SUMMAR'~
Soil-water regime under long-term wheat and gram sorghum variety trials at Bfloela, central Queensland, was simulated using fallow crop water balance models described in another study.
Analysis of the ratio of estimated actual to potential evapotransplratlon at defined phenological periods m both crops revealed a "critical" ontogenettc period approximating to the time from ear emergence through anthesls to early gram development. The calculated ratios for this "critical" period were more closely correlated with yield than a number of other indices used to characterize the crop water environment.
An index incorporating a,,allable soil-water supply and potential evaporative demand was developed to give more precise definition to the crop water environment at the ~'critical" period. This simple index gave highly significant correlations wLth grain ylelds~ accounting for 60-83 ยฐo of the yield '~armtlon within lndwldual wheat and gram sorghum varieties at Blloela. lntra-seasonal and year-to-year differences in varietal yield, in this environment, appear to be largely a function of anthesls date m relation to the environmental water stress prevailing at that time.
Farm wheat yield data for the period 1952-1965 inclusive were used to test the generality of the wheat-fallow model and stress index at five centres m Queensland extending through 5 : of latitude. Stress index/yield correlations from these data were highly significant at all centres, accounting for 60-70 ยฐ o of total yield varmtlon. The constants in the regression equations for each centre &d not differ significantly, and a combined regression accounted for 66ยฐo of the yleld variation over the whole range of sites.