RAINFALL-YIELD RELATIONSHIPS ACROSS THE AUSTRALIAN WHEAT-BELT

A network of rainfall stations was selected across the Australian whea tbelt and monthly rainfall regressed with wheat yields from the surrou nding shires for the period 1976-87. Yields were found to be strongly related to fluctuations in total rainfall amount and the seasonal dist ribution of rainfall through the year. These temporal relationships va ry spatially and appear to be regulated by the water-holding capacity of regional soils. Sixteen agrometeorological zones were defined with similar rainfall-yield relationships. In all these, autumn rains that permit an early sowing, and finishing rains after July, are most impor tant for higher yields. As the rainfall distribution becomes more wint er-dominant, both crop yield variability and the usefulness of high wi nter rainfall decreases. Heavy rainfall in the month after sowing can have a negative effect in southern Australia, as plants are more prone to suffer potential yield losses from a wet soil profile. Waterloggin g has a large negative effect in the south-west of Western Australia, such that the rainfall distribution can be more important than the rai nfall amount. Rainfall-yield correlations are generally more positive in drier regions, and are enhanced by persistent rainfall anomalies be tween April and November during El Nino Southern Oscillation years.