The Confluence Water Resources Planning Model

Evaluating Alternative Resource Strategies

The first alternative is to add 2 billion gallons in storage capacity in 2015, at a capital cost of $60 million and an incremental annual fixed operating cost of $3 million.

The second option is to add three increments of desalination capacity at 5 year intervals beginning in 2015.

There are many indicators that will help us choose among these two strategies, including cost and system reliability. This chart shows that the present value costs of the reservoir storage and desalination alternatives are comparable, with the reservoir storage strategy being slightly more expensive than the desalination alternative.

Both alternatives substantially improve the system's ability to serve demands. Rather than the 20% expected peak-season shortage of the base case, both of these strategies reduce this to a more acceptable 7-8% by the end of the planning period.

Based on these two criteria, there appears to be little difference between the two strategies. But a key question is how the two strategies perform if future streamflows and demands are affected by climate change. We would probably want to look at a range of climate change scenarios. This chart shows how expected peak-season shortages are affected by the single scenario we have already defined.

While the expected peak-season shortage of the desalinaton strategy is only affected slightly by climate change, the ability of the reservoir storage strategy to serve future demands is significantly compromised.

Another way to compare the performance of the two strategies if future climate were to change is to look at the peak-season shortage exceedance curves. For example, under the desalination strategy, by the end of the period, there is a 20% probability of a peak-season shortage of at least 750 million gallons.

Under the reservoir storage strategy, this 20% exceedance level almost doubles, to nearly 1.5 billion gallons.