Monitoring Agricultural Drought in Australia - Monitoring and Predicting Agricultural Drought: A Global Study - page 376

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available in digital format within three months. Maps of rainfall anomalies
for various periods from 24 h to more than 12 months are updated in near-
real time (http://www.bom.gov.au/cgi-bin/climate/rainmaps.cgi). Rainfall
anomalies are mapped as percentiles, as absolute or percentage departures
from the long-term average, and as drought severity levels.
An accepted approach to monitoring meteorological drought in Aus-
tralia is to rank, as a percentile, current rainfall against historical rainfall:
Percentile ( 0-100 ) =

[ ( r − 1 )/( n − 1 ) ] 100 [29.1]

where r is a rank in a series of n values. BoM's drought analysis indicates
regions in which rainfall has been below the 10th percentile (a serious rain-
fall deficiency) or the fifth percentile (a severe rainfall deficiency) for three
months or more. The rainfall deficiency is considered removed if, for the
past month alone, rainfall exceeds the 30th percentile for the three-month
period commencing that month or if, for the past three months, rainfall is
above the 70th percentile. This information is presented both on the Web
(above) and as a monthly publication ( Drought Review-Australia ).

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U se of Agronomic Models to Monitor Droughts

Anecdotal experience of graziers and the results from experimental trials
show that, for different years, different amounts of pasture growth are ex-
perienced with similar amounts of seasonal rainfall (Stafford Smith and
McKeon, 1998). Likewise, the relationship between crop yield and rainfall
during the growing season is generally poor because yield also depends
greatly on the timing of rain as shown, for example, by Nix and Fitz-
patrick (1969). Thus, agronomic models are probably the best indicators
of drought severity and the effectiveness of rain (White et al., 1998). The
following section describes the implementation of a national-scale frame-
work for modeling pasture and crop production in near-real time and the
adaptation of model output for drought monitoring and early warning.

[376

A National Drought Early Warning System
The development of a national drought early-warning system for Australia
commenced with a Queensland prototype in 1991. The need for such a sys-
tem was clear. A major land degradation episode in northeastern Queens-
land in the mid-1980s provided clear evidence that failure to reduce stock
numbers during drought can damage the land and pasture resource. A sur-
vey across northern Australia in 1991 indicated widespread deterioration
in pasture and land condition (Tothill and Gillies, 1992). An improved ca-
pacity for seasonal forecasting based on the ENSO phenomenon provided
hope that the impact of future droughts could be reduced if appropriate ac-
tion was taken in response to early warnings of drought. With the collabo-
ration of several state agencies and funding support from Land and Water
Australia, the Queensland Department of Natural Resources and Mines