Meteorology & Soil Physics details
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Meteorology & Soil Physics. Key words: energy balance, evapotranspiration, artificial
leaf. Measurement of soil-heat flux, soil temperature profiles, soil thermal
conductivity, leaf boundary layer conductance, soil heat capacity and sensor
calibration.
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In agricultural area's the use of water is an issue of major
importance. Often water is scarce, and one would like to use as little as possible, at the
same time minimizing the use of nutrients and fertilizers. In actively irrigated area's
there is the possibility to estimate the required amount of water and limiting the flow of
nutrients to the root zone of the plants, using information on the weather and on the
plants.
The study of meteorological phenomena for this purpose is called the
study of evapotranspiration. (evaporation from the soil, and transpiration from the
plants). Roughly speaking the process of evaporation follows the same rules as drying your
laundry; the higher the solar radiation and the wind speed, the quicker the process of
drying. In case of plants, they will require more water.
The study of evapotranspiration in fact is a study of energy fluxes
(one through heat transport, the other through vapor transport). These energy fluxes are
also interesting from a climatological point of view. In this case, the research is often
indicated with the term "energy balances".
The general principle behind the measurements is that the energy
source is radiation, either solar radiation coming in, or far infra red radiation going
out. This source can be used for three purposes; heating air (generating a thermal
gradient in the air) evaporating water (creating a humidity gradient in the air) or
heating the soil (creating a soil heat flux). Usually the first two purposes are what it
is really about, and the third, soil heat flux is measured to close the equation.
Typical station for the measurement of energy balances
and evapotranspiration. Parameters of primary importance are temperature and humidity at 2
levels and radiation (3). Usually 2 heat flux sensors are installed to measure soil heat
flux (1).
These can be complemented by a soil temperature profile and a soil thermal conductivity
measurement (2).
The soil heat flux is part of these studies. It can be measured
using a heat flux sensor. This sensor goes under different names, also it is called soil
heat flux plate, heat flux meter, etc.
For good scientific grade calculations, the soil heat flux is an
important parameter, offering the possibility to check the complete balance. The problem
with this measurement is that the calibration factor of most heat flux sensors strongly
depends on the kind of soil, its thermal conductivity and on the temperature of the
sensor. Also, once buried, the condition of the sensor is no longer known.
Hukseflux has developed two sensors for this application; the HFP01 for normal routine measurement application, and the
HFP01SC, for scientific application. See also the
section on heat flux.
The HFP01SC (patent pending) offers superior accuracy, improved
quality assurance and some redundant information on soil humidity. To achieve this, it
uses an on-line self-calibration, according to the Van den Bos-Hoeksema method.
Using Soil Temperature Profile Sensors for
measurement of the energy balance
Heat flux sensors offer a local view of the heat flux at the level
at which they are installed. Installation is typically at a depth of 5 to 10 cm. They are
not capable of measuring the changes of heat storage in the soil layers above and below
the point of installation. This is why in advanced scientific studies the measurement of
heat flux is often complemented with a measurement of the soil temperature
profile and volumetric heat capacity.
Typical depths of the temperature sensors are 2, 5, 10, 25 and 50 cm. For this purpose the
STP01 was designed. For the soil
heat capacity TP01 was designed.
In the design of STP-01 the same philosophy as the HFP-01 SC was
taken; that it is necessary to have some idea of how the sensor is performing. For this
reason a heating wire is incorporated. By switching it on one can see if the temperature
sensors react, and judge the condition of the sensor. Also (this is experimental)
one can get a rough idea of the thermal conductivity of the soil at the first 4 levels!
How this is done can be seen in the chapter on thermal conductivity.
Using Thermal Properties Sensor TP01 in
meteorological applications
To get the full picture of the energy balance of the soil, it is
useful to perform a high accuracy measurement of the soil volumetric heat
capacity and thermal conductivity. The new TP01 is especially designed for
this. This
measurement again introduces the possibility to increase the accuracy of the estimates of
the energy flows through the soil, and also it offers excellent complementary data for
soil moisture content measurements. Examples: knowing the temperature profile and the
thermal conductivity, one has at a fairly accurate estimate of the local heat flux. Second
example: when performing a measurement at one location with a soil moisture probe and a
thermal conductivity sensor, there should be a definite correlation between the measured
values, only depending on soil type and local soil moisture content.
More theory on the soil heat flux
measurement
In meteorological measurements the heat flux at the
surface is usually measured using a heat flux plate. This plate gives an output
that is directly proportional to the heat flux through it.
For various practical and theoretical reasons, the heat
flux plate cannot be installed directly at the surface. The main reason is that
it would distort the flow of moisture, and be no longer representative of the
surrounding soil, both from a moisture and from a thermal/spectral point of
view. Also in case of installation close to the surface, the sensor would be
more vulnerable and the stability of the installation becomes an uncertain
factor.
For these reasons the flux at the soil surface, F , is
estimated from the flux measured by the heat flux sensor, F heatflux
, plus the energy that is stored in the layer above it, S.
F = F heatflux + S
The parameter S is called the storage term.
The storage term is calculated using an averaged soil
temperature measurement combined with an estimate of the volumetric heat
capacity (of the volume above the sensor.
S = (T1-T2). Cv.d / (t1-t2)
Where S is the storage term, T1-T2
is the temperature change in the measurement interval, Cv the
volumetric heat capacity, d the depth of installation of the soil heat flux
sensors, t1-t2 the length of the measurement interval.
At an installation depth of 6 cm, the storage term
typically represents up to 50% of the total flux F . When the temperature is
measured closely below the surface, the response time of the storage term
measurement to a changing F is in the order of magnitude of 20 minutes, while
the heat flux sensor F heatflux (buried at twice the depth) is a
factor 4 slower (square of the depth). This implies that a correct measurement
of the storage term is essential to a correct measurement of F with a high time
resolution.
At present the volumetric heat capacity, Cv, is
estimated from the heat capacity of dry soil, Cd, the bulk density of
the dry soil r d, the water content (on mass basis), q m ,
and Cw, the heat capacity of water.
Cv = r d (C d + q m
Cw )
The heat capacity of water is known, but the other
parameters of the equation are much more difficult to determine, and are
dependent on location and time.
For determining bulk density and heat capacity one has to
take local samples and to perform careful analysis. The soil moisture content
measurement is difficult and suffers from various errors.
TP01 gets around these problems by performing a direct
measurement. This is a big advantage as such and sufficient reason for
application in Bowen Ratio systems. Additionally the TP01 measurement is quite
useful to create some redundancy for the soil moisture measurement that is often
done in such systems.
From the previous formula it can easily be seen that there
is a direct relationship between the soil moisture and the volumetric heat
capacity.
q m = (Cv / r d - C d
) / Cw
The latter formula gives water content on a mass basis.
For estimates on a volume basis, one has to multiply by r b
and divide by r w:
q v = (Cv - C d r d
) / r w Cw
As the properties of water are quite well known, an error
in q will stem from errors in Cv and in r d .
In studies of evaporation it is often very useful to have detailed
information on the "leaf boundary layer conductance", or in more
general terms, the transfer coefficient. This is an indicator for the ease with
which a leaf can exchange gasses with the atmosphere. WS01 can be used to study
the heat transfer coefficient. The heat transfer coefficient, which correlates
well with gas transfer coefficients. Because of the fact that the heat
transfer coefficient predominantly depends on the wind speed WS01 can also be
used as a wind speed sensor for ultra low wind speeds, for instance in
greenhouses. More information
about this application.
The most common application of TP02
Non-Steady-State Probe is for the analysis of soil
thermal conductivity.
This information is useful for
engineering purposes: calculation of the energy
balance of underground cables and pipelines,
calculation of the surface energy budget for
meteorology, analysis of the penetration of frost and
analysis of the thermal behavior of buildings.
The model TP02 is the method
that is recommended by ASTM for this purpose. The
standard is called ASTM D 5334.
The needle shaped TP02 is
primarily designed for analysis of sample material in
the laboratory, but can also be used in the field if
this is necessary. The TP02 is not suitable for long
term installation on one spot. For this purpose there
is a special model; TP01.
More information on TP02 and its
working principle can be obtained at Hukseflux: the
TP02 manual contains all the necessary background
information.
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