Seasonal change of evapotranspiration and crop coefficient in a rain fed
paddy field, cassava plantation and teak plantation in Thailand
Pedram ATTAROD*, Daisuke KOMORI*, Kazushi HAYASHI*, Masatoshi AOKI*, Tomoyasu ISHIDA**,
Kazunari FUKUMURA**, Samakkee BOONYAWAT***, Panya POLSAN**** , Piyapong TONGDEENOK***,Promsen SOMBOON***, Somnimitr PUNKNGUM****

Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, JAPAN**
Faculty of Agriculture, Utsunomiya University, Tochigi 321-8505, JAPAN***
Faculty of Forestry, Kassetsart University, Bangkok 10900, THAILAND****
Hydrology and Water Management of Northern, Royal Irrigation Department, Phisanulok 65000,THAILAND
Abstract
The evapotranspiration (ET) is an essential factor to estimate crop water use. It is also one of the factors in soil water
storage and water resource in a region. This study is to compare the trend of actual evapotranspiration (AET) and cropfactor (Kc) among three measurement sites in Thailand that are a rain fed paddy field, cassava plantation and teakplantation. Bowen ratio energy balance technique (BREB) was applied in this investigation to measure the AET andheat flux between ground surface and atmosphere. Penman-Monteith equation recommended by FAO was used tocalculate crop coefficients. Measurements were carried out during 1999-2003 in the paddy field, and teak plantation and2002-2003 in the cassava plantation. Results showed that the amount of daytime AET during the rainy season in thepaddy field and cassava plantation varied between 1 and 7 mm/daytime and in the teak plantation between 2 and 6mm/daytime. The average amount of daytime AET in the rainy season was about 4 mm in all sites although thevariations of AET were different. In the dry season, AET of cassava plantation was around 2.7 mm/daytime, slightly
lower than those of other sites. During the growing season, Kc factor varied in the paddy between 0.4 and 1.2 and in thecassava between 0.3 and 1.2. Variation of Kc factor in teak plantation was smaller than those of others. In the rainyseason, LE/Rn ratio in the cassava plantation and paddy field was around 70% but in the teak plantation was about 73%.
Kc factor in paddy field was more affected by soil moisture in paddy field than other sites.
Keywords: actual evapotranspiration, cassava plantation, crop factor, paddy field, teak plantation

1. Introduction
Thailand locates in tropical Monsoon Asian region.
The dry season begins in November and ends in April.
Rice, cassava and teak are one of the most important
crops and timber production tree which cover a large
portion of land use in the Chao Phraya River basin andNorth-East region of Thailand. In artificial and natural
ecosystems including agricultural lands, it is essential
to evaluate the evapotranspiration (ET) for developingmore efficient and sustainable water management
techniques and planning the irrigation scheduling.
Also vegetative productivity is closely related to ET.
The crop coefficient Kc which is the ratio of actual
evapotranspiration(AET)
tothereference
evapotranspiration (ET0) calculated by the FAO
Penman-Monteith method (Allen et al., 1998). Kc
values can be useful to plan the irrigation schedulingin tropical Monsoon Asian region. Since researches on
the ET and Kc values in the tropical region arerelatively scarce, the objective of the present study isthus to find out the trend of ET and crop factors inthree crop covers including a rain fed paddy field,
cassava plantation and teak plantation in Thailand andalso comparison of ET value among the mentioned
vegetations.

2. Materials and methods
2.1. Experimental sites and plants

The measurements were carried out in threemeasurement sites in Thailand that are a rain fed

paddy field in Sukhothai province (17° 03′ N, 99° 42′E, elevation 50 MSL), a cassava plantation near
Nakhonratchasima (14° 47′ N, 102° 38′ E, elevation
311MSL) and a teak plantation in Lampang province(18°, 40′ N, 99° 47′ E, elevation 241 MSL) which are
GAME-T and CEOP project sites. Measurements wereperformed during 1999-2003 in the paddy field and1999-2003 teak plantation and 2002-2003 in thecassava plantation. Measurement sites of paddy fieldand cassava were selected in farmer lands. Rice seeds
are sown every year by a farmer after starting therainfall period in the late July and harvested in the endof November. Cassava is also planted mostly everyyear in March and harvested in dry season and grownwithout irrigation. Teak, a 38-year deciduous
plantation, is mostly started to open leaves in May andthe leaves start to shed in January. At each field,
microclimate measurements have been carrying out
using an automatic weather station system developed
by AOKI, et al. (1996)
2.2. Evapotranspiration

Bowen ratio energy balance system (BREBS) wasemployed to measure AET. The dry and wet-bulb
temperatures were measured at two levels above
canopy (paddy, cassava: 1-3 and 10 m; teak: 12 and24m) using a hand made 10 paired copper-constantan
thermocouple thermometer (shielded and ventilated).
The instruments installed on a 10 m tower in paddy
field and cassava and on a 24 m tower in teakplantation. A net radiometer (MF -11 EKO) was used
above canopy to measure the net radiation flux density.

The fetches are around 500 m in paddy field, at least 2
km in cassava and 400 m in teak plantation. Two or
three heat flux plates (P-MF-81, EKO) were placed in
1 cm in the soil surface. Two tensiometers were also
set at 15 cm depth in soil near the tower of ETmeasurement. Their instantaneous data of every one
minutes was collected by a data logger to get ten
minutes average, and all data were finally averaged
into the daytime mean (Rn positive). Daytime latent
heat flux was calculated from the equation:from May 2002 to Dec. 2003 and for the teak forplantation from May 1999 to Dec. 2003. AET in Jan.
was lowest in three sites (Average; paddy: 2.7mm;
cassava: 1.7mm; teak: 2.9 mm). It started to increase

D. S

10R.S

D.S

9pF<2.3
2.3<pF<2.6pF>2.6
date with missing pF

876Paddy filed

5

LE = ( Rn − G − Gw ) /(1 + β )

4(1)

32

2.3. Reference evpotranspiration and Crop coefficient

Daily reference crop evapotranspiration (ET0) was
calculated using the form of Penman-Monteithequation recommended by the FAO (Allen, et al.,1998) for a hypothetical grass crop with an assumed
height of 0.12 m, a fixed surface resistance (70sm-1)
and an albedo (0.23) as follows:
ET0 =0.408 ∆ ( Rn − G − G w ) + γ (900 / T + 273))u 2 (e s − e a )∆ + γ (1 + 0.34u 2 )(2)where T the daily air temperature (0C), u2 wind speed
at the height of 2 m (ms-1), es the saturation vapor
pressure (kpa), ea the actual vapor pressure(Kpa), ∆ isthe slope of the vapor pressure curve (kPa°C-1), and γ
is the psychrometric constant (kPa°C-1). Daily averagevalues for each crop were used for Rn , T and ea. To
calculate the wind speed at 2 m above the surface, thewind speed data were converted by the following
equation recommended by FAO:
u 2 = u z (4.87 / ln(67.8 z − 5.42))(3)

where u2 wind speed at 2 m above ground surface(ms-1), uz measured wind speed at z m above groundsurface (ms-1) and z height of measurement aboveground surface (m). Crop coefficients (Kc) for paddy
field, cassava and teak plantation was calculated using
the relation Kc =AET/ET0.

3. Results and discussion
3.1. Seasonal change in evapotranspiration

Figure1. shows seasonal changes in daily AET
expressed in a year for the paddy field taken fromJune 1999 to Dec. 2003, for the cassava plantation

10109AET(mm/daytime)in which Rn (MJm-2/daytime) is the net radiation, G(MJm-2/daytime) soil heat flux, and Gw(MJm-2/daytime) heat flux in water (paddy field) .The
daytime latent heat flux (LE) then converted into thedaily actual evapotranspiration (AET) in mm per
daytime. The average contribution of the latent heat
flux to net radiation (LE/Rn) was calculated tounderstand the energy allocation to evapotranspiration
process.

8pF<2.3
2.3<pF<2.6
pF>2.6date with missing pF

Cassava plantation

76543
210

10

pF<2.392.3<pF<2.68pF>2.67date with missing pF

Teak plantation

6543210

JFMAMJJASONMonthFig.1. Seasonal change in daily evapotranspiration
for paddy field, cassava and teak plantations (D.S:Dry Season and R.S: Rainy Season ).
around May (Average: 3.2mm in all sites) when thegrowing season started, and reached its peak
(Average; paddy and cassava: 4.5mm, teak: 4.1 mm)
during the mid growing season in Sep. and Oct.
Increase in net radiation and LE/Rn ratio in May andApril might increase the evaporation from thesubmerged water in paddy field before starting thegrowing season. Increase in AET of cassava in Mayand April is explained by presence of the plants
before starting the rainy season. The peak of AET inthree sites synchronized by peak of net radiation andpeak of LE/Rn ratio. During the rainy season, AET
for paddy field and cassava ranged between 1 and 7D

mm. The range of AET for teak plantation wasbetween 2 and 6 mm. Data were classified to fourgroups based on soil moisture condition. Group 1(closed circles), Group 2 (open circles) and Group 3(crosses) correspond to the data obtained on days
when the pF values at 15 cm are less than 2.3,
between 2.3 and 2.6 and greater than 2.6, respectively.
Group 4 (triangles) represents the date with missing
pF value. Data showed that soil moisture had more
influence on AET in paddy field than two other sites.
The average contribution of latent heat flux to net
radiation (LE/Rn), actual evapotranspiration (AET)and reference evapotranspiration (ET0) in the threesites during the dry and rainy seasons are shown intable 1. In the rainy season, average of AET wasabout 4 mm in all sites, although the variations ofAET were different (paddy field: σ =1.28; cassava:
σ =1.42; and teak: σ = 0.88).

average values for paddy field, cassava and teakplantation were 0.66, 0.48 and 0.82, respectively.
During the growing season, Kc factor varied in paddy
field between 0.40 and 1.2, and in cassava between
0.3 and 1.2. Variation of Kc factor in teak plantation
was between 0.4 and 1.3. For cassava, our extreme
1.6(Mm/daytime)ET0(Mm/daytime)

1.2

date with missing pF

pF>2.6

0.8
0.6
0.4

Paddy filed

0.2
01.6
1.4
1.2

Vegetation coverage

pF<2.32.3<pF<2.6
pF>2.6date with missing pF

1Kc(LE/Rn) %2.6<pF<2.3

1

Table1. Daily average of actual
evapotranspiration (AET), LE/Rn ratio andreference evapotranspiration (ET0) in the threemeasurement sites (D.S: Dry Season and R.S:Rainy Season).
AET

Vegetation coverage

pF<2.3

1.4

0.8
0.6
0.4

D.S

R.S

D.S

R.S

D.S

R.S

0.2

Paddy3.0

4.1

6070

4.9

4.40Cassava

2.7

3.9

5472

4.5

5.1

Teak

3.0

3.8

67

73

4.1

4.6

This variation is partly relates to the variation of Rn.
In the dry season, AET of teak plantation showed lessvariation in compare with paddy field and cassava(paddy field: σ = 0.99; cassava: σ =1.25; teak:σ = 0.80). In the dry season, average amount of AET
in the paddy field and teak plantation were nearlysame around 3 mm and in cassava slightly lower thanthose of other sites, about 2.7 mm. The average ofLE/Rn which mostly depends on the variation of Rn
showed different values (paddy =60, cassava: 54 andteak: 67) in the dry season. LE/Rn was about 70%
during the rainy season for paddy field, whereas teakand cassava plantation had somewhat higher LE/Rn.
3.2. Crop coefficients for paddy field, cassava plantationand teak plantation

Figure2. shows the seasonal change of crop
coefficients (Kc) for paddy field, cassava and teak.
Extreme maximum of Kc factor in paddy field,cassava and teak plantation which observed in Sep.
and Oct. were approximately 1.2, 1.2 and 1.3,
respectively. It decreased gradually thereafter in allsites and reached to its minimum in Jan. And the1.6
1.4
1.2

Cassava plantation

Vegetation coverage

pF<2.32.3<pF<2.6

pF>2.6date with missing pF

10.8
0.6
0.4
0.2

Teak plantation

0JFM AM JJASON DMonthFig.2. Change in crop coefficients for paddy field,
cassava and teak plantations
maximum of Kc value was 1.2 in Sep. According toWatanabe et al., (2004), the Kc value in a cassava
plantation which planted in Nov. in Khon Kaen,
Northeast Thailand, was about 1.2 in June. The
difference of peak occurrence times may be attributed
to the difference of planting months in two cassava
fields. Data were classified to four groups same asprevious categorizations. Cassava and teakplantations showed no tendency of soil moisture
influence on Kc factor while in paddy field Kc factor
was affected by soil moisture. This fact implies that
cassava plants and teak trees uptake water fromdeeper soil layers because of deeper root zones andthat cassava and teak plantations are tolerable
vegetations to drought.

field, confirming again that soil moisture has more
effect on the Kc factor in paddy field than cassava andteak plantations, the tendency of soil moisture
influence on Kc factor in paddy field and teakplantation are nearly similar.

3.3. Dependency of kc factor on pF
Fig.3. shows pF value against kc factor in three sites.
Data were divided in two groups in paddy field andteak plantation, with crop coverage (filled triangles)
and without crop coverage (open circle) and one
group in cassava plantation, with crop coverage(filled triangles). Although the data points in cassava
and teak plantations are more scattered than in paddy

During the rainy season, range of AET in paddy
field and cassava plantation was between 1 and 7
mm/day time and in teak forest between 2 and 6mm/daytime. The average amount of AET in rainyseason was nearly same in the three sites. AET
initiate to increase when the growing season started
in May and reached its peak in Oct. or Sep. in all sites.
During the dry season, AET of cassava plantation
was around 2.7mm/daytime, slightly lower than othersites. In the dry season, average AET of teakplantation showed no significant trend. During therainy season, LE/Rn ratio in paddy field and cassava
plantation were less than teak plantation. Our results
showed that maximum Kc factor in paddy field,
cassava and teak plantations observed in Sep and Oct.
and that is different month with other reports. The
difference may be attributed to the difference ofplantation was smaller than those of others. Kc factor
in the paddy field was more affected by soil moisturethan cassava and teak plantations suggesting thatcassava plants and teak trees tolerate the water
shortage during the dry season.

1.6

With crop coverage

1.4

Without crop coverage

1.2
10.8
0.6
0.4

Paddy filed

0.2
00
1.6

0.5

11.5

2

2.5

3

3.5

With crop coverage

1.4

Acknowledgments

Kc

1.2

The authors would like to thank Professor Katumi
Musiake, Fukushima University, Professor Toshio
Koike, Tokyo University, and associate Professor
Taikan OKI, institute of industrial science, Universityof Tokyo for their financial support to accomplish thisresearch.

10.8
0.6
0.4
0.2

Cassava plantation

001.6

0.5

11.5

2

2.5

3

3.5

With crop coverage

1.4

Without crop coverage

1.2
10.8
0.6
0.4
0.2

Teak plantation

000.5

14. Conclusion

1.5

2

2.5

3

3.5

pF valueFig.3. Relation between pF and crop factor inpaddy field, cassava and teak plantation

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