Int. J. Water, Vol. 5, No. 4, 2010 311
Copyright © 2010 Inderscience Enterprises Ltd.
Solar energy dissipation and temperature control
by water and plants
Dukelská 145, CZ-379 01 TĜeboĖ, Czech Republic
Faculty of Agriculture,
Department of Landscape Management,
University of South Bohemia,
Studentská 13, CZ-370 05 ýeské BudČjovice, Czech Republic
Dukelská 145, CZ-379 01 TĜeboĖ, Czech Republic
Faculty of Forestry and Wood Technology,
Institute of Forest Botany,
Dendrology and Geobiocenology,
Mendel University in Brno,
ZemČdČlská 3, CZ-61300 Brno, Czech Republic
Dukelská 145, CZ-379 01 TĜeboĖ, Czech Republic
Faculty of Environmental Sciences,
Dept. of Applied Geoinformatics and Spatial Planning,
Czech University of Life Sciences Prague,
Kamýcká 129, Praha 6 – Suchdol, CZ-165 21, Czech Republic
312 J. Pokorný et al.
Institute of Physical Biology,
University of South Bohemia,
CZ-373 33 Nové Hrady, Czech Republic
Faculty of Forestry and Wood Technology,
Institute of Forest Botany,
Dendrology and Geobiocenology,
Mendel University of in Brno,
ZemČdČlská 3, CZ-61300 Brno, Czech Republic
Dukelská 145, CZ-379 01 TĜeboĖ, Czech Republic
Abstract: Ecosystems use solar energy for self-organisation and cool
themselves by exporting entropy to the atmosphere as heat. These energy
transformations are achieved through evapotranspiration, with plants as
‘heat valves’. In this study, the dissipative process is demonstrated at sites
in the Czech Republic and Belgium, using landscape temperature data from
thermovision and satellite images. While global warming is commonly
attributed to atmospheric CO2, the research shows water vapour has a
concentration two orders of magnitude higher than other greenhouse gases. It is
critical that landscape management protects the hydrological cycle with its
capacity for dissipation of incoming solar energy.
Keywords: ecosystems; evapotranspiration; sensible heat; albedo; radiative
forcing; temperature variation; remote sensing.
Reference to this paper should be made as follows: Pokorný, J., Brom, J.,
ýermák, J., Hesslerová, P., Huryna, H., Nadezhdina, N. and Rejšková, A.
(2010) ‘Solar energy dissipation and temperature control by water and plants’,
Int. J. Water, Vol. 5, No. 4, pp.311–336.
Biographical notes: Jan Pokorný took his PhD at the Charles University,
worked on photosynthetic processes at the Agricultural University, Prague, and
later headed the TĜeboĖ Section of the Institute of Botany at the Czech
Academy of Sciences. Among his many commitments, he directs ENKI, a
public benefit corporation for environmental research. Outside Europe, he has
worked in Africa and Australia, and in 1998 was elected to the Scientific
Technology Review Panel of the Ramsar Convention. His 200 publications
include Macrophyte Photosynthesis and Aquatic Environment, Rozpravy of
Czechoslovak Academy of Sciences. Academia: Praha (1991).
Jakub Brom is a researcher in the Faculty of Agriculture at the University of
South Bohemia in ýeské BudČjovice, Czech Republic; he also contributes to
Solar energy dissipation and temperature control by water and plants 313
the public benefit corporation ENKI in TĜeboĖ. His interests are the
physiological ecology of plants and bioclimatology, mostly researched using
meteorological methods, thermometry and remote sensing. He is the co-author
of several papers in peer-reviewed journals, including ‘Temperature and
humidity characteristics of two willow stands, a peaty meadow and a drained
pasture and their impact on landscape functioning’ (Boreal Env. Res., 2009).
Jan ýermák studied forestry at the Mendel University of Brno Czech Republic
and received his PhD from the same university. He has over 30 years of
teaching experience in whole tree water relations, architecture and growth.
He has performed collaborative research throughout Europe, Harvard,
Washington University and the US Forest Service. He has served as Vice-Chair
of the Whole Tree Physiology IUFRO association and on the editorial review
board of Tree Physiology. His 270 published papers and 60 reports are widely
cited (see: ‘Application of sap flow technique for characterising the whole tree
architecture, especially root distribution’, Acta Horticulturae, 2009).
Petra Hesslerová graduated in Physical Geography and Geoecology at the
Charles University, Prague, where she also received her PhD in 2008.
She currently works for the ENKI public benefit corporation and is an
Assistant Lecturer at the Czech University of Life Sciences, Prague.
Her specialisations are geography, remote sensing and landscape ecology,
carried out in cross-national contexts. She has co-authored a number
of publications including: ‘The synergy of solar radiation, plant biomass, and
humidity as an indicator of ecological functions of the landscape’, Integr. Env.
Assessm. Managem. (2010).
Hanna Huryna graduated in Natural Resources and Ecology at the Belorussian
National Technical University, Minsk. She later worked with the Belarusian
Scientific Institute for Transport Research. She is currently a PhD student in
the Institute for Physical Biology at the University of South Bohemia, Nove
Hrady, Czech Republic. Her publications include: ‘Comparison of reflected
solar radiation, air temperature and relative air humidity in different
ecosystems’ in Vymazal, J. (Ed.): Water and Nutrient Management in Natural
and Constructed Wetlands (forthcoming 2010).
Nadia Nadezhdina studied in Archangelsk and Gorky, Russia, and has a PhD
from the Ukrainian Academy of Sciences, Kiev. She is currently Professor in
the Institute for Forest Botany at the Mendel University, Brno, Czech Republic.
She has joined many international research collaborations including Europe,
China and the USA. She holds 11 patents on physiological equipment and has
120 papers and 20 research reports on eco-physiology and innovative methods
for the study of tree water relations and architecture (see: ‘Integration of water
transport pathways in a maple tree: responses of sap flow to branch severing’,
Ann. For. Sci., 2010).
AlžbČta Rejšková took her MA at the Charles University in Prague, and in
2009 completed a PhD at the University of South Bohemia. Her main research
interest is the eco-physiology of plants and the role of vegetation in landscape
functioning. She currently works for the public benefit corporation ENKI and
is an external lecturer in the Department of Plant Physiology at the Charles
University. She has co-authored ‘Temperature distribution in light-coloured
flowers and inflorescences of early spring temperate species measured by
infrared camera’, Flora (2010).
314 J. Pokorný et al.
This paper demonstrates the fundamental role played by water and vegetation in
maintaining local climate. Human modification of a landscape by deforestation,
industrial agriculture and urbanisation may destroy the capacity of an ecosystem to
dissipate – digest and distribute – solar energy fluxes. On a hard dry surface, sensible
heat can be released to several hundred watts/m2, but water or vegetated surfaces have
the capacity to convert solar energy into the latent heat flux of vaporisation. Evaporation
allows latent heat, in the form of water vapour, to be carried to cooler places where it
condenses and releases heat. The amount of energy and heat moved in this way should
not be underestimated. On a sunny day, a flux of solar energy falling on 2 km2 of ground
may be equivalent to the power generated by a large nuclear power station (2000 MW).
Ultimately, it becomes a matter of public education and political choice as to how these
natural processes are treated. Either the solar flux is permitted to provide the natural
air-conditioning of water vaporisation and plant growth or it is converted into sensible
heat and environmental temperatures soar to 40–50°C.
1.1 Solar energy flux between Sun and Earth
It is the Sun’s energy that drives the water cycle, plant production and other processes,
which cause equilibrium shifts in the biosphere and which enable its development. Solar
energy warms the Earth to an average temperature of around 15°C or 288 K. For a mean
distance between the Sun and the Earth, the intensity of solar radiation incident upon a
surface perpendicular to the Sun’s rays measured above the atmosphere is approximately
1367 W m–2. This quantity is called the solar constant. The solar constant is very stable,
fluctuating ±1.3 W m–2 (0.1%) during the 11-year period of sun spot cycles. The actual
direct solar irradiance at the top of the Earth’s atmosphere fluctuates during a year from
1412 W m–2 in early January to 1321 W m−2 in early July due to the Earth’s varying
distance from the Sun. The amount of solar energy changes over the year by about ±3.2%
(45 W m–2) depending on the position of Earth in its elliptic orbit around the Sun (Geiger
et al., 2003; Kopp et al., 2005). The Sun as a blackbody with a surface temperature of
5900 K emits a maximum radiation in the visible range of 400–700 nm. The surface of
the Earth with its temperature ca. 288 K emits a maximum radiation in the infrared (IR)
part of the spectrum of about 10 µm. The atmosphere influences the spectrum of incident
light both quantitatively and qualitatively. The spectrum of solar radiation incident on
both the Earth’s atmosphere and the surface at sea level is shown in Figure 1 where the
absorption of radiation through the main atmospheric gases is indicated.
The amount of received solar energy varies across the world and varies in daily and
seasonal pulses. The mean distributions of global insulation for different months are to be
found in the pages of NASA SSE (http://eosweb.larc.nasa.gov/sse). According to NASA
SSE data, the maximum annual average direct normal radiation incident on the Earth’s
surface is up to 10 GJ m−2 (2770 kWh m−2), with maximum monthly average direct
normal radiation 30 MJ m−2 day−1 (8.5 kWh m−2 day−1). This intensity of solar radiation is
reached, for example, along the northwest coast of Australia. In the Czech Republic,
which is a temperate zone, the annual income of solar radiation is about 4.14 GJ m−2
(1150 kWh m−2); in Helsinki (Finland) 3.49 GJ m−2 (970 kWh m−2) and in Giza (Egypt)
7.45 GJ m−2 (2070 kWh m−2).
Solar energy dissipation and temperature control by water and plants 315
The maximum irradiance commonly lies between 800 W m–2 and 1000 W m–2 in the
tropics and subtropics and during the growing season in temperate zones. This indicates
that approximately 25–40% of energy incident on the upper layer of the atmosphere is
reflected, scattered or absorbed in the atmosphere and does not reach the Earth’s surface.
The transmission of solar radiation is a function of two variables:
• path length through the atmosphere across which the solar beam travels
• the content of absorbers present in the atmosphere.
Figure 1 Solar spectrum incident on the atmosphere and on the Earth’s surface at sea level.
Radiation of the blackbody of 5900 K is also shown. Seven gases in the Earth’s
atmosphere produce observable absorption features in the 0.4–2.5 mm range: water
vapour, carbon dioxide, ozone, nitrous oxide, carbon monoxide, methane and oxygen.
(http://www.csr.utexas.edu/projects/rs/hrs/process.html). 1 ǖ = m−10
By far, the most variable and dynamic absorber is water vapour. The amount of incoming
energy differs significantly also with weather conditions. The difference between the
amounts of incoming radiation on a clear day (e.g., 29.3 MJ m−2 and maximum flux
1000 W m–2) can be an order of magnitude higher than the amount of incoming radiation
on an overcast day (e.g., 0.78 kWh m–2, maximum flux 100 W m–2). The distribution of
solar energy on the Earth depends on its surface characteristics. Part of the energy is
reflected straight away after incidence. The ratio of reflected to incident radiation is
called albedo. Dark surfaces such as water, wet soil and wet vegetation absorb solar
radiation whereas light surfaces like snow or sand are more reflective. The sum of all
incoming radiation minus all outgoing radiation across a unit area of the plane is called
net radiation (Rn). The IR part of radiation contributes to radiation balance – the sky is
commonly colder than the Earth’s surface, which results in energy loss from the Earth’s
1.2 Distribution of solar energy in ecosystems
There is a big difference between the distribution of net radiation in functioning natural
ecosystems of high plant biomass well supplied with water vs. dry non-living physical
surfaces. In ecosystems, net radiation (Rn) is divided in varying proportion into the
316 J. Pokorný et al.
following four parts: latent heat flux (LE), sensible heat flux (H), ground heat flux (G)
and storage of energy (S). It can be described using the following equation:
LE H G S.
The latent heat flux (LE) represents the energy used for evaporation of water from the
surface. The transition of liquid into a gas phase is an endothermic reaction, i.e., energy
consumption, and thus local cooling accompanies it. On the contrary, condensation is an
exothermic process attended by energy release and local warming. An amount of 2.45 MJ
(0.68 kWh) of energy is needed for the evaporation of 1 kg water at 20°C; the same
amount of energy is released during condensation of 1 kg water vapour. The dynamic
system of phase transitions of water plays a crucial role in thermoregulation of living
systems and in damping temperature differences on the Earth.
Plants transport water with nutrients from soil via their roots and stems into leaves
from where water evaporates – a process called transpiration. The evaporation of water
from plant stands is called evapotranspiration, as it consists of transpiration and
evaporation both from soil and from intercepted water from the plant’s surface. The rate
of ET from stands of vegetation supplied with water reaches several litres of water per
day per square metre. ET of 4 kg m−2 day−1 represents mean daily flux of latent heat
225 W m–2 (calculated for 12 daylight hours). The horizontal movement of energy, called
advection, amplifies ET substantially. Advection is horizontal convective heat transport
both of sensible and of latent heat (Oke, 1987) and it can have significant effects on
energy exchange and on a vegetation-water regime (see e.g., Li and Yu, 2007). A hot, dry
wind blowing on to a moist oasis evaporates more water than can be calculated by the
vertical accounting of energy budgets (Monteith, 1975). Because of the effect of
advection, ET values can exceed 15 mm per day (Kuþerová et al., 2001).
The sensible heat flux (H) represents the sum of all heat exchanges between the
surface of a landscape and its surroundings by conduction or convection. It is positive
when the surface is warmer than surroundings and heat is lost from it. The sensible heat
flux is negative when heat moves in the opposite way. The proportion of sensible heat in
the energy balance of an ecosystem increases when water is not present, since the
capacity for evaporative cooling by latent heat is diminished. On dry surfaces, the
sensible heat flux may reach values of several hundreds of Wm−2. The sensible heat of an
overheated surface warms air, which rises up in a turbulent motion creating atmospheric
The ground heat flux (G) is positive when the ground is warming. G is commonly
positive during the day and negative at night. The heat balance of the ground is usually
moderately positive during warm periods and negative during the cold periods, making
the net yearly ground heat-flux approach zero. During the plant growing period in
daylight hours, G ranges from 2% of Rn in a dense vegetation canopy to more than 30%
of Rn in sparse canopies with little shading of the soil (Jones, 1992). A layer of organic
matter like dry leaves and bark will insulate soil from energy coming in from its
surroundings. The heat conductivity of soil increases with its water content
(Peters-Lidard et al., 1998). The energy stored in vegetation is the smallest part of Rn.
Principally, there are two energy sinks within a plant stand
• metabolic sink, i.e., photosynthesis with consequent biomass production
• a physical sink, which represents heating of the plant material itself.
Solar energy dissipation and temperature control by water and plants 317
In closed canopy ecosystems, the maximum net photosynthesis rate is 1.0–1.4 mg m−2 s−1
of sucrose (Cooper, 1975), which corresponds to 17.0–23.4 J of energy fixed every
second, i.e., energy flux 17.0–23.4 W m–2. This represents about 2% of total summer
irradiance and about 3% of Rn, on the assumption that it should be approximately
700 W m–2. The physical sink of energy depends on the amount of living biomass, which
contains about 90% of water. Assuming that 2.2 kg m–2 of fresh biomass containing
approximately 2 kg of water was warmed up to 4°C per hour, then the heat-flux warming
the biomass would be approximately 10 W m–2 (heat capacity of water is 4184 J kg–1 K–1
at 20°C). Contrary to biomass production, the decomposition of organic matter is
associated with release of energy. The rate of decomposition can be several times higher
than the rate of biomass production, i.e., release of heat due to decomposition can reach
several tens of W m–2. The decomposition of organic matter in soil is accelerated by
1.3 Exchange of water and CO2 in plant stands
Most plant tissues contain large amounts of water. The biomass of non-woody tissues
typically comprises 80–95% of water. Most water taken up by roots is transported
through plants in the Soil-Plant-Atmosphere Continuum (SPAC) and transpired into the
air. Transpiration is vital to the uptake of CO2 in the intercellular air spaces of stomata
(Berry et al., 2005). The cooling process of transpiration is often considered a side effect
rather than a mechanism to control leaf temperature (Lambers et al., 1998). Transpiration
is also perceived as a rather negative process. Plant physiology and hydrology may
use negative terms such as ‘transpiration loss’ and ‘evapotranspiration losses’.
Transpiration Efficiency (TE) is defined as the amount of water lost through transpiration
per unit of dry matter produced. TE normally reaches a value of several hundred
kilograms of water consumed per kilogram of dry biomass produced. The amount of
water molecules exchanged by plants is at least two orders of magnitude higher than the
amount of carbon dioxide fixed to biomass (You et al., 2009).
The average concentration of CO2 in the air is about 391 ppm (NOAA, 2010).
In plant stands, the concentration of CO2 varies both by vertical profile measures and by
day and night, depending on the photosynthetic and respiration activities of biota. On the
leaf surface of C4 plants such as corn, the concentration of CO2 falls to zero whereas on a
forest floor, CO2 concentration can be over 500 ppm (Jassal et al., 2007). Conventional
climate models, as well as calculations of the global carbon budget, treat the
concentration of CO2 in the atmosphere as homogeneous (Randerson et al., 2006).
In contrast to CO2 as a greenhouse gas, the amount of water vapour as a greenhouse
gas found in plant stands and in the atmosphere is many times higher and it changes
dramatically across time and space. For example, air saturated with water at 21°C
contains 18 g m–3 of water vapour, i.e., 22,400 ppm. Air saturated with water at 40°C
contains 50 g m–3 of water vapour, i.e., 62,200 ppm. The amount of water vapour in air is
often two orders of magnitude higher than that of CO2. Water absorbs solar energy in
visible, near and far IR. The energy absorption spectrum of water is broader than that of
CO2 (see Figure 1). The content of water vapour in the atmosphere is highly variable, and
furthermore, water exists in three phases (solid, liquid and gaseous). The transitions
between these phases are linked with the uptake or release of high amounts of energy.
Climate change and global warming are reputedly caused by an increase in CO2
318 J. Pokorný et al.
concentration from 250 ppm to 390 ppm. The present research highlights the dynamic
role of water vapour, with its concentration two orders of magnitude higher than that of
other greenhouse gases. The implication is that human landscape management affects the
behaviour of water vapour and its role in the dissipation of solar energy.
2 Site description and methods
Seven different sites were monitored in this study: five non-forested and two forested
areas. Non-forested areas were located in the TĜeboĖ Biosphere Reserve, Czech
Republic; the forested areas were in South Moravia, Czech Republic and in Antwerp
Exact methods for the monitoring and evaluation of energy fluxes were used to
• seasonal series of incoming and reflected solar radiation, albedo and temperature
• daily series of main fluxes of solar energy (incoming and reflected solar energy,
sensible, latent and ground heat fluxes)
• temperature distribution in landscape monitored by remote sensing
• transpiration of trees measured by sap flow method and expressed in terms of energy
2.1 TĜeboĖ Basin Biosphere Reserve (TBBR)
This research area is situated in the southern part of Bohemia at the Austrian–Czech
border. The basin is mostly filled with sand and clay sediments while the marginal parts
consist of igneous crystalline rocks. The flat bottom of the basin is 410–470 m altitude;
the undulating marginal area reaches 550 m. The TĜeboĖ Basin belongs to a moderately
warm region with an annual mean temperature of 8°C and a mean annual precipitation
total of 650 mm. Forest covers about 50% of the territory. The Basin is marked by a
sophisticated network of human-made canals and watercourses built since the Middle
Ages for drainage of wetlands and the construction of fish ponds. The TBBR with a total
area of 700 km2 has about 500 fish ponds making a total area 7500 ha. Forestry,
agriculture and fishing are main activities in TBBR (Jeník and KvČt, 2002). The study
sites are several km apart.
Data were collected at the following sites:
• Autumn barley field (area of 22 ha).
• Wet Meadow (area of c. 200 ha) in the Rožmberk fish pond (450 ha) floodplain.
Dominant macrophyte species include high sedges (Carex gracilis (L.), Carex
vesicaria (L.)), Calamagrostis canescens (Weber), Phalaris arundinacea (L.) and
Urtica dioica (L.) The area surrounding the meteorological station is not managed;
tall vegetation is cut only close to the station. The dryer parts of the Wet Meadow are
mowed once a year.
• Concrete surface (area of 400 m2) within the area of the Wastewater Treatment Plant
of the city of TĜeboĖ.
Solar energy dissipation and temperature control by water and plants 319
• Open water surface of the Ruda fish pond (72 ha, depth c. 2 m).
• Meadow is represented mostly by mesic vascular plant species: Alopecurus pratensis
(L.), Arrhenatherum elatius (L.), Phleum pratense (L.), Tanacetum vulgare (L.) and
Taraxacum sect. Ruderalia. This locality is mulched two or three times per year.
2.1.1 Forested sites in Moravia and Belgium
The first forested site is near Lednice in the southern part of Moravia in the alluvium of
the Dyje River and classified as Ulmeto-fraxinetum carpineum. The soils of this locality
originate from sedimentation during spring floods. The stand is composed mostly of oak
(Quercus robur (L.) 78%, Fraxinus excelsior (L.) 18%, Tilia cordata Mill. 3% and 1% of
other species) with the mean age of dominant trees 95 years and stand density of 90%.
The leaf area index is 5 for the tree layer and 2 for the shrub layer (Vasicek, 1985;
The second experimental plot of Scots pine (Pinus sylvestris (L.)) is a forest
plantation in Brasschaat, Campine region of the province of Antwerp, Belgium, in a
plateau of the northern lower plain basin of the Scheldt River. The original climax
vegetation in the area is a Querceto-Betuletum. Soil is characterised as moderately wet
and sandy with a distinct humus or iron B-horizon (psammenti haplumbrept in the USDA
classification, umbric regosol or haplic podzol in the FAO classification).
The experimental pine stand was 66-years old at the time of study. The original,
homogeneous stocking density was relatively high for pine and the stand had
been frequently thinned, most recently in 1993. Leaf area index (one side) was 3
(ýermák et al., 1998).
2.2 Meteorological data and energy balance
All measurements were made in the TĜeboĖ Biosphere Reserve except measurements of
tree transpiration. Meteorological data were recorded at 10-minute intervals during the
growing season, 1 April–30 September 2008. Air temperature was measured 2 m above
the soil surface and at the vegetation surface (Ta and Tc, °C; T+Rh probes, accuracy
±0.1°C). Temperature at the soil surface was measured by Pt 100 thermometer (Ts, °C;
accuracy ±0.1°C). Incoming (RsĻ) and reflected shortwave (global) radiation (RsĹ) was
measured on each site by CM3 pyranometers (Kipp & Zonen, the Netherlands, spectral
range from 310 nm to 2800 nm). Incoming and emitted long-wave radiation was
measured on the Meadow site by a CNR1 Net radiometer (Kipp & Zonen, the
Netherlands, spectral range from 5 µm to 50 µm). The volumetric content of liquid water
in the soil at 0.05 m below the surface was measured by (ș, %; Wirrib, AMET, Czech
Republic, accuracy ±0.01 m3 m–3). Emitted long-wave radiation (RLĹ, W m–2) was
computed using Stefan-Boltzmann’s law from temperature measured at the vegetation
surface. Sky temperature calculated from net radiometer data was as follows (Monteith
and Unsworth, 1990):
where İ is the emissivity of the surface (tabulated values were used according to
Gates (1980)), ı is Stefan-Boltzman constant (5.6703 × 10–8 W m–2 K–4) and T is
320 J. Pokorný et al.
, rel.) was computed as a ratio between reflected and incoming short-wave
Net radiation was computed from the short- and long-wave parts of radiation using
equation (Brutsaert, 1982):
RR R R R
Ground heat flux was computed using the vertical profile method described in Monteith
and Unsworth (1990).
where k is the soil thermal conductivity (W m–1 K–1), ǻT is the temperature profile in the
soil and ǻz is the depth of this profile (m).
Latent and sensible heat fluxes were computed using the Bowen ratio method
(Brutsaert, 1982; Monteith and Unsworth, 1990):
where the Bowen ratio (
, unitless) is
where Ȗ is psychrometric constant (kPa K−1), Ta and Tc are the temperature of the air 2 m
above and at the canopy layer (°C), respectively, and ea and ec are the water vapour
pressure (kPa) 2 m above and at the canopy layer, respectively.
2.3 Processing of seasonal data
The data were collected for 183 days during the growing season of 2008. Albedo (ratio of
reflected to incoming short-wave solar radiation) and air temperature at 0.3 m were
evaluated. The data were divided according to the amount of total incoming short-
wave solar irradiance per day into three groups: overcast (0–3000 Wh m−2), cloudy
(3000–6000 Wh m−2) and clear (over 6000 Wh m−2). For a detailed description of data
processing and seasonal data on incoming, reflected radiation, air temperature and
relative air humidity (0.3 m, 2 m) at five different sites, see Huryna and Pokorný (2010).
2.4 Application of satellite data
Satellite data were used to evaluate the relationship between land surface temperature and
land cover. Remotely sensed data were obtained by processing Landsat 5 TM scene,
which was acquired on 29 July 2008, 9:38 GMT + 1. The satellite data with resolution of
30 m in VIS, NIR and SWIR spectral bands and 120 m in thermal spectral band were
Solar energy dissipation and temperature control by water and plants 321
rectified with an S-JTSK coordinate system and resampled by applying the nearest
neighbour method to preserve the original radiometric values for subsequent data
processing. The data was radiometrically corrected using the Cos(t) model (Chavez,
1996) with calibration coefficients described by Chander et al. (2009). Land surface
temperature was computed using the THERMAL Idrisi 15 Andes module (Clark Labs,
2006). To get the real surface temperature, the surface emissivity, (İ), was computed
from NDVI using the NDVI Threshold Method (NDVITHM) described by Sobrino (2004):
for NDVI < 0.2 İ was obtained from red Landsat 5 TM band, for NDVI > 0.5 İ was set
0.99 and for rest of the NDVI scale (0.2 NDVI 0.5)İ was computed as follows:
where NDVImin = 0.2 and NDVImax = 0.5.
2.5 Land surface temperatures by thermovision
A thermographic IR FPA camera ThermaCAMTM PM695 (Flir System Sweden) installed
on the aircraft Cessna TU 206F was used for temperature mapping of landscape. The
aircraft operated by the ArgusGeo s.r.o. company is equipped with gyro-stabilisation
and other instruments allowing precise navigation. Pixel size of thermo-images is 2 m.
The thermal pictures were done at about 1 pm on 29 July 2008. For more details,
see Jirka et al. (2009).
2.6 Conversion of solar energy via transpiration
The transpiration of forest stands was estimated on the basis of sap flow measurements in
tree stems, allowing continuous records over long periods under any terrain and
environmental conditions. The Trunk Heat Balance (THB) method was applied for oak
and the Heat Field Deformation (HFD) method for pine in studies of tree sap flow rates.
Stand transpiration was calculated by upscaling the flow data measured in a series of
well-selected trees over a range of tree size, using the method of quantiles of total
(ýermák et al., 2004).
The THB method is characterised by direct electric heating and internal sensing
of temperature and sensors integrating the radial sap flow profile by technical averaging
within wide stem sections, applying two to four such sensors per sample tree (ýermák
et al., 2004; Tatarinov et al., 2005). The current is automatically distributed in the xylem
along the electrodes, reaching the heartwood edge even when the electrodes in the
sapwood do not reach it. The THB sensors integrate the radial sap flow profile by
technical averaging within the wide stem sections.
The HFD method is based on measurement of the deformation of the heat field
around a needle-like linear heater inserted into the stem in a radial direction (Nadezhdina
et al., 2006). The HFD method has unique capabilities for measuring the real vector of
sap flow rate zero flow as well as reversed flow. A series of multi-point sensors were
322 J. Pokorný et al.
used, allowing measurements of radial patterns of sap flow (ýermák and Nadezhdina,
1998; Nadezhdina et al., 2007), so altogether 48 points were used for characterising the
conductive system of each sample tree.
3.1 Solar energy and temperature in TĜeboĖ ecosystems
The average daily courses of incoming solar radiation were similar at all sites in TĜeboĖ
(Figure 2). The average maximum solar radiation fluxes reached about 250 W m–2,
600 W m–2 and 890 W m–2 on overcast, cloudy and clear days, respectively. The
incoming solar radiation on the Wet Meadow was slightly lower than that at other sites
on clear days, perhaps due to higher fog formation in the terrain depression of this
habitat. The mean daily fluxes ranged from 80.4 W m–2 to 83.5 W m–2, from 191.3 W m–2
to 293.9 W m–2, and from 286.3 W m–2 to 293.9 W m–2 on overcast, cloudy and clear
days, respectively. The mean daily values of incoming solar energy at five sites on clear
days ranged from 7.06 kWh m–2 in the field to 6.87 kWh m−2 in the Wet Meadow. The
amount of reflected short-wave radiation differed markedly between the sites (Figure 3).
The reflection of the solar radiation was always lowest at the open water surface, not
exceeding 50 W m–2, even on clear days. On clear days, reflection as high as 220 W m–2
was measured on the concrete surface. On clear days, the highest mean flux of reflected
solar radiation (76.93 W m–2) was at the concrete surface. The fish pond showed the
lowest value of mean reflected solar radiation (24.66 W m–2). The mean fluxes of
reflected solar energy at the remaining sites were close to each other (about 60 W m–2).
The highest daily average of reflected solar radiation on clear days was 1.85 kWh m–2 at
the concrete surface and the lowest average value of the daily-reflected solar radiation
was 0.59 kWh m−2 at the fish pond.
Figure 2 The average daily series of the incoming short-wave solar radiation on overcast,
cloudy and clear days on five different sites in vegetation season 2008
Solar energy dissipation and temperature control by water and plants 323
Figure 3 Average daily series of reflected short-wave radiation on overcast, cloudy and clear
days on five different sites in vegetation season 2008
The daily mean time courses of albedo – defined as the ratio of reflected to incoming
solar radiation – are shown in Figure 4. The average albedo of the water surface was
always the lowest of evaluated sites (about 10%). The albedo of the concrete surface was
about 25% on both cloudy and clear days. On overcast days, it was about 20% at this site.
Albedo for other sites was about 20% under all irradiance conditions. The high values of
albedo in the early morning and late afternoon hours were caused by dividing very small
numbers of both variables of the ratio, i.e., incoming and reflected solar radiation,
therefore do not provide much useful information.
Figure 4 Average daily series of albedo on overcast, cloudy and clear days on five different sites
in vegetation season 2008
324 J. Pokorný et al.
The daily mean courses of the air temperature above ground (30 cm) for overcast,
cloudy and clear days are plotted in Figure 5. On clear days at 30 cm, the lowest average
midday temperature was at the fish pond; the other sites had similar midday
temperatures. On overcast days, the mean temperature was similar at all sites during the
whole day. Low early morning temperature in the Wet Meadow can be explained by the
terrain depression of the flood area of Rožmberk fish pond. The early morning
temperature was on average highest in the fish pond due to the high heat capacity of its
water. On overcast days, the difference of mean temperature between the sites ranged
from 10.2°C (Wet Meadow) to 11.8°C (fish pond). On cloudy and clear days, the lowest
mean temperature was measured at the Wet Meadow (14.08 and 15.58°C, respectively).
On clear days, the average difference of temperature between the Wet Meadow and the
concrete surface was 3.58°C.
Figure 5 Average daily series of air temperature above ground (30 cm) for overcast, cloudy
and clear days on five different sites in vegetation season 2008
3.2 Energy fluxes at four sites on a sunny day
The daily series of incoming short-wave radiation measured on a sunny day of 29 July
2008 on four sites are plotted in Figure 6(A). The maximum values of incoming radiation
are over 800 W m–2. Maximum values on Meadow and Wet Meadow were lower due to
local small clouds. Reflected radiation is plotted in Figure 6(B), maximum values range
from 150 W m–2 (Wet Meadow) to 250 W m–2 (concrete). The highest albedo was
recorded on concrete, the lowest albedo (less than 20%) was measured on the field and
Wet Meadow (Figure 7). Net radiation corrected for IR fluxes is plotted in Figure 8(A).
The highest maximum of net radiation was shown by the field and the lowest maximum
was estimated for the Meadow. Values of ground heat fluxes differ markedly
(Figure 8(B)): the highest ground heat fluxes were shown by concrete and field soil;
estimates for the Meadow and the Wet Meadow were lower. Sensible and latent heat
fluxes plotted in Figure 8(C) and (D) reach values several hundred W m–2. The highest
sensible heat was shown by the concrete and the field. In both Meadows, solar energy
Solar energy dissipation and temperature control by water and plants 325
was converted mostly into ET – sensible heat flux was relatively low whereas latent heat
fluxes reached a maximum between 400 and 500 W m–2, which corresponds to ET rate
up to 0.2 g m−2 s−1.
Figure 6 Daily series of incoming short-wave radiation: (A) and outgoing (reflected) short-wave
radiation and (B) measured on a sunny day of 29 July 2008 on four sites
Figure 7 Daily series of albedo of short-wave radiation measured on a sunny day of 29 July 2008
on four sites
326 J. Pokorný et al.
Figure 8 Daily series of energy balance components, total net radiation: (A) ground heat flux;
(B) sensible heat flux; (C) latent heat flux and evapotranspiration intensity and
(D) on a sunny day of 29 July 2008 on four sites
3.3 Satellite images of the TĜeboĖ Landscape
Satellite map of NDVI (Normalised Difference Vegetation Index) highlighting vegetation
cover and water bodies and map of surface temperature of the TĜeboĖ Basin are shown in
Figure 9. The relatively lower temperature of water bodies and forests (under 20°C) is
evident in comparison with higher temperatures of the fields and town of TĜeboĖ (about
25°C). The satellite picture was acquired at 9:38 GMT + 1 as temperature differences
between land covers start to develop.
Solar energy dissipation and temperature control by water and plants 327
Figure 9 Satellite image of the TĜeboĖ Basin region: (A) a map of vegetation cover using
NDVI (Normalised Difference Vegetation Index) and a map of surface temperature
and (B) with highlighted water bodies
3.4 Detailed map of landscape surface temperature
Surface temperature scanned at about 1 pm GMT + 1 by the thermovision camera around
the town TĜeboĖ ranged from 18°C to 42°C, i.e., a 24°C variation (Figure 10). The lowest
temperature of around 20°C was shown by water bodies, woods (town park with adult
trees) and a large complex of Wet Meadow. The highest temperatures were shown by
drained sealed surfaces of the town and mowed Meadow. Drained and harvested fields
also showed a very high surface temperature (36–38°C) whereas measures located near
water bodies and adult trees gave lower temperatures (about 22°C (Figure 11)).
Figure 10 Surface temperature scanned at c. 1 pm GMT + 1 by the thermovision camera around
the town of TĜeboĖ on 29 July 2008
328 J. Pokorný et al.
Figure 11 Surface temperature of drained and harvested fields scanned by thermovision camera
at c. 1 pm GMT + 1 on 29 July 2008
3.5 Conversion of solar energy in forested sites
Direct measurements of dissipated solar energy via transpiration of trees from flood-plain
forest and from relatively dry pine forest show marked differences in the amount of
transpired water. Flood-plain oaks (Quercus) in the growing season transpired about
910 MJ m−2 (250 kWh m−2), which corresponds to c. 365 l m−2. Pines in plantation
converted via transpiration about 325 MJ m−2 (90 kWh m−2), which corresponds to c.
130 l m−2 (Figure 13). Quercus trees in the flood plain could not transpire much more
because of high humidity of the air. The large complex of flood-plain forest in Moravia
maintains a humid local climate and therefore PET is about only 40% of incoming
radiation. Pine trees are adapted to dry conditions, their needles have several times lower
mesophyll cell wall surface than broadleaf species (Nobel, 1991) and the crowns of pines
are sparser than that of oaks (ýermák and Prax, 2001; ýermák, 1998; ýermák et al.,
Values for the transpiration of individual oak trees in a flood-plain forest are
expressed as MJ m−2 per month as shown in Figure 12(A) together with monthly values
of potential ET and the amount of incoming solar energy. The amount of solar energy
converted into the latent heat of vaporisation of water is about 18% in May and about
30% in June, July, August and September. A proportional relationship between incoming
solar radiation and transpiration was evident – trees well supplied with water increased
their conversion of radiation according to the amount of incident solar energy and air
Solar energy dissipation and temperature control by water and plants 329
humidity (PET). The total amount of solar energy converted during the growing season
by means of the latent heat of vaporisation of water (measured as rate of transpiration) is
about 910 MJ m−2 (250 kWh m−2), which corresponds to about 365 l m−2 (Figure 13).
Figure 12 Transpiration of oak (Quercus) trees (A) and pines (B) potential evapotranspiration
of flood-plain forest (A) and pine plantation (B) and incoming solar radiation to
flood-plain forest (A) and pine forest (B) expressed as MJ m−2per month
Figure 13 Amount of solar energy converted via transpiration of pine plantation (Belgium) and
oak (Quercus) flood-plain forest (Moravia), potential evapotranspiration and income
of solar energy to the both sites (MJ m–2)
The sap flow method for assessment of transpiration measures the flow of water in a
tree trunk. It does not measure the so-called micro-watercycle – evaporation of water
from leaves and consequent condensation of water vapour in the night. This water can
also be considered as intercepted. However, recent studies show that a certain amount of
water can be absorbed by forest canopies and transported via stems, measured as a
reverse flow down to roots. This amount of water is much smaller than transpiration,
but it is very important under severe drought, and even after short rains, because it
allows roots to survive and absorb water as soon as soil water supply is renewed
(Nadezhdina et al., 2006).
330 J. Pokorný et al.
The transpiration values of individual pine trees in the plantation were expressed as
MJ m–2 per month as shown in Figure 12(B). The amount of solar energy converted
in latent heat of vaporisation of water (measured as transpiration) was about 23%.
A relatively higher difference between transpiration and PET indicates either a shortage
of water or low ability of the pine to transpire water. The amount of water transpired by
Pinus during the growing season is equal to 130 l m−2 corresponding to the latent heat of
vaporisation of water about 325 MJ m−2 (90 kWh m−2), see Figure 13.
Transpiration as a fraction of the potential ET for pine and oak is shown in Figure 14.
In summer months, there was an evident difference between Pinus and Quercus – the
transpiration from trees in the flood-plain forest is about 90% of PET, i.e., Quercus trees
are usually not limited by shortage of water there. In the pine plantation, the transpiration
of pine trees did not exceed 55% of PET. Potential ET of Quercus and Pinus as a fraction
of incoming solar radiation are plotted in Figure 15. A remarkably similar PET fraction
for Quercus from May to September (about 45%) indicates that flood-plain forest
maintains or controls the humidity of the air by transpiration from trees.
Figure 14 Transpiration of pine and oak (Quercus) as a fraction of potential evapotranspiration
for individual months of vegetation season (April–October)
Figure 15 Potential evapotranspiration as a fraction of seasonal income of solar radiation
for the flood-plain forest and pine plantation
Solar energy dissipation and temperature control by water and plants 331
The surface temperature of a landscape results from the interplay of albedo and
distribution of net radiation into the latent heat flux, sensible heat flux, ground heat flux
and energy storage in the ecosystem. Surface temperatures on different land covers were
found to be similar on overcast days when most of the solar radiation is absorbed
and reflected by clouds and there is less than 3 kWh incident on square metre of land
surface per day. On cloudy days when incoming radiation ranged between 3 and
6 kWh m–2 day–1, the daily series of temperatures differed at early morning and midday:
the lowest daily temperature amplitude was shown by fish-pond water. Differences
among sites in midday maxima reached 4°C. On clear days when incident solar radiation
exceeded 6 kWh m–2 day–1, mean temperature differences among sites reached c. 7°C at
both early morning and midday. The lowest daily amplitude, highest early in the morning
and lowest at midday values, was shown at the fish pond. The high midday temperature
of the Meadow can be caused by the mulch, which insulates soil and reduces ground heat
flux. The average temperature calculated from data measured for the whole growing
season was high at the concrete surface both at 30 cm (16.09°C) and 2 m (15.73°C).
The concrete surface was the site of highest albedo (25%), but it was not albedo that
decreased temperature. Low temperature was measured at the Wet Meadow and other
vegetated sites. The fish pond, which was the site of lowest albedo c. 10% (see Figures 3
and 4), had the lowest temperature reading 0.3 m above water level (Figure 5). Dense
concrete has a relatively high thermal conductivity (1.5 W m–1 K
–1) and high heat
capacity: 2.11 J m−3 K−1× 106 (Oke, 1987). The heat capacity of water is twice as high as
dense concrete (4.18 J m−3 K−1× 106). Despite a high albedo, the concrete surface had
highest temperature maxima and highest daily temperature amplitude. The latent heat of
vaporisation of water is three orders of magnitude higher than the heat capacity of dense
concrete (2.5 J m−3 K−1× 109). Thus, surfaces are cooled during vaporisation and warmed
during condensation of water. Water bodies and forests have the lowest surface
temperature in a landscape on a sunny day (both thermovision and satellite thermal
pictures confirm this (Figures 9–11)).
Some scientific papers argue that the low albedo of forests contributes to global
warming (Gibbard et al., 2005). For example, Randerson et al. (2006) conclude that
“future increases in boreal fire may not accelerate climate warming”. Bala et al. (2007)
“global-scale deforestation has a net cooling influence on climate, because the
warming carbon-cycle effects of deforestation are overwhelmed by the net
cooling associated with changes of albedo and ET.”
However, the cooling effect of forests on hot sunny days is confirmed by Figures 9–11.
Likewise, thermal satellite pictures from Hesslerová and Pokorný (2010a, 2010b) show
the low surface temperature of forested areas. The data strongly suggest that it is not
albedo that controls the damping of temperature with high incoming solar radiation. It is
the interaction of water and plants that dampens temperature maxima.
The detailed study of solar energy distribution on a summer sunny day in the four
TĜeboĖ sites has shown that the most vigorous fluxes are linked with sensible heat or
with the latent heat of ET. In dry places, the sensible heat flux reached
values over 300 W m–2 whereas vegetation well supplied with water will convert
over 400 W m–2 into cooling ET. The daily curve of latent heat or ET shows a feedback
332 J. Pokorný et al.
coupling between incoming solar radiation and the rate of ET – ET increases with
incoming solar energy reaching its maximum at about midday. These effects both concur
with everyday experience, and are described quantitatively by Budyko (1974), Monteith
and Unsworth (1990) and Bonan (2008) among others. However, findings on the
principal role of water and plants in the distribution of solar energy and temperature
control of ecosystems are not emphasised in policy recommendations issued by the
Intergovernmental Panel on Climate Change (IPCC, 2007).
In the present investigation, temperature maps of the landscape taken both by satellite
images and by airborne thermo-camera clearly evidence temperature differences between
land covers. On a sunny day, the highest temperatures were in sealed urban areas and on
harvested drained fields. At about midday, the difference between vegetated areas and
sealed surfaces reached 14°C over an area of several square kilometre in the flat TĜeboĖ
Basin region. The satellite image taken, the same day at 9:30, showed smaller
temperature differences with forests, wet Meadow and water bodies being the coolest
sites. Satellite pictures taken in Northwest Bohemia at the same time showed temperature
differences of almost 20°C between large drained areas such as open cast mines and
fields vs. forests (Hesslerová and Pokorný, 2010a). Likewise, the deforestation of large
areas in tropical regions may result in a temperature increase of about 20°C (Hesslerová
and Pokorný, 2010b). The consequent turbulent flow of hot air of low relative humidity
affects the surrounding ecosystems and may contribute to the melting of mountain
Differential heating caused by sensible heat gradients across adjacent regions of
intensively transpiring vegetation and dry, bare, soil can generate a sea breeze-like
circulation, called ‘vegetation breeze’ (Eltahir and Bras, 1996; Pielke, 2001, 2005;
McPherson, 2007). Lawton et al. (2001) have shown that land-use change in tropical
lowlands has serious impacts on ecosystems in adjacent mountains. Landsat and
Geostationary Operational Environmental Satellite imagery shows deforested parts of
Costa Rica’s Caribbean lowlands remain relatively cloud-free, whereas forested regions
develop dry season cumulus cloud fields. These conditions facilitate maintenance of the
short water circuit essential to re-coupling the water and carbon cycles (Ripl, 2003).
Land drainage for agriculture or urbanisation usually means a loss of vegetation,
resulting in a shift from the self-regulating dissipative structures described earlier, to
negative consequences such as temperature swings leading to turbulent motion in warm
dry air. In relation to global warming, it is recognised that while humans induce CO2
emissions by land clearing and burning fossil fuels, ecosystems bind CO2 in the biomass
of plants and soil. What is less often realised is the fact that the annual increase in
humanly induced carbon in the atmosphere is an amount equivalent to only 0.6% of the
carbon contained in vegetation and 0.2% of the carbon contained in soils. Studies by
Beran (1994) and IPCC (2007) put the annual increment of carbon in the atmosphere
from CO2 emissions at 3.5 GT. In soil, there is c. 2000 GT of naturally occurring carbon;
in vegetation 610 GT and in the atmosphere 750 GT of carbon. These various sources
exchange carbon in a functional relation to each other, a dynamic that is uncoupled when
local water cycles are damaged. This lost functioning is observed on a global scale in the
Millennium Environmental Assessment (2005), which notes that every year, some
60,000 km2 of badly managed land is becoming desert. About 200,000 km2 of land loses
agricultural productivity as people in development projects or farmers themselves cut
down plants and drain soils. The drying out and loss of ecosystem function now affects
30–40% of the global landmass.
Solar energy dissipation and temperature control by water and plants 333
When carbon is fixed in biomass via photosynthesis, this consumes a relatively
small amount of solar energy. However, photosynthesis is accompanied by transpiration
of water, which transfers solar energy at two orders of magnitude higher than
photosynthesis. The restoration of water and plants to arid areas can sequester substantial
amounts of emitted carbon, as well as simultaneously improve land productivity and
temperature extremes. The largest data collection on the regenerative capacity of global
ecosystems is the International Biological Programme (Cooper, 1975), which estimates
that plants produce annually 1 kg dry biomass per m2 in temperate zones and up to
5 kg m2 in the tropics. Now, 1 kg of dry biomass contains 0.4 kg of carbon. Thus, to
sequester an annual increment of 3.5 GT of carbon (IPCC, 2007), each m2 of land would
need to bind an additional amount of 0.02 kg of carbon. This could be achieved if the
40% of global landmass that currently lacks water and vegetation capacity were restored,
because plant binding of an additional 58 metric tons of carbon per km2 year
(0.058 kg m−2) can compensate for 3.5 GT. Given wide public concern about CO2 levels,
it makes sense to enhance ecological functioning to its maximum capacity.
The control of human impacts on landscapes is desirable for a number of reasons and
this study clarifies one of these – the self-regulatory damping of solar radiation in a
healthy ecosystem. Plant stands supplied with water are able to respond to incoming solar
energy to an order of magnitude of hundreds W m–2. This dissipative structure receives
energy pulses from outside and reacts with a positive feedback loop, which is unlike the
classic amplifying feedback regarded as destructive in cybernetics. The responses of an
ecosystem to incoming solar energy function as a source of new order and complexity
(Capra, 1996). Currently, the IPCC (2007) does not engage adequately with the complex
interplay of water, sunlight and vegetation. It tends to focus on correlations between
artificially generated average Earth temperatures, on the one hand, and concentrations of
specific greenhouse gases, on the other. Levels of atmospheric CO2 increased during the
past 250 years from c. 280 ppm to 360 ppm. The IPCC documents the radiative
forcing caused by an increase in greenhouse gases in the atmosphere from 1750 is equal
to 1–3 W m–2. In the next 10 years, the radiative forcing is expected to increase
by 0.2 W m–2. Comparing these values with the amount of water in the atmosphere, the
ability of ecosystems to distribute solar energy, and the value and time variability
of the solar constant (1345–1438 W m–2 during one year), it is difficult to understand
why the IPCC does not give more attention to the climatic effects of ecosystem
Functioning ecosystems cool themselves; they use solar energy for self-organisation and
export entropy to the atmosphere as heat. These energy transformations are achieved by
water and plants in the activity of ET. In this study, the dissipative process is
demonstrated by data collected at seven sites in the Czech Republic and Belgium, with
analysis of landscape temperatures assisted by thermovision and satellite images.
Implications of the research for understanding climate change are discussed.
334 J. Pokorný et al.
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