a complex shape. Ponds with a high shoreline index,
especially due to the steep shoreline, are suitable to a spatial heterogeneity. The study of a
smaller and round shaped pond (Etang du Château, Rilhac-Rancon, France, 0.4ha, 4.5m
max depth) is in progress to learn the spatial repartition of temperature over the pond.
The differences in the thermal dynamic and the heat budget are various and important
during the springtime. The first stratifications, during springtime, are weak and the wind
can easily break it. During this time, even a small differentiation in the fetch can lead to a
consequent difference into the thermal stratification over the different basins. B1, which is
the source of calculation of BT1, is sheltered by a great part of the wind directions, at the
opposite of B2, which concentrates the biggest volume, at the junction of almost all the
winds. The differences between B1 and B2’s temperatures are mainly due to the wind
exposure. The wind’s effect is also visible on B3, well sheltered by winds due to the
narrowness of the basin, but also probably due to the vegetation sheltering, compared to B4,
a bit less narrow but mainly less sheltered by vegetation (especially on the East part totally
devoid of vegetation. See Fig. 1). Consequently, B4 concentrates the coldest water, weakly
and rarely stratified, while B3 is frequently stratified in spite of the shallow water column.
The localisation according to the dominant winds is important. The wind drive the warm
surface water to the direction of the opposite shore. The differences between the two shores
(and more broadly the basins) can reach 4°C between the windward basin cooler and the lee
basin. The distribution of the heat deeper by the wind enhance the heat content of the pond
at the lee basin. In contrast, a upwelling occur in the windward coast, reducing the heat
content. In the central part, there is no such effect of upwelling and downwelling but the
basin is more often affected by wind from any directions.
5.2 The spatial and temporal scales: a solution to improve the heat budget?
The heat budget is traditionally calculated from the deepest point, considering the
representativeness of this point to the whole waterbody. When this budget is calculated
from four points around the pond, according to the bathymetry and the fetch map, the
differences between the two heat budgets can be consequent. These differences vary with
time and meteorological conditions. It is highest when a strong stratification occurs during
summer. It can be partly explained by the difference in the temperature of B1 and B2, the
two main volumetric parts of the pond. When B2 is colder than B1, which happens v ery
often, especially under windy conditions, BT1 is overestimated. With a wind from the
North, the 19th of July for example, the underestimation of the BT1 is not due only to B2-
B1, but mainly to the increase of the heat content of B3 and B4, sheltered by North-wind.
B1 and B2 play a major role into the precision of the heat content of the pond. B3 and
B4, for their part, can play, in some events, an important role improving the heat content.
All of these factors seems to be due to the wind heterogeneity around the pond.
The temperature and heat content of a polymmictic lake is changing fast. The
temperature can be different from a place to another but also from a day to another or
within a day. At a daily scale, the main heat content differences between t he two methods
seem to take place at the end of the day, when the stratification and the warming are at their
maximum. On the opposite, it is during the morning, at 8:00 principally, when the warming
has done nothing yet and the previous heat content is attenuated by the cooling, that the
marge of error of BT1 is the lowest.
In order to enhance the knowledge of the heat content of the waterbodies, it would be
useful to take continuous temperature data, for as long as possible. The days with no wind
Quentin CHOFFEL, Laurent TOUCHART, Pascal BARTOUT and Mohammad AL DOMANY / … 21
are useful for this type of waterbody. The wind-induced variations are the main factor on
La Ramade pond because of it complex morphology and well-developed shoreline. It is
different with great lakes and also with very small ponds, as L’Etang du Château (0.4ha,
4.5m max depth), where the first results shows the importance of the direct radiation, where
temperature differences are caused by a shaded area of the pond (Choffel, ongoing study).
6. CONCLUSION
The heat content is a very important component of the evaluation of water quality. This
methodology allows to understand the heat dynamic of the pond taking into account the
vertical and horizontal variations. In other studies the evaluation of the heat content is based
on data from one point, usually the deepest point of the pond. The temporal and spatial
variation of temperature can causes a high percentage of error between traditional and
spatially-based methods. Because of the wind heterogeneity through the pond, it is suitable
to take temperature data on several points of the pond. Temporal variations, daily or
weekly, linked to the meteorological conditions lead to take continuous data in order to
reduce the percentage of error. The pond of La Ramade, with a complex morphology and a
well-developed shoreline induces the spatial heterogeneity. This kind of morphology is
common in France and Europe when a dam bar a river. The forthcoming work will be to
compare with other ponds of different and simpler morphology and have an estimation of
the heat budget of the French ponds, according to the inventory work and volume
correlation of Bartout & Touchart (2013).
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