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Comparison of Various Methods for Estimating Reference Crop Evapotranspiration

Dr. M. H. Ali
SSO
Agril. Engg. Div

MGM Amin
Assist. Prof.
IWM, BAU

AKMR Islam
Assist. Prof.
GTI, BAU

Abstract/ Executive Summary:

The reference crop evapotranspiration (ET₀) was estimated for three locations of Bangladesh using the standard Penman-Monteith (P-M) method as recommended by the Food and Agriculture Organization of the United Nations (FAO). The same was also estimated using four other emperical methods and compared with the standard method. The regression equations developed were evaluated with independent data sets. The superiority order found as: FAO temperature, Radiation, Hargreaves and pan evaporation, respectively. It is revealed from the study that the regression equation developed herein with FAO temperature method can be used to estimate ET₀ more accurately than the original FAO temperature method, as well as regression equations with other methods.

Keywords: Evapotranspiration, Crop water requirement, ET¬0 methods

Location: Three locations of Bangladesh

Start Date: 01-01-2003

End Date: 31-12-2003

Program Area: Crop-Soil-Water Management

Commodity: Irrigation scheduling

Objectives:

The purpose of this study was to compare a few of the most popular (FAO suggested) ET₀ equations against the FAO P–M equation, which is considered the standard approach to define and compute reference crop evapotranspiration (Walter et al., 2000), and to correlate the same with P-M equation.

Methodology:

Selection of ET₀ estimation methods

The methods for estimating ET₀ from the numerous methods reported in the literature were chosen based on the report of Smith et al. (1992). They reported that the P-M method gives the best estimation of ET₀, but also recommended the Blaney-Cridle (referred as FAO temperature), Radiation (FAO radiation), Hargreaves method and Class-A pan evaporation.

Calculation of ET₀

The daily values of the meteorological variables were used to compute reference crop evapotranspiration (ET₀) with Penman-Monteith (ET_P-M), FAO temperature (ET_BC), Hargreaves (ET_HAR) equations. The ET₀ values were calculated using a software package ‘ET₀’, developed by Katholic University of Leuven, Belgium. In P-M method, the model parameter angstrom coefficients a_s and b_s were taken as 0.25 and 0.50, respectively.

Correlating the methods with standard (P-M) method

The computed daily ET₀ values for three years (2001-03) from the different methods were analyzed to find the components of the following linear regression equations:

y = mx + c

y = mx

where y represents ET_PM and x is the ET₀ estimates from the FAO temperature, FAO radiation, Hargreaves methods and pan evaporation; and m and c are constants representing the slope and intercept respectively.

Source of Information: J. Bangladesh Agril. Univ. 2(2): 313-324, (2004)

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The FAO temperature showed lowest bias, root mean square error (RMSE) and relative error (RE) than the other methods. From the simulation graph and performance indicator, it can be concluded that FAO temperature method is best over other methods studied in simulating actual ET₀. Radiation method showed second position and the Hargreaves method showed third position in superiority judgement. The pan evaporation method ranked fourth. From the study, it is revealed that the regression equation developed herein with FAO temperature method can be used to estimate ET₀ more accurately than the original FAO temperature method, as well as regression equations with other methods.

Knowledge Management: Journal

Selection of ET0 estimation methods

Calculation of ET0

Correlating the methods with standard (P-M) method

Selection of ET₀ estimation methods

Calculation of ET₀