OBJECTIVE CRITERIA FOR ESTIMATING THE SEVERITY OF DROUGHT IN THE WHEAT CROPPING AREAS OF AUSTRALIA

The Australian National Drought Policy requires that exceptional droug hts be determined in a consistent yet regionally sensitive way across different agro-climatic zones. Due to the inadequacy of rainfall defic its in assessing agricultural drought, this paper outlines a modelling approach that uniformly assesses the severity of drought through its impact on shire level production across the Australian wheatbelt. For a defined 'cropping year', an agro-climatic model (STIN) estimated win ter-season wheat yields and an index of summer soil moisture condition s. These were added and weighted (by the proportion of crop grown in w inter and summer) to form a yearly productivity index (YPI). Exception al droughts were defined when sequences of low YPIs occurred in associ ation with low rainfall. A summation of YPI (or yield) deviations belo w an acceptable threshold (40th percentile) formed a drought exception al circumstance index (DECI). This index integrates drought duration a nd severity and utilizes the historical rainfall record to directly ra nk dry cropping years. The varying frequency of severe droughts in dif ferent regions was highlighted by this approach. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.