USING THE GRASSGRO DECISION-SUPPORT TOOL TO EVALUATE SOME OBJECTIVE CRITERIA FOR THE DEFINITION OF EXCEPTIONAL DROUGHT

Some objective criteria for defining conditions of exceptional drought were evaluated using the GrassGro decision support tool. In two analy ses, pasture and animal production were simulated from historical dail y weather records for the last 95 years at sites in central New South Wales. The first analysis successfully discriminated between two sites 40 km apart, only one of which had received an effective fall of rain during a long dry spell. In the second analysis, production was simul ated at one of these sites to identify the exceptional droughts over t he 95-year period from the percentage ranking of moving averages of mo nthly rainfall, available green herbage and the supplementary feed req uired for sheep survival. Greater summer rainfall in the second half o f the period meant that most of the exceptional droughts were in the f irst half. We suggest that a monitoring system based on shire-by-shire simulations of appropriate grazing systems, using 18-month moving ave rages of the weight of supplementary feed required for the survival of the animals, may be a practicable way of establishing exceptional dro ught circumstances. (C) 1998 Elsevier Science Ltd. All rights reserved .