An index of crop water stress related to wheat and grain sorghum yields

Abstract Soil-water regime under long-term wheat and grain sorghum variety trials at Biloela, central Queensland, was simulated using fallow-crop water balance models described in another study. Analysis of the ratio of estimated actual to potential evapotranspiration at defined phenological periods in both crops revealed a “critical” ontogenetic period approximating to the time from ear emergence through anthesis to early grain 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 available 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 with grain yields, accounting for 60–83% of the yield variation within individual wheat and grain sorghum varieties at Biloela. Intra-seasonal and year-to-year differences in varietal yield, in this environment, appear to be largely a function of anthesis date in 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 in Queensland extending through 5° of latitude. Stress index/yield correlations from these data were highly significant at all centres, accounting for 60–70% of total yield variation. The constants in the regression equations for each centre did not differ significantly, and a combined regression accounted for 66% of the yield variation over the whole range of sites.