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Cropet0: a Model for Determination of Reference Crop Evapo-transpiration (et0) from Weather Variables

Dr. M. H. Ali
SSO
Agril. Engg. Division, BINA, Mymensingh 2202, Bangladesh

Abstract/ Executive Summary:

An accurate estimate of crop water requirement is of vital importance in planning, design and operation of irrigation and water resources systems. Relationships between the evapo-transpiration of a pre-selected crop, which is referred to as reference evapo-transpiration (ET₀), and other crops are established multiplying ET₀ by crop coefficients. The Penman-Monteith (P-M) equation with its new definition of ET₀ is recommended by FAO experts as the standard method of ET₀calculation. A model, called “ CropET₀ ” was developed to calculate monthly ET₀ using the FAO Penmen-Monteith approach. The model was developed in Microsoft Excel, which is flexible in use and user friendly. The ‘CropET₀’ calculate the monthly ET₀ similar to that of ‘CROPWAT’ model already developed by FAO.

Keywords: Penman-Monteith equation, CropET0, Reference evapotranspiration, CROPWAT

Location:

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Irrigation scheduling

Objectives:

The objective of this article is to report a ET₀ model, called “ CropET0 ”, which is simple but efficient to calculate monthly ET₀ values.

Methodology:

The model “CropET0” was developed following the FAO Penman-Monteith approach as described by Smith et al. (1992) and Allen et al. (1998).

Model Input

The basic input of the model are the monthly average values of the weather variables (i) maximum temperature, (ii) minimum temperature, (iii) humidity, (iv) wind speed, (v) sunshine hour, (vi) latitude, and (vii) altitude of the location. For north latitude, the value would be positive, and for south latitude, the value would be negative.

The user can adjust the parameter ‘Angstrom coefficients’ (used in solar energy calculation) if the local calibrated values are available. Average values (a_s = 0.25, b_s = 0.50) as suggested by FAO can be used if local calibrated values are not available.

Input data should be provided through ‘Model Interface’, a data input platform.

Model Output

There is dynamic interaction between input and output in ‘CropET₀’. The ‘CropET₀’ produces:

The monthly average reference evapotranspiration (mm/d) throughout the months of the year; and
Display graph. As the model output ET₀ values are linked to a graphical display, the display graph respond instantaneously with the input value .

Source of Information: Bangladesh J. of Agril. Engg., 16(1 & 2): 25 - 37, (2005)

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Based on the results of statistical analyses and the graphical display of CROPWAT output versus ‘CropET₀’ output value, it can be concluded that ‘CropET₀’ was successful in calculating reference evapotranspiration accurately, and hence can be used to calculate reference crop evapotranspiration for crop water requirement estimation purpose.

Knowledge Management: Journal