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Figure 4 Total profit (annual gross margin) over 86-year simulation period for each catchment using different decision methods for pump threshold at the 50th percentile of flow.

• The forecast methods provide a substantial return in gross margin relative to the total achievable gross margin (via the perfect decision model) in each case (on average, 55% of the possible maximum).

To investigate the consistency of the forecast skill, we derived the percent of time during the simulation period during which different income levels were exceeded for each decision model and forecast method. Results for a single catchment (410033) and the 20th percentile pumping threshold are presented in Figure 5.

Several observations can be made about the consistency of the forecasts:

• Negative gross margins (losses) are experienced in a greater number of years for both the average and naïve decision methods (>7% of time) than for any of the seasonal forecast methods (<3.5%).

$800,000

$800,000

$700,000

$600,000

c $500,000

I $400,000

2 $300,000

$300,000

$100,000

120%

Time Exceeded

Figure 5 Exceedance probability for annual gross margin for one catchment (410033) with a pumping threshold at the 20th percentile of flow.

• The naïve and average decision methods give a lower income at almost all exceedance probabilities, and for those areas where they are greater, the difference is very small.

• The naïve decision method gives a greater gross margin for very high gross margin years (2.4% of the time).

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