In order to study the effect of irrigation interval on the yield of winter wheat (triticum aestiuum) cultivar Ro-shan an experiment was conducted on the Experimental Farm of the faculty of Agriculture in Karaj in the years 1989-91. Seven treatments, including on irrigation after planting, and the rest six at a 10,30, 40, 60, 80 and 100% of ETm. Et was calcuated on the basis of Blaney and Criddle equation and effective rainfall was estimated using S.C.S method. Irrigation was carried out at 10, 25, 50, 75% soil moisture depletion for each treatment. The results indicated that the amount of irrigation water affected: total biological yield, grain yield, spike length , 1000 grain weight, and other yield factors. Yield varied from 1710 to 2630 Kg ha’. In autumn 1998 all plots were irrigated to field capacity. The plots were next irrigated after depiction of 10, 25, 50 and 75% available moisture. The grain yield was 3384, 3050, 3094 and 2273 Kg/ha respectively. Water consumption effeciency was 1.13, 1.05, 0.82 and 0.86 Kgm3. Grain yield in the second year was 2187, 2630, 2518, 1803, 2136, 1970 and 1710 Kg/ha.
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