In order to study the effect of moisture defecit on grain yield and seasonal water use of four different sorghum cultivars, an experiment was conducted in the experimental station of Isfahan University situated in Zaghmar, Isfahan, in 1990. A split plot design with three replications in a complete randomised block was used in the study. Main plots were irrigation water application after 100, 150,200 mm evaporation from class A pan represented by 11, 12, 13, respectively. Subplots consisted of four sorghum cultivars, namely, numbers 651,394, 10555 and 18082.
Irrigation frequency in the above irrigated treatments increased from 100 to 200 mm pan evaporation. Grain yield in treatment II (5110.6 Kg/ha) was more than that of 12 and 13. Grain yield. of cultivars in treatment 12 and 13 was 60 and 49.5 percent of Ii, respectively. Increased weight of 1000 grain, under soil moisture depletion, in treatment 12 and 13 mainly contributed to grain yield.. Statistically, grain yield difference between treatment 12 and 13 was not significant; but the differnce between Ii and other irrigated treatments was significant.
The average seasonal water use measuered in tratments Ii, 12 and 13 was 538, 452 and 419.2 mm, respectively. In general, soil moisture defecit in the field caused the seasonal water use of sorghum to decrease and the difference between seasonal water use in irrigated treatments was statistically significant. Also, decreased seasonal water use due to deficient water in the soil decreased seasonal water use due to deficient water in the soil caused a reduction in the water use efficiency in all cultivars; the decrease in the efficiency was not identical, however.
Comparison of measured and estimated seasonal water use under depleting soil water indicated that, apart from crop coefficeint, a coefficient representing soil water depletion should be used when estimating real evapotranspiration. Water requirement for production of one Kg of dry matter in treatment Ii was more than that of 12 and 13.
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