Development of daily spatial heat unit mapping from monthly climatic surfaces for the Australian continent

In absence of other limitations, the growth rate of a plant is dependent up on the amount of heat it receives. Each species, whether a crop, weed or di sease organism, is adapted to grow at its optimum rate within a specific te mperature range. Within this range, the growing degree days (GDD) is the he at accumulation above a given base temperature for a specific time period, such as a crop's growing season or phenological stage. In this paper we det ail a methodology to predict GDD for synthetically generated average growin g seasons derived from long term average climate data over the Australian c ontinent. An application of these techniques has been made using the GEODAT A 9 second DEM, with temperature threshold values estimated to characterize optimum growth in citrus (Citrus sinensis (L.) Osbeck). Three major determ inants of the annual growth cycle of Citrus sp. were established and predic ted on a spatial basis including the starting day of the growing season, th e GDD for a growing season, and the time required to accumulate an arbitrar ily selected 2000 GDD from the estimated starting day. When these critical environmental factors are expressed on a spatial basis, covering the Austra lian continent, the combination can be used to identify locations where new crop varieties can most effectively be grown to maximize fruit quality and productivity, or to extend the harvest season. Likewise, new germplasm int roduced to Australia from overseas can be horticulturally assessed at sites climatically matched to the source location.