Comparative Actual Water Consumption of Irrigated and Rainfed Paddy Rice Field Using Bowen

COMPARATIVE ACTUAL WATER CONSUMPTION OF IRRIGATED AND RAINFED PADDY RICE FIELD USING BOWEN RATIO METHOD

PANYA POLSAN1, MASATOSHI AOKI2, SA-NGUAN PATAMATAMKUL3

1.  Royal Irrigation Department, Ph.D student of Water Resources

Engineering, Khon Kaen University

2.  Tokyo University of Agriculture and Technology, Japan

3.  Khon Kaen University, Thailand

Knowledge of Evapotranspiration of paddy rice is a very important factor supporting irrigation management decisions. In this study, water consumption in irrigated and rainfed area were compared base on actual evapotranspiration using Bowen Ratio. The result of these methods were compared which the other method such Penman- Monteith. The result shows that water consumption by Bowen ratio more economic than other methods. Thus for further irrigation planning be better use Bowen method to estimated and evaluated irrigation management.

INTRODUCTION

Thailand is a agriculture country, total area of Thailand approximately is 500,000 square kilometers covering agriculture area approx. 200,000 sq.km. The most economics and famous plant in Thailand is paddy rice. Precipitation is the main source of water for agriculture in Thailand and study area with average 1,135 mm per year. With the monsoon climate, the precipitation in the study area were fall distribution mainly in rainy season. During rainy season (June – October) the rainfall average is 831 mm, according for 73.2 % of the annual precipitation amount. In winter and summer season are limited with winter (November – February) according for only 4.7 % of annual amount and summer (March- April) according for less than 22.7 % of the annual total. The shortage of rainfall in paddy rice field is unfavorable for production, the rice growing period is from June (seeding) to November (harvesting), the average rainfall amount during this period in the study area is average 831 mm. The paddy rice need much more water, suitable water consumption average 843 – 1125 mm per one crop (need 1,350-1800 cubic meter of water per crop). In fact, for rained paddy rice, we cannot control the distribution of rainfall, many times the water shorted were occurred and paddy rice field were damaged.

In irrigation paddy rice field, according to sufficient of water from reservoir or other sources, we can control the water supply to the field and possible to fix crop calendar. The crop calendar of irrigated paddy rice field shorter than rained paddy rice field (non irrigated area). For irrigated paddy rice can be planting more than 2 crops per year.

For this perpose the research, to determine crop water consumption or to find evaoptranspiration of both rainfed and irrigated paddy rice field. To compare evapotranspiration which were estimated by Penman-Montieth method and Bowen ratio method. To compare evapotranspiration between rainfed and irrigated paddy rice field. The study area stress at middle of Chao Praya Basin in Sukhthai Province (Rainfed paddy rice) and Phitsanulok Province (irrigaed paddy rice ) during the period year 2002.

METERIALS AND METHODS

Figure 1 shows the studies area are located at Sukhothai Province (for rainfed paddy rice), 450 Km. North from Bangkok (latitude 17-06-59N, longitude 100-25-04 E ), and at Phitsanulok Province(for irrigated paddy rice), 80 Km. East from Sukhothai. (lat: 16-50-10 long: 100-22-55 E, altitude 44 m) . The climate is the tropical monsoon.

Figure 1. The studies area is located at Sukhothai Province (for rainfed paddy rice)

Automatic Weather Stations (AWS) are constructed at both sites, one is 10 meters tower high at rainfed paddy rice Sukhothai Province, another one is constructed at irrigated paddy field in Phitsanulok Province is 6 meters tower high. There are consist of upper and lower wind speed, wind direction, upper and lower wet and dry bulb thermometer (psycrometre), net radionmeter, 3- soil heat flux plate, water temperature thermometer, soil thermometer sensor and raingage.

Data collecting, field data will be collected every 10 minutes average and were stored in data logger. Field data will be converted to daily data by computer.

METHODS

1.  Estimated evapotranspiration based on Penman- Montieth Combination Equation ( Allen et al., 1989, Montieth,1965)

Etp = …Δ….. x(Rn-G) + …..ﻻ….. x Ea

(Δ + ﻻ ) (Δ + ﻻ )

Δ = Slope of saturation vapor pressure –temperature curve

ﻻ = Psychrometric con stant

Rn = Net radiation, G = Ground heat flux

Ea = 0.262(Es – Esd)(1 + 0.0062 U2)

Es = Saturation of vapor pressture of air

Esd = Water vapor pressure in air = RHx Es

U2 = Average wind velocity at 2 metres high/day

2. Estimated evapotranspiration based on Bowen ratio method (Bowen 1942)

Estimated evapotranspiration (Et) in this study are used Bowen ratio technique through sensible heat flux latent heat flux and energy balance;

Rn – G – H – LE = 0

B = H/LE = ﻻ (ΔT/Δe)

Et = (Rn-G)/(1+B)

Rn = Net radiation

G = Ground heat flux

H = Sensible heat flux

LE = Latent heat flux

B = Bowen ratio

ﻻ = Psychrometric contant = cpP/6.22λ

ΔT = Different of up and low air temperature

Δe = Different of vapor pressure up and low level

DISCUSSION AND RESULT

From Figure 2, Evapotranspiration (ET) of irrigated paddy rice much more than no irrigated (rainfed) paddy rice, amount of ET of irrigated paddy rice was 1843 mm and amount of ET of rainfed was 1320 mm. The 2- crops water consumption of irrigated paddy rice were 540 mm and 449 mm, the crop water consumption of rainfed paddy rice (one crop) was 659.3 mm. Amount of rainfall at irrigated paddy rice was 1297 mm and 1331 mm at rainfed paddy rice.

Figure 2. Comparison Et of irrigated & non-irrigated paddy rice by Bowen

Figure 3. Comparison Et of non-irrigated paddy rice by P-M vs. Bowen

Figure 4. Comparison Et of irrigated paddy rice by P-M vs Bowen

From Figure 3, it was shown that ET estimated by Penman-Montieth more than Et estimated by Bowen ratio techniques. Amount of Et by Penman-Montieth(P-M) was 1633.3 mm, by Bowen Ration techniques was only 1320.8 mm. Et by P-M method were very high in dry season (March- May) but by Bowen ratio, Et was high in rainy season.

From figure 4, the amount of Et in both methods were closed value, Et by Bowen ratio method was little higher than by P-M method at 1843 mm for Bowen Ratio and 1756 mm for P-M method. The shape of Et by P-M method same as figure A but shape of Et by Bowen ratio method in figure B different with figure A, Et by Bowen ratio was fluctuated because irrigated paddy rice was plenty of water.

CONCLUSION

1. Evapotranspiration (Et) estimated by Bowen ratio method both irrigated and rainfed paddy rice, Et at irrigated paddy rice was more than rainfed paddy rice, because enough water for plant to growing until harvesting stage.

2. Evapotranspiration which estimated by Penman- Montieth method and Bowen Ratio method at rainfed paddy rice, Et estedmated by Penman – Montieth more than by Bowen ratio approx. 20%

3. Evapotraspiration at irrigation paddy rice, ET by Penman- Montieth method lower than by Bowen method approx. 5%

COMMEND

1. The estimated evapotranspiration by Bowen ratio method need to use accuracy equipments, sensor, especially for the maintenance because raw data will be collecting every minute and its take average every ten minute before collected into data logger, if there are some problem with sensor, estimated Et will be error.

2. Data collecting will be continued more long time to adjust and finding suitable evapotranspiration at paddy rice and its more useful in future.

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