Example 1: Sorghum in Central Queensland
To illustrate seasonal crop forecasting using Whopper Cropper, consider a grower in the Callide valley planning summer cropping options. Let's say it is November and that the SOI phase for September–October was positive (as it was in 1998). The producer has heard that a positive SOI could mean a better-than-usual season, and is considering sowing sorghum earlier than usual. The SOI phase was negative at the same time last year, and a poor season was predicted. The current season is expected to be much more favourable, and may even be good enough to sow maize, which can attract a higher price but is a riskier crop.

The clay soil on this property has a PAWC of 170 mm, and the soil profile is full of moisture after good spring rain. We will assume that a medium-maturing hybrid of sorghum is sown at a density of 10 plants/m² and with total available nitrogen of 100 kg/ha.

A time series analysis shows the year to year variability of yields compared to the long term median, and the years in which the October SOI phase was negative, positive, falling, rising or near zero.

Box plots can be used to analyse the distribution of sorghum yields for each group of analogue years. The distribution of yields for years with a positive phase at the end of September-October has a higher median and lower risk of low yields than the distributions for the other phases.

A box plot analysis can also be used to select the best sowing date based on SOI phase. For a positive phase in September–October, an early sowing in November produces the highest median potential yields, and early sowing is less risky (the distributions are tighter) than sowing in January.

The farmer may decide to sow in November, and then want to know what combination of other management options to use. Whopper Cropper can be used to assess any combination of simulations in the database. For example, a full soil profile and favourable expected seasonal conditions mean that higher sowing densities increase the probability of high potential yields, without significantly increasing risk. The additional costs of increased inputs must also be considered, and future versions of Whopper Cropper will provide a facility to analyse gross margins.

With a November sowing and a density of 10 plants/m², sorghum is potentially a better and less risky crop than maize, at least in terms of yield.