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A Comparative Study of the Commonly Used Methods for Calculation of Evapotranspiration of Rice

Abul Fazal M. Saleh
Associate Professor
Institute of Flood Control and Drainage Research, BUET, Bangladesh

Kaniz Fatema
Assistant Engineer
Bangladesh Water Development Board, Bangladesh

Abstract/ Executive Summary:

Mean monthly evapotranspirations of rice have been calculated by three commonly used prediction methods for ten stations in Bangladesh using long term weather data. The stations selected were representative of the whole country considering their latitudes. The accuracy of a method in predicting the evapotranspiration of rice was tested by calculating the coefficient of efficiency of each method. The results of the study showed that of the three methods, Modified Penmen method gives the best prediction, followed by Radiation and Blaney-Criddle methods.

Keywords: Evapotranspiration, Lysimeter, Rice,

Location: Institute of Flood Control and Drainage Research, BUET, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Water management

Objectives:

The main purpose of this study was to check the accuracy of commonly used methods in estimating the evapotranspiration of rice.

Methodology:

Evapotranspiration can be measured directly or can be predicted indirectly. The principal methods of direct measurement are Lysimeter Experiments, Soil Moisture Depletion Studies and the Water Balance Method. Direct methods are laborious, time consuming and expensive. Prediction methods for evapotranspiration are often used in irrigation planning studies owing to the difficulty ot obtaining accurate field data. The prediction equations can be applied under climatic and agronomic conditions very different from those under which they were originally developed. But, testing the accuracy of the methods under a new set of conditions is essential. The prediction methods use meterological parameters for estimating ET. Several methods are available for computation of ET. Primarily, the choice of a method depends upon the meteorological data available and the needed accuracy of estimation. Of the several prediction methods available for estimating ET. three approaches have been used by different investigators in Bangladesh. They are: 1) Blaney-Criddle method 2) Radiation method, and 3) Modified Penman method. The reference crop evapotranspirations (ETo) for different months were calculated by the three commonly used prediction methods in Bangladesh mentioned in the previous section. For the calculation of ETo (mm/day) by each of these methods, the methodology suggested by Doorenbos and Pruitt (1984). In order to study the effectiveness of the methods used in predicting ET, it is necessary to compare the predictions with the actual ET. But the process is complicated by the fact that in Bangladesh the actual ET is not measured and such data are not commonly found in literature. The only two Iysimeter studies that have been carried out in the past gave grossly erroneous results. Several types of statistics provide useful numerical measures of the degree of agreement between modeled and recorded quantities. One such measure is coefficient of efficiency which can be used for evaluating the results of hydrologic simulations (James and Burges, 1982).

Source of Information: Bangladesh Journal of Water Resource Research Vol. 9, No. 1, December 1988

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The Modified Penman method best predicted the actual ET of rice, followed by Radiation and Blaney-Criddle methods. The Modified Penman method is recommend for the calculation of ET of rice. Even with minimal field data (temperature, wind speed and length of day time hours), the Modified Penman method gave better prediction than the other two methods.

Knowledge Management: Journal