Abstract
Crop evapotranspiration estimation is critical for sustainable water management under the semiarid and arid climate in New Mexico. While irrigation scheduling is based on daily evapotranspiration through the two step approach, reference evapotranspiration and crop coefficients should be determined with accuracy. The utilization of grass reference evapotranspiration (ETo) or alfalfa reference evapotranspiration (ETr) and the conversion from one to the other calls for an understanding of the relationship between ETr and ETo. The objectives of this study were to develop the ratio of ETr to ETo values (Kr) and to determine the variability of Kr during the year for six locations across the State of New Mexico (USA). The results showed long term annual Kr values to vary from 1.28 to 1.45 and the coefficient of determination varied from 0.96 to 0.98. Kr value decreases from January to July and increases from August to December and was at its lowest Kr value at the peak daily reference evapotranspiration. Annual average Kr values were 1.31, 1.38, 1.43, 1.38, 1.38 and 1.48 at Alcalde, Fabian Garcia, Farmington, Leyendecker, Las Cruces and Tucumcari locations and the growing season average Kr values were 1.24, 1.32, 1.38, 1.31, 1.32 and 1.42 at the respective locations. The strong correlation between ETr and ETo allows the conversion of the grass reference evapotranspiration to alfalfa reference evapotranspiration and vice versa however, caution should be taken when using the crop coefficient values to convert either ETo or ETr to actual crop evapotranspiration for irrigation scheduling and water management.
Highlights
While crop actual evapotranspiration could be measured via lysimeter [1], scintillometer [2,3], Bowen ratio energy balance system [4-7], or eddy covariance [6,8-11], among other methods, it is estimated through the two step approach by multiplying crop reference evapotranspiration by the crop coefficients [12,13]
Irmak et al [5] stated that there is no consensus on the choice of reference crop for a particular region they indicated that alfalfa may be preferable for semiarid or arid climates because alfalfa has a vigorous and deeper root structure and tends to transpire water at potential rates even under adventive environments, and is less likely to suffer water stress while grass has a shallow-rooted system and grass crop is subjected to suffer water stress
The strong correlation between ETr and ETo allows the conversion of the grass reference evapotranspiration to alfalfa reference evapotranspiration and vice versa caution should be taken when using the crop coefficient values to convert either ETo or ETr to actual crop evapotranspiration for irrigation scheduling and water management
Summary
Crop water use is generally the main source of water losses from the hydrological cycle and is an important parameter in hydrological, agricultural and environmental studies. Reference evapotranspiration is the rate at which readily available soil water is vaporized from specified vegetated surfaces [12]. Reference evapotranspiration is defined as the ET rate from a uniform surface of dense, actively growing vegetation having specified height and surface resistance, not short of soil water, and representing an expanse of at least 100 m of the same or similar vegetation [14]. Grass and alfalfa are considered as reference crops grown under humid and semiarid/arid climates respectively. Irmak et al [5] stated that there is no consensus on the choice of reference crop for a particular region they indicated that alfalfa may be preferable for semiarid or arid climates because alfalfa has a vigorous and deeper root structure and tends to transpire water at potential rates even under adventive environments, and is less likely to suffer water stress while grass has a shallow-rooted system and grass crop is subjected to suffer water stress. The grass may be preferable under humid, subtropical climates where alfalfa is not commonly grown [5]. Wright et al [16] reported that alfalfa has greater aerodynamic and surface conductance
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