Evapotranspiration Estimation of Paddy Rices in Thailand
Utilizing Limited Meteorological Data
Panya Polsan1, Masatoshi Aoki2, Sanguan Patamatamkul3
1. Royal Irrigation Department, Ph.D student of Water Resources Engineering,
Khon Kaen University, Thailand
2. Tokyo University of Agriculture and Technology(TUAT), Japan
3. Khon Kaen University, Thailand

ABSTRACT
Evapotranspiration (Et) information used for irrigation scheduling may benefit farmers inincreasing the irrigations area and evaluating amount of water applied. In Thailand, there areseveral methods of evapotranspiration estimating. The Bowen ration technique is the method ofevapotranspiration estimating which is firstly applied to paddy field in Thailand. This research isattempted to generate the basic meteorological data which are necessary for formulating simple
model to determine evapotranspiration. The model is simplified evapotranspiration from Bowen
ratio technique with linear regression and non linear regression method using basic meteorological
data. By comparing evapotranspiration from simple model with evapotranspiration from Bowen
ratio method, it shows the similar pattern of distribution over a year. Therefore, in the paddy fieldarea of which limited meteorological data, the simple model can be applied for evapotranspiration
estimating.

Keywords : Evapotranspiration, Bowen ratio, Simple model
INTRODUCTION
Evapotranspiration(Et) is a major
component of the hydrologic cycle. It isrepresented the loss of water from both soil andplant surface and is very especially importantfactor in planning and developing river basin andwater resources. Evapotranspiration is the
combined processes by which water is transferredfrom the earth’s surface to the atmosphere, itsmeans evaporation of liquid or solid water plustranspiration from plants. Evapotranspiration
forms the foundation for the planning anddesigning of most irrigation projects andirrigation development. The evapotranspiration is
the same as consumptive use which includes
evaporation of water from land and water surface
and transpiration by vegetation. Consumptive use
has been important in arid and semi-arid irrigatedareas of the world and also its importance humid
areas is increasing with expansion of
supplemental irrigation.
There are two types of

evapotranspiration have been widely expressedin irrigation management, i.e , actual
evapotranspiration(Ea)andpotentialevapotranspiration(Ep).
Evapotranspiration
data are essential for estimating irrigation waterrequirement especially for paddy rice field. In
Thailand, the most important and economic
plant is the rice. The rice usually is consumes
great amount of water. The irrigated area in
Thailand approximately
35,200 square
kilometers(7% of all area). Those are total
water used by rice will be very high. The
estimated evapotranspiration is the one method
to define the minimum of water used or water
consumption in paddy field.
Many Thai
Scientist and Hydrologist try to computed andestimated evapotranspiration of paddy ricefrom Ep, are not from by Ea methods such as
the Pan evaporation method, the Lysimeter
method, the Blaney-Criddle method.

The Thornthwaite method, the Penman
method,the Jensen-Haise method, theCombination method and others. Each method
weregivendifferent
valuesofevapotranspiration.
Theestimatedevapotranspiration of paddy rice field is very
important and should be studied delicately andintensively for water management especially in acondition of insufficient water.
Bowen ratio techniques are employs theBowen ratio method
estimate actual
evapotranspiration(Eta) and used to estimatedactual evapotranspiration in paddy rice field.
The Bowen ratio method give high accurate valueof estimated evapotranspiration.
How ever, the Bowen ratio method used many
complex data and quite expensive in maintenanceand data collecting. This research try to find thesimple method by using basic meteorological data
which be given the nearest value ofevapotranspiration with the Bowen ratio method
and can be used in at any paddy rices field ofThailand.
MATERIALS AND METHODSThe two Automatics Weather Station
are located at two places. One is located in nonirrigated paddy rice field at Sukhothai provine inlower northern region of Thailand. The other one
is located in irrigated area of paddy rice field atPhitsanulok province 80 kilometers at the east offirst site. The AWS are consisted of two dry andwet bulb thermometers in different level, ( onewas placed at 2 meters height above the ricecrops), a net radiometer, three soil heat fluxmeters(at 1 cm depth in soil), the thermister
sensor for the soil temperature, wind speed, winddirection, rain guage, water level sensor.
The instantaneous data of every one
minute is collected by the data logger to get ten
minutes average and finally averaged in to thedaytime mean( from 8 a.m .to 4 pm).

Then evaluated actual evapotranspirationand get the equation as:
Eta = - (Rn+G)/(1+γdT/de)

Where Eta is actual evapotranspiration
γ is psychrometric constant= Cp.P/0. 622L
Cp is specific heat of air
P is atmospheric pressuredT/de is gradient of temperature andhumidity in air
L is latent heat of vaporization
From evapotranspiration which computed fromequation (3) at both sites of AWS, then using
linear and non linear regression related withmeteorological data and gets the simple models
as:Et = a + bT + c RH + d WS

(4)

Et = a + bTc(5)

Et = a + bRHc

(6)

Et = a + b WSc(7)

Where Et is evapotranspiration
T is air temperature
RH is relative humidity
WS is wind speed
RESULTS

0.7
0.6
0.5
0.4
0.3
0.2

Ett,rh,wd0.1

From the energy balance

ET01

Rn + H + LE + G = 0(1)

And from the Bowen ratio technique
B = H/ LE
B = Bowen ratio, H = sensible heat,
G = ground heatflux, Rn = net radiation
LE = latent heat flux

(3)

(2)

7 13 19 25 31 37 43 49 55 61 67 73 79 85

R=0.833
Et(t,rh,ws)=-23.1+4.2x10-3T +2.76x10-4RH+ 8.829x10-2WS
The hourly evapotranspiration calibrated fromthe Bowen method are following in the same
shape and the values of hourly Et are quite equal.

The relation between Simple Et from windspeed and temperature with evapotranspirationfrom Bowen ratio as follow;

0.70.6mm

0.5

0.7
0.6

ET
0.3Et,ws

0.2

0.5mm

Et,rh

0.4

0.1

0.4

ET

0.3Et,t,ws

01 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

0.2

Et(ws) = -11.0587 + 0.00016WS2Et(rh) = -1.3119 + 0.068RH- 0.000645RH2

0.1

R=.83301 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

R are 0.83, 0.619 respectivelyEt(t,ws) = -23.57-.0904T+0.00681WS

The graph above showed that hourly
evapotranspiration which used wind speed factor
and humidity respectively are following with Et
computed by Bowen ratio. Evapotranspiration
computed by Bowen ratio method, the peak indaytime were higher than two Et.

0.7
0.6

ETEt,rh,ws

0.5

MM

0.4
0.3

Average Monthly Evapotranspiration

0.2

R=.8330.1

8706
7 13 19 25 31 37 43 49 55 61 67 73 79 85

Etb

5mm

1Et(rh,ws) = -22.822+0.0867WS+0.001RH

Et(t,rh,ws)

43Et(rh,ws)
Et(t,ws)

21JulAug
Se
pAug
Nov
DecJun0JanFebM
arAprMay

This relation of Et from Bowen ratio
method and Et calibrated with wind speed
and humidity going in the same direction.

Figure above is shown that the seasonalevaspotranspiration of each methods are nearly
different. So wind speed, temperature and humidity
are important for evapotranspiration.

0.7
0.6mm

0.5
0.4Et,t0.3

ET

0.2
0.1

Average Monthly Evapotranspiration
8R=.7457
60Et(t) = -17.42634+1.116T- 0.01735T2Etb

5mm

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86Et(t)43Et(rh)Et(ws)

2
1Jul
Aug
Se
p
Aug
Nov
De
cJan
Feb
M
ar
Ap
r
M
ayJu
n0The air temperature is the important factor
forevapotranspiration.
Thehourlyevapotranspirationfrom Bowen ratio
method and temperature calibrating are look
best fit.

The average monthly evapotranspiration whichused only one meteorological data, Et more different.

LIMITED CONDITIONS

mm/yearTotal evapotranspiration
900800700
600
500400300200
1000b
Et( t,Ets)
,wrh)
s,wws
( t,( rhEtEt( t)Et)( rhEts)(wEt

The total evapotranspiration of each method
are shown at above. The evapotranspiration
estimated from Bowen ration method(Etb),Et(t,rh,ws),Et(rh,ws),
Et(t,ws),Et(t),Et(rh)andEt(ws) are 735.9,785.4,762,770.9,688.9,828 and768 mm/year respectively. The amount of each
evapotranspirations value are not much different.
The highest of evapotranspiration is Et computed by
using
only
humidity.
Thelowest
isevapotranspiration
computed
using
only
temperature data. However, all data that used tocalibrated and calculated are used data from 2002
year at the irrigated paddy field. Many data are
missing, are damaged and also many problem. The
Et that we estimated are insufficient data needed.

!. Using data in year 2002
2. Using data only at irrigated paddy field
3. Estimated during 8.00am to 16.00 pm
4. Each Et data which get from Et formula
should be multiplied by 18 to convert to daily
Et if using average daily data.

CONCLUSION
The simple model to estimatedevapotranspiration in paddy field can beused under limited conditions. The
evapotranspirations getting from simple
model are fit curve. However, those simple
models equation should be
accuratechecking with more data and try to apply
which other year. The simple models areconvenient to used and estimated water
consumption at paddy field in Thailand
with simple meteorological data. The
average total evapotransiration from simple
models are 767.2 mm/year with the Bowen
ratio method is 735.9 mm/year

REFERENCES
Ahmad A. Al-Shooshan and El-Sayed E-S. Ismail. Alfalfa Microclimate and Evapotranspiration
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Evapotranspiration and Irrigation Schedule, Proceeding of the internal Conference. November 36,1996. San Antonio, Texas, USA.
Aoki, M., T Machimura, Y. Hidashima, N.Obase and S. Maruya. 1996. Estimated of Bowen ratio
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Evapotranspiration and Irrigation Schedule, Proceeding of the internal Conference. November 36, 1996. San Antonio, Texas, USA.
Ibrahim, M.A.M. 1966. Assesment of Differential Potential Evapotranspiration(Etp)
Methods inthe Nile Delta. pp 387-393. In C.R. Camp, E.J Sadler and R.E.Yoder(eds.) Evapotranspirationand Irrigation Schedule, Proceeding of the internal Conference. November 3-6,1996. San
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