Readers asked a very good question last week about the proposed $2.6 billion reservoir project that would more than double the capacity of Millerton Lake.
Here’s the setup: A new Temperance Flat Reservoir upstream of Millerton would add a net 1.26 million acre-feet of capacity. Millerton’s capacity is 520,000 acre-feet.
The question: So why does this add only an average of 70,000 acre-feet of water to the yield each year? Readers basically were saying: “We thought you just said there would be more than double the capacity.”
They’re right. This is confusing.
First, you need to know one acre-foot is 325,851.4 gallons of water. I call it 326,000 gallons. To most of us, it’s not a small amount of water. Now let’s talk about those big numbers.
The 1.26 million acre-foot figure refers to how much the reservoir would hold at any one time when it’s full. The 70,000 figure is not referring directly to capacity. Instead, it is called additional yield over a one-year period. It is the additional water the bigger reservoir would add to the amount that the project produces each year.
Still confusing, I know.
Think of it this way. The San Joaquin River pours out an average of about 1.7 million acre-feet of snowmelt and rainfall in a year. Most of it just stops briefly at Millerton Lake on its way out to a farm field or down the river channel as part of the restoration.
So if you have a bigger reservoir, you can stop more water, hold it longer and use it in different ways. And that’s where the numbers and the debate take place.