ῐ
J. Jpn. Soc. Soil Phys.
ῑ῏῎ῒΐῐ
No. ++*, p. 01ῌ11 ῍,**2῎Thermal Properties and Shrinkage-Swelling Characteristicof Clay Soil in a Tropical Paddy Field
ARDIANSYAH*, Sho SHIOZAWA* and Kazuhiro NISHIDA** Graduate School of Agricultural and Life Sciences, The University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ20/1, Japan
Abstract
Thermal conductivity (K) as functions of water content (w) is not so well known for swellingsoils as that for non-swelling soils. We measured K and thermal di#usivity of a swelling soil takenfrom paddy ﬁelds in Indonesia, together with its shrinkage-swelling characteristic. The soil
showed remarkable shrinkage-swelling nature ; void ratio of the soil in-situ exceeded ,.* when
ﬂooded condition whereas it became less then +.* when the soil was dry. Observed K increased
from *.0* to +.,2 W mῌ+ Kῌ+ with w increased from zero to *.,. g gῌ+. However, for w greater than*.,. g gῌ+, K decreased gradually with increase in w, di#erently from non-swelling soil that showsmonotonic K increase with w. The decrease in K with increase in w must be produced by increased
void ratio. As a result, the value of K for the soil of most dry condition in the ﬁeld (K῍*.33 W mῌ+Kῌ+) was almost the same as the value of K for ﬂooded condition. Consequently, drying of theswelling soil has no e#ects of reducing heat conduction from surface into the soil when it is
exposed to sunshine.
Key words : thermal conductivity, thermal di#usivity, swelling soil, paddy ﬁeld, shrinkageswelling characteristic+.

Introduction

Thermal properties of soil are necessary to
predict heat ﬂow and temperature changes insoil. The heat ﬂow in soil is proportional to thetemperature gradient in soil, and the proportionality coe$cient is thermal conductivity. Another soil thermal property, thermal
di#usivity, is deﬁned as the ratio of thermal
conductivity to volumetric heat capacity, andis a measure of the rate of transmission oftemperature change into soil, when the surface
temperature changes with time. Soil thermal
conductivity and di#usivity depend on soil
composition, bulk density, particle shape, andespecially water content. A number of measurements have been conducted for thermal
conductivity of non-swelling soils. Generaltendency of them as functions of water content

is well known ; the thermal conductivity of
non-swelling soil monotonically increases withincrease in water content (q). Thermal conductivity of water is about twenty times greaterthan that of air ; if soil air is replaced by waterin the process of increasing soil water content,
thermal conductivity must increase with watercontent (de Vries D.A., +30-). On the other hand,increase in bulk density (by compaction) increases thermal conductivity. Empirical equations for these relations have been also proposed (Campbell, +32/).
In paddy ﬁelds, there often exists surface soil
containing swelling clay minerals. Because of
puddling practice of every time of paddy culture, its original soil structure has been lostand single-grained structure is formed. Thesesoils in paddy ﬁeld remarkably swell by wetting and shrink by drying. The relation of ther-

῏



68

mal conductivity with water content of theswelling soil is signiﬁcantly di#erent from thatof non-swelling soil because water content
change of the swelling soil is accompanied by
the change in bulk density. However, thermal
conductivity of the swelling soil with relation
to water content or bulk density is not wellknown.
We measured thermal conductivity of a swelling soil in paddy ﬁeld in Indonesia in the laboratory. The paddy ﬁeld becomes uncultivatedand dry bare soil with cracks is exposed during
dry season of the tropical climate. Therefore,
in order to estimate evaporation from the
paddy ﬁeld, thermal properties of the paddy
ﬁeld soil, as functions of water content or bulk
density, are necessary to calculate the heatﬂow into or out of the soil that is a componentof heat exchange at the soil surface.
,.

Material and Methods,. + Location of soil sample
Soil sample was taken from a paddy ﬁeld inCidanau watershed, located northwest of Javaisland, Indonesia (between +*/ /,῍Eῐ+*0 *-῍E
and 0 2῍Sῐ0 +1῍S). This area is part of the tropical monsoon region, in which the rainy season
and the dry season are explicitly distinguishable. This watershed receives a great deal ofprecipitation : average annual rainfall is about,,/** mm. Since average daily temperature ishigh and constant all year around (,0ῐ,1),
rice can be cultivated in the paddy ﬁelds at any
time of the year as long as water is available ;up to three times cultivation of rice in a paddy
ﬁeld in a year is feasible if water is available in
the paddy ﬁeld during dry season. The ratio of
cultivated area of the paddy ﬁelds to the total
area of the paddy ﬁelds during dry season andwet season are /.ῌ and 2*ῌ, respectively (Yoshikawa and Shiozawa, ,**0). The uncultivatedpaddy ﬁelds expose dry bare soil with cracks
during dry season.
The soil has 0,.3ῌ clay, ,3.1,ῌ silt, and /.3ῌsand content (Yoshino and Ishioka, ,**/). Remarkable swelling-shrinkage and cracking phe-++*,**2

nomena observed in the paddy ﬁelds are responsible for the swelling property of the clay.
To obtain ﬁeld water content, we selected two
paddy ﬁelds and sampled soils of both ﬁelds.
One soil sample was taken from a paddy ﬁeld
in ﬂooded condition (wet ﬁeld), and the othersample was taken from uncultivated paddy ﬁeld
in dry condition (dry ﬁeld). The water contentsof the sampled soils were determined by ovendrying (+*/) and weighting in laboratory.
,. , Shrinkage-swelling characteristic
Shrinkage-swelling characteristic is expressedby void ratio (e) as a function of moisture ratio()). ) and e are deﬁned as follows,
)

Vw
Vs

ῌ

Vf
῍
Vs
where Vs is volume of solid, Vw is volume ofwater, and Vf is volume of void (ﬂuid ; air andwater). Volumetric water content (q) is given
from ) and e by
e

Vw
)
῎

VsVf +e
For saturated condition, VwVf, and )e.
Therefore, q of saturated condition (qs) is given
by
q

qs

e
+e

῏

Shrinkage-swelling characteristic was determined by measuring volume and mass of thesoil (including water inside soil) and soil volume
on changing water content. Initially soil pastewith *.01 g g+ water content ()+.1 cm- cm-)was made and put in a container to have a sizeof 0./ cm wide, ++ cm long, and -./ cm deep. The
soil paste was dried slowly by evaporating waterin a room (the room temperature was +1ῐ,*,
but not controlled) to prevent unequal verticaldeformation due to quick evaporation. Several
times during evaporation, volume of shrunken
soil and mass of lost water were determined bymeasuring length of three size of the soil block,and weighing. Bulk density was calculatedusing the soil volume mass of water and massof dry soil that was obtained by oven-drying῕῔ : Thermal Properties and Shrinkage-Swelling Characteristic of Clay Soil in a Tropical Paddy Field

after the evaporation experiment.
,. - Crack area measurement
Due to strong shrinkage-swelling nature of
the soil, large cracks are formed in uncultivated
dry paddy ﬁeld. To measure cracks area, imageof dry paddy ﬁeld with crack taken by a digital
camera was analyzed. Image analysis withthresholding methods of pixels (using ImageJ)
separated object into soil surfaces and thecracks only.
,. . Soil thermal conductivity measurement
Thermal conductivity of the soil was measured by probe method described by Shiozawa
and Campbell (+33*). Thermal conductivity
probe (Decagon Devices Inc., USA) is 0* mm
length-+ mm diameter stainless steel pipe inwhich a resistive heating wire and a copperconstantan thermocouple is inside.
Soil of higher water content was preparedfrom mixing initially dry soil (oven-dried) andwater to make clay paste with certain watercontent. The clay paste was then put into
container size of +,./ cm long, /./ cm wide, and /cm height. The thermal conductivity probe
was inserted to the soil. During wet condition,
the thermal conductivity probe can be inserted
easily into the soil. Soil block of lower watercontent than *.0 g g῏+ was prepared by gradually and slowly dried the soil in the container,
keeping it in a room temperature in the same
way as the shrinkage experiment. As the soil
drying, the heat probe was keep inside the soil.
Heat was generated in the probe for a shorttime by a constant current through a heating
wire and thermal conductivity was determinedby measuring the probe temperature change
during the heating period and/or subsequentcooling period. For the heating period, temperature of the probe, T, is given byT῏Toῐῌqῌ.pK῍lnῌt῎to῍῎d for tῑt+ ῌ
where K is thermal conductivity of the soil [Wm῏+ K῏+], q is heat generated per unit length ofthe probe, To is initial probe temperature [ΐ], t+
the heating time [second], t* and d is empiricalconstants. Temperature in the soil will fall if

69

the heating current is switched o#. During thisperiod, an imaginary heat sink removes thesame heat which exists previously. The temperature in the cooling period is the result ofsuperposition of e#ects of the imaginary heatsource and sink. Therefore probe temperatureduring the cooling period is given byT῏Toῐῌqῌ.pK῍lnῌt῎tῌo῍῏lnῌt῏t+῎tῌo῍῎dῌ for tῒt+῍where the parameters toῌ and dῌ may be di#erent from to and d in the equation /.
A data logger/controller CR,+X (Campbell
Scientiﬁc Inc. Logan, USA) was used for thethermal conductivity probe measurement. CR,+X controls switching of the heater and measures temperature of the probe. Heating period
was +,* seconds with the voltage of heating in
the probe up to 0** mV (q is ../ W m῏+).
K, to, d in equations (/) and (0) were determined separately for heating and cooling periods by using least squares optimization procedure. The data in the ﬁrst ﬁve seconds ofheating and cooling are excluded from theanalysis to eliminate the bias caused by e#ects
of probe thermal properties. Examples of themeasured and ﬁtted heating and cooling T-t
curves are shown in Fig. +. The observed values
ﬁt theoretical curves quite well. Resulting
thermal conductivity is acquired by averaging
K values from heating and cooling periods.
Thermal di#usivity (D) is deﬁned as the ratioof thermal conductivity (K) to heat capacity ofthe soil (Ch) :K
῎
ChThe volumetric heat capacity of a soil can becalculated as the sum of the heat capacities ofthe soil components. Soil is made up of minerals,
water, and organic matter. The soil heat capacity is therefore computed from (Campbell, +32/) :Dῐ

ChῐCmfm῎Cwq῎Cafa῎Cofo

῏

where f is the volume fraction of the component [m- m῏-] indicated by the subscript. Subscripts, m, w, a, and o indicate mineral, water,



70

Fig. +,.-+fm..+2q

ῌ

where Ch is expressed in MJ m- K+. For swelling soil, fm and q are not easy to obtain due tothe shrinkage-swelling characteristic, but canbe calculated from ) and e. q is given by equation (-), and fm is given byfm

,**2

Examples of temperature rise and down of a heat probe during heating and cooling period.
Heating period was +,* seconds with heat of ../ W mῌ+. Water content was *.1+ g gῌ+.

air, and organic constituents. C is volumetricheat capacity [MJ m- K+]. Values of C foreach of substances are ,.-+ [MJ m- K+] for soil
minerals, ..+2 [MJ m- K+] for water, and ,./[MJ m- K+] for organic matter. Volumetricheat capacity of air (Ca) is given as function of
air temperature (Campbell and Norman, ,***).
In this experiment only two terms of theequation were used to calculate volumetricheat capacity, namely mineral and water substances. Neglected parts of equation 2 is because air considered to contributes only small
amount to heat capacity, while organic mattersfound to be few (,ῌ) in the soil. Thus, the heat
capacity is given by
Ch

++*

+
+e

῍
῎

-.

Results and Discussion-. + Soil shrinkage-swelling characteristic
Figure , shows plot of void ratio (e) vs. moisture ratio ()). General form of the swellingshrinkage characteristic (SSC) curve contains
normal shrinkage phase, residual shrinkage
phase, and zero-shrinkage phase (Haines, +3,- in
Bronswijk, +322). On the other hand, Braudeau(+322) modeled )-e curve based on the assumption that the soil consists of clayey microaggregates separated from each other and fromthe other soil constituents by a network ofmacropores. Braudeau (+322) divides swellingshrinkage curve by four point ; shrinkage limit(SL), “air entry” in the microaggregates (AE),the limit of contribution of macroporosity toshrinkage (LM), and the maximum swelling of
the microaggregates (MS) (Garnier et al., +331).
However, it is apparent that the observed )-e
curve shows the typical three phases of normal,
residual and zero shrinkage. Since the soil was
made into puddle with water as it was in theﬁeld, it had no microaggregates or structure.
Field observation in dry season indicatedthat surface soil of a su$ciently wet paddy
ﬁeld (irrigated and slightly ponded) had waterΐῒ : Thermal Properties and Shrinkage-Swelling Characteristic of Clay Soil in a Tropical Paddy Field

Fig. ,

Shrinkage-swelling characteristic of the soil with three phases. Solid arrows indicate
moisture ratios observed in ﬁelds of wet and dry conditions, and that gives the thermal
conductivity peak in Fig. /. Dashed lines and arrows indicate shrinkage limit and air entry.

content of *.3/ g g῏+ and that of a dry paddy
ﬁeld (uncultivated) had water content of *.+0 g
g῏+. Mapping these values of moisture ratio
and void ratio in the )-e curve (Fig. ,) indicatesthat soil of dry paddy ﬁeld lies in the residual
shrinkage zone, while soil of wet paddy ﬁeldlies in the normal shrinkage zone where watercontent is saturated.
SSC curve for the curve range *-SL, SL-AE,
and after AE is expressed by the following
equations :a. Zero-shrinkage
eῐeSL
b.

71

ῌ
῍

Residual shrinkage)AE῏)SLeῐeSL῎Krῌ
expῌVn῍῏+῏Vn῍῎῍expῌ+῍῏+ ῌ
῏
῎
ῌ
)῏)SL
Vnῐ
ῌ
῏
)AE῏)SLc.

Normal shrinkageeῐKrῌ)῏)AE῍῎eAEῌῐValues of parameters in equations obtained byﬁtting them to observed data are :)SLῐ*.,, [cm- cm῏-], eSLῐ*.22 [cm- cm῏-], )AEῐ+.+3 [cm- cm῏-], eAEῐ+.+3 [cm- cm῏-], and Krῐ +.
Subscript SL indicates “Shrinkage Limits”, andsubscript AE indicates “Air Entry”.
The coe$cient Kr which equal to + showsthat during normal shrinkage the volume change
is equal to moisture change. In addition eῐ) of
equation (+.) means that soil is saturated. This
gives straight line of ./-degree slope thatpasses (*, *) in Fig. ,. Three observed points
that lie on the straight line indicate that thissoil has the volume change of normal shrinkage for )ῑ+., cm- cm῏-. Residual shrinkage isthe process of reducing water content follows
entering of air into soil and shrinkage simul-

72



++*

,**2

Fig. - Relationship between bulk density and water content of the swelling soil.

Fig. . Crack pattern in paddy ﬁeld. (a.) Dry paddy ﬁeld with crop plant residue and cracks. (b.)
Image of a. which thresholded for soil surface and cracks only. Crack opening is +0ῌ ofsurface area.

taneously. In zero shrinkage, reducing watercontent occurs without shrinkage.
The relationship of bulk density and mass
base water content is shown in Fig. -. In rigid
soil, the values of bulk density vary with structural condition of the soil, particularly related
to packing. Unlike rigid soil, bulk density ofthe swelling soil is variable with water content.
The bulk densities vary from +.. Mg m- to *..
Mg m- with increasing water content up to +.1
g g + .
Figure . shows observed crack pattern for+.*/ m, of paddy ﬁeld surface. As a result, areaof soil surface in the image was *.22 m, (2.ῌ)

and that of crack opening was *.+1 m, (+0ῌ).
Neglecting cracks in the edges of the image, theaverage surface area of one block surroundedby cracks is 010 cm,.
-. , Thermal conductivity
Figure / shows the relation of thermal conductivity (K) to mass base water content (w).
For water contents lower than *.,., K increases
steeply with increase in w. By contrast, for watercontents larger than *.,., K decreases with increase in w due to increase in bulk density.
This relation is di#erent from K-w relation of
non-swelling soils, which generally showsmonotonic K increase with the increase in῏῎ : Thermal Properties and Shrinkage-Swelling Characteristic of Clay Soil in a Tropical Paddy Field

Fig. /

73

Relationship between thermal conductivity and water content of the swelling soil. Solidarrows indicate water contents observed in ﬁelds of wet and dry conditions.

water content at an ordinary temperature (e.g.
Campbell, +32/ ; Hiraiwa and Kasubuchi, ,***).
The thermal conductivity value corresponds to
w of the wet (cultivated and ﬂooded) paddy
ﬁeld during ﬁeld observation (w῍*.3/ g gῌ+)was +.+0 W mῌ+ Kῌ+, while that correspond to w
of dry paddy ﬁeld (w῍*.+0 g gῌ+) was *.33 W
mῌ+ Kῌ+. K value of the ﬂooded paddy ﬁeld soil
was similar to that observed in a ﬂooded paddy
ﬁeld in Japan by Mowjood et al. (+331), whichwas *.3* W mῌ+ Kῌ+. It is surprising that thethermal conductivity value for very dry soil
and that for su$ciently wet soil are almost thesame ; generally for non-swelling soil, K ofwater saturated condition is several times asmuch as K of most dry condition. The maximum value of thermal conductivity is +.,2 W
mῌ+ Kῌ+ at water content value of *.,. g gῌ+.
This water content exists in the middle position between zero shrinkage and normal shrinkage in Fig. /.
These phenomena can be explained by comparing three values of thermal conductivity ofsoil constituents. Thermal conductivity of air,water and soil mineral are *.*,., *./0, and ,./ [Wmῌ+ Kῌ+] (de Vries +30- ; Campbell and Norman,,***). Roughly the ratio of air : water : soil particle is + : ,* : +**. If void ratio is constant withchanging water content (for non-swelling soil
or zero shrinkage zone of swelling soil), increasing water content occurs by replacing air(lowest K) in pores by water (the second highest K) of the same volume and it results inincreasing thermal conductivity. This e#ect ofincreasing K with increasing w is signiﬁcant
when air exists at contact point of solid particles (highest K) is replaced by water. For nonswelling soils, of which void ratio is constant, K
must increase monotonically with increase inw. However, for water saturated or nearly water
saturated condition of the swelling soil, increasing water content occurs by increasing
void ratio, that is decreasing solid volume ofthe same (or similar) amount of the increased
water ; therefore, it must result in decrease inK.
Regression equation that expresses thermal
conductivity as function of w is :



74

++*

,**2

Fig. 0Relationship between thermal di#usivity and water content of the swelling soil. Solidarrows indicate water contents observed in the paddy ﬁelds of wet and dry conditions.

Fig. 1

Relationship between volumetric heat capacity and water content of the swelling soil.
Solid arrows indicate water contents observed in the paddy ﬁelds of wet and dry conditions.

for * w *.,.
K1..1w,..2-w*.//῍ῌ

for *.,. w
K*.,,.lnw*.33

῎ῌ

Figure 0 shows the di#usivity of the soil thatis calculated by equation (1) and (3). The di#usi-

vity (D) basically decreases with increase in w
except for very dry condition in which di#usivity slightly increase with w. Its peak value isobserved at water content of *.+, g g+. The
value of di#usivity ranges between ,.0+*1
and /.-+*1 m, s+. At water content less thanthe peak value of thermal di#usivity, the rateof increase in thermal conductivity exceeds theῐ῏ : Thermal Properties and Shrinkage-Swelling Characteristic of Clay Soil in a Tropical Paddy Field

Fig. 275

Soil surface condition and heat ﬂow in soil during daytime ; (a) wet condition of bothswelling and non-swelling soil that have high thermal conductivity (K) ; K is high and heatﬂow is large. (b) dry surface of swelling soil with cracks ; K is high and heat ﬂow is large
even if cracks exist. (c) dry surface of non-swelling soil without cracks ; K is low and heatﬂow is small. Heat ﬂows in the reverse direction during nighttime.

rate of increase in volumetric heat capacity(Fig. 1). At higher water content but less thanthat of the peak of K (w῍*.+,ῌ*.,. g gῌ+), theincrease in heat capacity (C) exceeds increase
in K and thus D starts decreasing. At watercontents higher than that of the peak of K, decrease of K and increase of C resulted in steep
decreasing of D with increase in w. However,for higher water content of w῎*.1, K decreaseslightly while C is nearly constant values, thus
D decreases gradually with increase in w.
-. - E#ects of thermal conductivity of surface soil on heat balance
Thermal conductivity of the swelling soil asa function of water content is remarkably di#erent from those of non-swelling soils. The valuesof K for the swelling soil of very dry soil andthose of su$ciently wet (water saturated) soil
are almost the same ; for non-swelling soil, K ofnearly dry soil is much smaller than that of
su$ciently wet soil. When the soils are dry,the values of K for the swelling soil are muchlarger than those of non-swelling soil becauseof high bulk density and probably good thermal contacts among clay particles for the
shirked soil.
Thermal conductivity of the surface soil has
important e#ects on heat balance at soil surface and formation of the surface temperaturewhen the soil surface is exposed to sunshine.
The higher soil thermal conductivity the more
proportion of the heat from the sun ﬂows into
soil and stored in soil during day time ; surface
temperature rise during daytime is thus restrained. On the contrary, when the surface
soil has a low thermal conductivity, the low
conductivity prevents heat conduction into
soil and surface temperature rises more during
daytime ; apparent heat ﬂux from soil surface
to air thus increases. In the uncultivated dry
paddy ﬁeld, surface temperature rise during
the daytime would be less than that expected
in upland dry ﬁelds, by a larger heat conduction into the soil due to higher thermal conduc-

76



tivity of the swelling clay soil.
-. . E#ect of cracks on heat ﬂow in soil
Cracks are formed in the paddy ﬁeld as soil
dries. We consider that the cracks can be regarded as insulated spaces for heat conduction
and also for heat convection because thermal
conductivity of air is low and because convection of air should be limited in the narrow
space of cracks (at most a couple of centimeters). However, the formation of the crack is
vertical direction that is parallel to heat ﬂow insoil (Fig. 2-b). It creates system of parallel heat
resistance with crack resistance as one component. Resistance for heat ﬂow parallel to heat
ﬂow has only minor e#ect on the heat ﬂow,
even if the resistance is extremely large. Opening area of cracks was about +0ῌ of surface
area and cracks closed at about +* cm depth ;even assuming crack as insulator, the e#ect of
cracks on thermal conduction is at most only+0ῌ reduction within a few cm surface soil
layer. Therefore, we think that heat ﬂow in soilin the paddy ﬁeld with cracks is approximately
the same as the heat ﬂow that would be observed in the soil of the same thermal properties but with no crack.
Existence of air in soil that is series to heat
ﬂow signiﬁcantly resists heat ﬂow in soil. Thissituation is expressed by the remarkably lower
thermal conductivity of non-swelling soil oflower water saturation. In general, thermal conductivity of dry soil is about one order smaller
in magnitude than that of water saturated soil.
Due to such lower K, drying of the surface of
non-swelling soil interrupt heat conduction(Fig. 2-c). On the other hand, drying of theswelling soil with forming of cracks does not
reduce heat conduction signiﬁcantly (Fig. 2-b).

++*

,**2

References
Braudeau, E. (+322) : Equation generalisee des courbes
de retrait d’echantillons de sols structures. C.
R. Acad. Sci. Ser, -*1 : +1-+ῌ+1-..
Bronswijk, J. J.B. (+332) : Modelling water balance,
cracking and subsidence of clay soils. Journalof Hydrology, 31 : +33ῌ,+,.
Campbell, G.S. (+32/) : Soil physics with Basic ;Transport Models for Soil-Plant Systems, Elsevier,
Amsterdam.
Campbell, G.S. and Norman, J.M. (,***) : An Introduction to Environmental Biophysics ,nd ed.,Springer.
de Vries, D.A. (+30-) Thermal properties of soils. pp.
,+*ῌ,-/, In van Wijk W.R.(ed) Physics of Plant
Environment. North-Holland Publishing Co.,
Amsterdam.
Garnier, P., Perrier, E., Jaramillo, R.A. and Baveye,
P. (+331) : Numerical model of --dimensional
anisotropic deformation and +-Dimensional
water ﬂow in swelling soils. Soil Science, +0, :.+*ῌ.,*.
Haines, W.B., (+3,-) : The volume changes associatedwith variations of water content in soil. J.
Agric. Sci. Camb., +- : ,30ῌ-++.
Hiraiwa, Y. and Kasubuchi, T. (,***) : Temperaturedependence of thermal conductivity of soil
over a wide range of temperature (/ῌ1/). European Journal of Soil Science, /+ : ,++ῌ,+2.
Mowjood, M.M., Ishiguro, K. and Kasubuchi, T.
(+331) : E#ect of convection in ponded wateron the thermal regime of a paddy ﬁeld. Soil
Science, +0, : /2-ῌ/21.
Shiozawa, S., and Campbell, G.S. (+33*) : Soil thermal conductivity. Remote Sens. Rev. / : -*+ῌ-+*.
Yoshikawa, N. and Shiozawa, S. (,**0) : Estimating
variable acreage of cultivated paddy ﬁelds
from preceding precipitation in a tropical watershed utilizing Landsat TM/ETM. Agricultural Water Management, 2/ : ,30ῌ-*..
Yoshino, K. and Ishioka, Y. (,**/) : Guidelines forsoil conservation towards integrated basin
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and Water Environment, - : ,-/ῌ,.1.

: Thermal Properties and Shrinkage-Swelling Characteristic of Clay Soil in a Tropical Paddy Fieldῌ

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77