Monitoring Agricultural Drought in Australia - Monitoring and Predicting Agricultural Drought: A Global Study - page 380

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Figure 29.5 Percentage of rural holdings officially drought declared in Queensland and the
percentage of land calculated to be in drought (line) based on thresholds of simulated pasture
growth.

discrepancy in both periods. In this region pastures are of poor quality due
to high rainfall and low soil nutrient status. As a result, cattle numbers
are generally low in relation to the amount of forage available, and ani-
mal condition is influenced more by the length of the growing season than
by the amount of pasture grown per se. Thus, a model based on pasture
growth alone may not adequately represent drought for this region.
In the late 1960s, model calculations also indicated more droughts in
far-western Queensland than were officially declared. In this region pasture
growth is highly variable from one year to the next (coefficient of variation
> 100%), and extremely low pasture growth is more the norm than the
exception. Stocking rates are, in general, extremely light, and some larger
companies have the capacity to move livestock in and out of the region as
seasonal conditions dictate. Hence, industry is more adapted to drought in
this region than in regions with more reliable pasture growth.

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D rought and Pasture Condition Alerts

The GRASP model is run forward in time to estimate the probability of fu-
ture drought. The probability is dependent both on current conditions (e.g.,
soil moisture) and the likelihood of future rainfall. Current conditions are
determined by operationally running the model up to the current month.
The model is then run forward for several months using rainfall and cli-
mate for each past year for which historical rainfall surfaces are available.
A subset of historical years are objectively chosen as being analogous to
the current year in terms of the Southern Oscillation Index (SOI) or global
sea-surface temperatures (SSTs). The likelihood of drought is based on the
proportion of model runs based on these analogue years for which pro-
jected pasture growth for a 12-month period is less than the 10th percentile.
When combined with calculations of pasture utilization by livestock and
kangaroos, the drought alert can be modified to provide pasture condition
alerts as first envisaged by Pressland and McKeon (1990). The alert is trig-
gered when high grazing pressure (i.e., > 30% utilization of pasture grown
over a 12-month period) is likely to occur during the periods of low pasture
growth (i.e., less than the 30th percentile). Such circumstances are likely to
cause losses of perennial grasses and pasture cover.