Comparison of Irrigation Schedules Based on Pan Evaporation and Growth Stages in Winter Wheat1

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Abstract
Most of the schedules suggested by researchers for irrigating wheat (Triticum aestivum L. em Thell) are not sufficiently simple to be adopted by farmers in general. Recently a more practicable approach based on the ratio of a fixed amount of irrigation water (IW) to pan evaporation, PAN‐E, (cumulative evaporation from US Weather Bureau class A pan less rain since previous irrigation) has been suggested. Several workers have advocated irrigation of wheat at definite growth stages. One possibility of further improving the water use efficiency could be a combination of these two approaches. We compared, in a two year field study, IW/PAN‐E ratios of 0.75 and 0.9 for scheduling irrigation to winter wheat irrespective of growth stage with (i) a combination of these ratios with growth and (ii) irrigation at five growth stages.IW/PAN‐E of 0.75 irrespective of growth stage produced as much grain yield as irrigation at five growth stages. But the former, as an average, received 12 cm less irrigation. There was no gain in the yield by combining the IW/PAN‐E with growth stages. These results indicate that irrigating wheat, sown after a presowing irrigation, on the basis of IW/PAN‐E, irrespective of growth stage, offers a practical means to economize irrigation water without reduction in yield.

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