The City’s Water Supply Advisory Committee (WSAC) is trying to analyze strategies that would reduce the City’s water supply shortfall during drought years. The question is, just how big a shortfall are we facing during drought years? Climate changes could result in drought years that are worse than 1977, which is the current benchmark worst-case.
Whatever the WSAC settles on in terms of a future climate change scenario, the good news is that conservation measures are a powerful tool to reduce and even eliminate the shortfall in most drought years. I presented the following information to the WSAC that demonstrates that a small amount of reduction in our water demand has a multiplier effect on reducing our shortfall. That’s because every gallon of water we don’t consume in a normal year is saved in Loch Lomond Reservoir and available in future droughts.
Desal Alternatives has suggested for years that the City’s dramatic reduction in water demand—25% in the ten years ending in 2010—has improved our shortfall picture. But until now we never had access to the Confluence Model spreadsheet the City uses to calculate its shortfall. Access to the Confluence Model allows us to plug in the amount of actual current water demand, instead of using outdated demand estimates. The result is a very encouraging picture of our drought shortfall.
The Outdated Demand Scenario
First let’s look at the large shortfall resulting from the outdated demand estimates. Water Dept. staff presented the graph below to the WSAC. The graph depicts the water supply shortage under stream flow conditions equivalent to 1977. The scenario assumes that the City will leave enough water in the streams and river to satisfy a fish-flow regime proposed by the Department of Fish and Wildlife, known as DFG-5. The demand estimates used in this model are 350 million gallons higher in the months of April-October than actual current demand. Actual demand amounts come from a City memo averaging the demand from 2012 and 2013.
The graph depicts an enormous shortfall during the dry season: the water supply would be 1530 million gallons less than “normal” demand. It’s hard to imagine the City being able to cope with such an enormous shortfall.
Updated Demand Scenario
Here’s what happens when you plug in actual demand into the Model’s spreadsheet. As you would expect, the shortfall diminishes by 350 million gallons in the 1977-type year. But there’s more. You may notice that the graph above shows zero water available from Loch Lomond in the months of June-October—when the reservoir water is needed the most. That’s because in the outdated demand scenario, reservoir levels are depleted over the two previous years.
When you use actual demand, the unused water is saved in Loch Lomond Reservoir and is available for use in the 1977-type year. As a result, the shortfall predicted by the spreadsheet drops from 1530 million gallons to 480 million gallons. This dramatic difference is due to the multiplier effect: a little demand reduction goes a long way.
What would it take to get from 480mg to zero shortfall in a 1977-type year? That would be accomplished by a drop of 160 million gallons from our current peak-season demand, (according to the same multiplier effect).
How realistic is it to reduce our current demand by 160 million gallons? That’s a 7% reduction from the 2012/2013 peak season demand. Compare that to the 25% reduction in demand that City customers accomplished in 2014. A drought such as we just experienced results in lasting changes in behavior, as well as investments in efficient fixtures and appliances. Those changes will be reinforced by a 61% water price increase over the next five years. It seems highly likely that demand will remain at least 7% lower than 2012-13.
I’m not saying that conservation is the only thing we need to do to solve our water supply challenges. On the contrary, Desal Alternatives strongly advocates that the City increase its water storage capacity, by collaborating with neighboring districts in recharging depleted aquifers. If we don’t invest in recharging the aquifers, we could lose a valuable water source to salt water intrusion. That’s unthinkable.
Enjoy the rain!
You can check out the Confluence Model spreadsheet by clicking here.