A national benchmark for the Australian wheat intdustry: accounting for overlooked climate drivers

Abstract

In Australia wheat is produced in environments of predominantly winter rainfall (Victoria, South Australia and Western Australia), summer rainfall (central and northern New South Wales, and Queensland), and a transition region (central New South Wales) where rainfall is more evenly distributed throughout the year. Regional differences in rainfall patterns (seasonality and size of events) vapour pressure deficit (VPD), mean temperature (T) photosynthetically active radiation (PAR) and fraction of diffuse radiation (FDR) during critical stages in wheat, can dramatically affect potential values of water use efficiency (WUE) across Australia. Models that do not account for these factors, e.g. French and Schultz with fixed parameters, are therefore bound to be biased. In this work we analysed wheat WUE at the shire level for the whole of Australia during the period 1975-2006; we used (a) actual census yields from the Australian Bureau of Statistics, and (b) modelled water use (Oz-Wheat model) to derive WUE = yield/water use, and (c) a climate index accounting for T, VPD, PAR and FDR to produce a measure of actual water use efficiency normalised by these climate drivers (nWUE). The newly developed nWUE index can be used to more fairly compare the regional performance of wheat production across Australia, irrespective of important differences in environmental potential. In general, nWUE from the northern GRDC region is as high as that from the southern or western GRDC regions. Potential causes for lower nWUE values in underperforming regions include the presence of subsoil constraints (e.g. South Australian and Victorian Mallee, south eastern Queensland, southern Western Australia), water logging (e.g. high rainfall zones of Victoria, and southern Western Australia), and management issues in regions with high proportion of mixed grain and livestock farm businesses (e.g. the Western Downs and Maranoa regions in Queensland, and central and eastern New South Wales).