Current flow from the lake is averaging 2500 cfs or 18701 gallons per second.
This is a fairly typical healthy summertime flow for the American River
I assume the average Folsom resident uses 5000 gallons of water per month.
Therefore:
Every 3.208 seconds one year of water supply per Folsom resident flows from the Lake.
The population of Folsom is ~73,000.
It takes 234212 seconds for an entire year's supply for the population of Folsom to flow from the lake.
That's 65 hours or 2.7 days.
Conversely one could say that the draw of water by Folsom reduces the reservoir capacity by 2.7 days per year.
Switching that around to reservoir talk that's 13441 acre feet.
The Capacity of Folsom Lake is 1,010,000 acre feet.
Folsom currently uses 1.3 % of the capacity of Folsom Lake however this says nothing about the total water flow through the system.
In normal rainfall years I suspect the flow through the reservoir is 2-3 times the reservoir capacity.
Given that, one could say that the Folsom water usage of the available water in the reservoir system is somewhere between .43% and .65% in normal rain years, topping out at 1.3% in a drought year like this year.
When the new spillway is finished, hopefully the Bureau of Reclamation will reassign the purpose of Folsom Lake from a flood protection reservoir to a water reservoir. This would allow them to maintain a higher lake level during the rainy season giving an even greater summer time capacity.
JC