ENERGETICKÁ BILANCE POROSTU KUKUŘICE ZA JASNÉHO A
ZATAŽENÉHO DNE
THE ENERGY BALANCE FOR A MAIZE CANOPY DURING CLEAR AND
CLOUDY DAY
Chalupníková Blanka – Rožnovský Jaroslav – Hurtalová Tatiana – Matejka František
Mendel University of Agriculture and Forestry in Brno, Změdělská 1, 613 00 Brno, ČR
Geophysical Institute of Slovak Academy of Sciences, Dúbravská cesta 9, 842 28 Bratislava
Abstract
Příspěvek představuje výsledky stanovení energetické bilance porostu kukuřice (Zea maize L.)
pro vybraný den jasný (21. srpna 2000) a zatažený (31. srpna 2000) ve vegetační sezóně 2000.
Potřebné experimentální podklady byly získány na základě mikroklimatologických měření, které
probíhaly v roce 2000 v porostu kukuřice na pokusných plochách Mendelovy zemědělské a lesnické
university v Brně. Radiační bilance byla stanovena na základě statisticky významného, lineárněregresního vztahu s globálním zářením. Tok tepla do půdy byl měřen. Pro určení latentního a
turbulentního toku tepla z měřených meteorologických prvků, jako je teplota a vlhkost vzduchu,
rychlost větru atd., byl použit SVAT model.
Key words: energy balance; maize canopy;
Introduction
Land surface is coupled with the atmosphere through the exchange of mass and energy. In our
case we investigate continuous exchange of mass and energy between a maize canopy and atmosphere.
Within the process of this exchange, turbulent fluxes transfer considerable amounts of heat and water
vapour into the lowest layers of atmosphere which results in changes in heat and moisture conditions
not only in the maize canopy but also in the air above (Rožnovský et al., 2000). The ratio in which the
radiation balance of the stand is divided into its individual parts, i.e. turbulent flux of heat, flux of
latent heat and flux of heat into soil, determines the structure of energy balance of a canopy (Matejka
et al., 1999). Regarding the fact that the active surface represents a single source of water vapour and a
dominant source of heat for the atmosphere, it can be said that from the formation point of view and
canopy microclimate dynamics, the structure of the equation of energy balance is very important. It
also determines to a great extent both the temperature of canopy and the amount of energy available
for hydrological processes.
The energy budget of the active surface can be described by the energy balance equation in a
steady state form by (Brutsaert, 1982).
Rn = λE + H + G
There are essentially four types of energy fluxes, which are described by this equation. Namely,
the net radiation (Rn) going into or from the surface, the sensible or turbulent heat flux (H), latent heat
flux (λE) into or from the atmosphere, and the heat flux going into or out of ground (G). The net
radiation flux is a result of the radiation balance at the surface. During the day, the net radiation is
usually dominated by the solar radiation and is always directed toward the surface. On the other side at
night the net radiation is much less weaker and directed away from the surface. As a result, the surface
warms up during the daytime, while it cools during the evening and night hours, especially under clear
sky and undisturbed weather conditions.
The soil heat exchange depends primarily on the soil heat conduction. Thermal properties
relevant to the transfer of heat into a soil and it effects the mean temperature or influence of
temperatures in the soil density, specific heat, heat capacity and thermal conductivity of the soil (Arya,
1988).
The sensible heat flux at the surface and above arises as a result of the difference in the
temperatures of the surface. Actually, the temperature in the atmospheric surface layer varies
continuously with height of the canopy. The magnitude of the vertical temperature gradient usually
decreasing with height. The latent heat or water vapour flux is a result of evaporation and
transpiration, or water condensation at the surface and it is given as the product of the latent heat of
evaporation or condensation process. Evaporation emerge from water surfaces as well as moist soil
and vegetative surface, whenever the air above is drier than a medium. This is fact usually during the
daytime. On the other hand, condensation in the form of dew may appear on relatively colder surfaces
at nighttime.
The latent and the sensible (turbulent) heat fluxes are the most important components of the
energy balance equation of the active surface such as vegetation (Hurtalova, 1997). These components
exhibit a strong variation during the daily course, mostly in response to the daily variation of radiative
energy input at the surface. This fact is more expressive above the surface formed by plant canopies.
Methods
Data were obtained at the fields of the experimental station Žabčice, which belongs to the
Mendel University of Agriculture and Forestry in Brno. The Žabčice is located in the southern part of
the Moravia region of the Czech Republic (altitude 49° 01´ N, longitude 16° 37´ E and 179 m above
the see level). The soil type is described as Oxyaquic Cryofluvents along the USDA (U.S. Department
of Agriculture, 1975) Classification. The measurements of microclimatological, pedological and plant
parameters were carried out.
Measurements were performed by the automatic measuring and registration system Campbell
Scientific, type CR10 and CR10x. The micrometeorological station and all related sensors were
located in the middle of the maize plot. The Kipp&Zonen pyranometer was situated at a tower at the
height of 12 m above the soil surface. The heat flux sensor was put under the soil surface in maize, in
the middle between the plant rows. The gradient method was used to achieve the related
micrometeorological vertical profiles. The anemometers (A100L) and thermo-hydrometers (HMP45C)
for gradient measurements of the wind speed (V), the air temperature (T) and relative humidity (Rh)
were fixed at a metal construction at height levels of 0.5 m, 1.0 m and 2.0 m above the zero plane
displacement level (d) (Valentová and Rožnovský, 1999). The zero plane displacement is the mean
height of the active surface (h), where the radiation is reflected and transformed. It is a very important
parameter for vegetated surfaces, d is assumed as zero for surfaces with low vegetation. With the
growth of vegetation the zero plane displacement increases as well. The best fit presented in literature
is d = 0.68h with extremes of 0.53 and 0.83 (Brutsaert, 1982). Hence, d=2/3h appears to be fairly
representative for natural crop covered surfaces. The reference level of the gradient measurement was
set 2.0 m above the zero plane displacement. The sensors were lifted once a week according to the
height of the canopy. Fifteen-minute averages of measured data were used for calculations of
aerodynamic characteristics and energy balance fluxes of the maize canopy. Soil water content profiles
(w) in the root zone of maize stand were estimated gravimetrically during season 2000. Equally the
leaf area index (LAI) were estimated at the beginning and as well at the end of the period of
micrometeorological measurements.
For quantification and mathematical description of interrelationships among soil – maize canopy
– atmosphere we used the SVAT model as published by Matejka (1997). The latent heat flux and the
sensible heat flux were calculated by the SVAT model. The net radiation was determined based on its
dependence on the global radiation McCaughey (1978), Nkemdirim (1973) etc. The ground heat flux
was measured by soil heat flux plate. Based on these determined heat fluxes of the maize stand we
quantified the related energy balance.
The micrometeorological measurements in the stand of maize were carried out during the
growing season from the 3rd of May till the middle of October 2000. One clear and cloudy day were
chosen from the period in 2000. Specifically August 21 2000 as clear day and August 31 2000 as
cloudy day.
Results and discussions
Two days were selected as representative days to demonstrate the variation of energy balance
under the given conditions in 2000. For presentation was chosen 21st August 2000 as clear day and 31st
August 2000 as cloudy day. Fig. 1 and Fig. 2 presents these days. Figures are accompanied by Tab. 1
and 2 with basic statistics.
600
Energy fluxes (W m-2 )
500
h = 2.25 m
z0 = 0.06 m
400
-2
Rn
G
-2
LAI = 2.30 m m
w = 26.1 %vol
300

H
200
100
0
-100
9:
15
10
:1
5
11
:1
5
12
:1
5
13
:1
5
14
:1
5
15
:1
5
16
:1
5
17
:1
5
18
:1
5
19
:1
5
20
:1
5
21
:1
5
22
:1
5
23
:1
5
8:
15
7:
15
6:
15
5:
15
4:
15
3:
15
2:
15
1:
15
0:
15
-200
Fig. 1. Diurnal energy budget of the maize stand at Žabčice, Czech Republic,
on 21st August 2000.
Tab. 1. Daily statistics of energy fluxes on 21st August 2000.
Daily statistics of energy fluxes on 21st August 2000
G
λE
H
Rn
(W m-2)
(W m-2)
(W m-2)
(W m-2)
Max
Min
Average
Daily Sum
Amplitude
464
-22
139
13 364
486
110
-28
20
1 928
138
250
1
84
8 089
249
162
-18
35
3 347
180
The maize stand was 110 days old. Plant mean high h was 2.25 m, leaf area index reached value
of 2.30 m-2 m-2, roughness length z0 was 0.06 m and soil water content in the root zone was assessed as
26.10 %vol. The maize canopy was fully developed and closed. This fact influenced heat fluxes of the
canopy significantly. The net radiation reached a maximum of 464 W m-2 and a minimum of –22 W m2
and the variation of net radiation had the typical shape for clear days. Daily sum of net radiation is 13
364 W m-2. The latent heat flux varied from 1.0 to 250 W m-2 and is the most significant flux after net
radiation (Tab.1.).
Ground heat flux is much more reduced then at the start of growing season, which is typical for
developed and closed canopies. The daily sum of the ground heat flux is 1 928 W m-2. Positive G
values occurred during the daytime, especially from 6:45 am to 6:15 pm and some negative values
could be observed during the night.
An interesting phenomenon of the sensible heat flux variation can be seen in the early morning
st
on 21 August 2000. Values oscillated around zero during that time, even at 1:15, 2:30, 4:15, and 4:45
am zero was reached. This is an interesting indication from the point of view of the energy exchange.
It means that the temperature of the surrounding air was the same as the temperature of the active
surface. No exchange of energy took place between these media. The reason for that behavior could be
cloudy conditions combined with strong wind, keeping the surface temperatures in equilibrium with
the atmosphere.
600
Energy fluxes (W m-2)
500
400
300
Rn
h = 2.45 m
z0 = 0.05 m
G
LAI = 0.90 m-2 m-2
w = 23.3 %vol

H
200
100
0
-100
0:
15
1:
15
2:
15
3:
15
4:
15
5:
15
6:
15
7:
15
8:
15
9:
15
10
:1
5
11
:1
5
12
:1
5
13
:1
5
14
:1
5
15
:1
5
16
:1
5
17
:1
5
18
:1
5
19
:1
5
20
:1
5
21
:1
5
22
:1
5
23
:1
5
-200
Fig. 2. Diurnal energy budget of the maize stand at Žabčice, Czech Republic,
on 31st August 2000.
Tab. 2. Daily statistics of energy fluxes on 31st August 2000.
Daily statistics of energy fluxes on 31st August 2000
Rn
G
λE
H
(W m-2)
(W m-2)
(W m-2)
(W m-2)
Max
Min
Average
Daily Sum
Amplitude
132
-22
9
817
154
20
-22
-1
-107
42
76
0
14
1 299
76
47
-27
-4
-375
74
An example of a cloudy day could be seen in Fig. 2. From the Fig. 2 is evident a small
differences among the energy fluxes in comparison with the clear day. The maize stand is fully ripe
and relatively dry. No distinct fluctuation of the heat fluxes can be observed. The biggest values were
reached by the net radiation with a maximum of 132 W m-2. The highest value of a daily sum is shown
by the latent heat flux (although low compared to the previous period), probably caused by
evaporation mostly. The amplitudes of net radiation, ground heat flux, latent and sensible heat fluxes
are only 154, 42, 76, and 74 W m-2 (Tab. 2.). The net radiation and latent heat flux were mostly
positive, namely during the daytime. Energy balance fluxes oscillated around zero during the night.
They reached positive values during the daytime.
Conclusion
Energy balance components were determined for selected days in 2000. The results were in
accord with generally known facts about courses of energy balance components. The net radiation, as
the most dominant flow during daytime, is the main contributor to the available energy, partitioned
between H and λE. The daily course of Rn is mainly the result of the short-wave incoming radiation.
The daily course of ground heat flux was subject to the some principles as the course of net radiation.
Soil surface was receiving energy during the day and releasing thermal energy during the night, as
typically for the selected periods of the year. The intensity of the soil heat flux was considerably
changed by vegetation or the development stage of the maize crop. When the soil in the root zone
became dry, the canopy resistance increased leading to decreasing of evapotranspiration and
increasing of the sensible heat flux. Partitioning of latent and sensible heat flux of the maize stand was
determined by soil water storage, by evaporative demand of the atmosphere, by leaf area index, and
globally by meteorological conditions.
A strong variability of energy balance components was observed for clear days. Small
differences among the energy fluxes in comparison with clear days occurred at cloudy conditions.
References
Arya, S. P., 1988: Introduction to micrometeorology. Academic Press. San Diego. California. 307p.
Brustaert, W., 1982: Evaporation into the atmosphere. London. D.Reidel Publishing Company. 269p.
Hurtalova, T., 1997: Self-preservation in the daily course of the surface energy balance. Contr.
Geophys. Inst. SAS. Vol.17. 35-43.
Matejka, F., 1997: A Three-layer SVAT model for homogenous land surface. Contr. Geophys. Inst.
SAS. Ser. Meteteorol. Vol.17. 44-53
Matejka, F., Roznovský, J., Hurtalová, T., 1999: Structure of the energy balance equation of a forest
stand from the viewpoint of a potential climatic change. Journal of Forest Science. Vol.45. 385391.
McCaughey, J. H., 1978: Estimation of net radiation for a coniferous forest and the effect of logging
on net radiation and the reflection coefficient. Canadian Journal of Forest Research. Vol. 8. No.
4. 450-455.
Nkemdirim, L. C., 1973: Radiative flux relations over crops. Agric Meteorol. Vol. 11. 229-242.
Rožnovský, J., Valentová (Chalupníková, B.), Hurtalová, T., Matejka, F., 2000: Influence of soil water
content reduction to evapotranspiration of maize stand. Transport of water, chemical, and energy
in the soil – plant – atmosphere system. Ed: J. Majerčál, T. Hurtalová. VIII. Posters day with
international presence. Bratislava.CD-ROM
USDA (U.S. Department of Agriculture) Soil Conservation Service, 1975: Soil taxonomy: A basic
system of soil classification for making and interpreting soil surveys. Agricultural Handbook
No. 436p.
Valentová (Chalupníková), B., Rožnovský, J., 1999: Comparison of air temperature and humidity
courses between potato stand and meteorological station. Agrometeorological prognosis and
models. Czech Bioclimatotlogical Company. Brno. 46-52.
Kontaktná adresa:
Ing. Blanka Chalupníková
Czech Hydrometeorological Institute
Kroftova 43
616 00 Brno, Czech Republic
tel: +42 – 05 – 41321214
e-mail: chalupnikova@chmi.cz