Are you assuming that everything that is in the lake is only for the City of Folsom? That is not the case, which makes the calculation even scarier.
Are you assuming that water will be held back and not released?
Folsom's population is approx. 72,000. According to epa.gov, a family of four uses 400 gallons per day (many of us are probably well below that though in winter months). If you break up our population into families of four, that would be approximately 18,000 households. 18,000 households using 400 gallons per day is 7,200,000 gallons. If an acre foot of water is 325,851 gallons, wouldn't that be 22.10 acre feet a month just for residential households?
I know it doesn't include parks, schools, businesses, but that doesn't seem like much when our contracts add up to 34,000 acre feet (assuming the water is there to take out and not below the intake). The lake level is showing storage of 262,874 acre feet right now.
There's got to be something wrong with my calculations.
OK, I'll bite. I think you forgot to multiply by 30. You started with gallons per day, so 22.1 is acre feet per day. The number would be 663 acre feet per month, or 7,960 acre feet per year. The city has been, on average, treating about 23,000 acre feet per year (the city does not use reclaimed water to my knowledge).Currently, about 65% of city revenue water use has been for residential, the rest for commercial, industrial and parks. That word, revenue, is important, because 25% of water use in the city has been "non-revenue", which includes leakage, construction use, fire-fighting, hydrant flushing and other unknown losses or apparent losses. So, if you remove 25% of the 23,000 acre feet total, that leaves 17,250 acre feet. Sixty-five percent of that would be 11,200 acre feet, which is not too far off of your calculation. Which begs the question of why you are low, given that your assumption of 4 people per household is high (for the south of 50, they assume about 2.8 per household, less for apartments), and your estimate of population was also high (for water purposes, the city does not include the prison population, so they used 65,000). I think the problem is in your 400 gallons per day per household number. That is an average for CA. Folsom use is significantly higher because we have traditionally had non-metered water to go with our hot summers. In Folsom, for single family residential, outdoor use is about double indoor use, and indoor use is about 70 gallons per capia per day. So, that's about 210 gallons per capita, or 588 gallons per day per household (for single family, assuming 2.8 people per household). By comparison, many southern CA cities use less than 150 gallons per capita per day, or 420 gallons per household. So, back to the totals, if you use 65,000 people and 210 gallons per day, that comes out to 15,290 acre feet for residential. This is high because a significant percentage of the population lives in small lot single family or multi-family, which use much less water. So, take the average of the corrected version of your calculation and my version, and, vualla (sp?), you get 11,600 acre feet as the residential use in Folsom, which agrees with the city's numbers. BTW, all this info is taken from the CITY OF FOLSOM 2010 Urban Water Management Plan (although none of the numbers are simple to extract from their tables). Also, please note that the 4,000 or so San Juan water users mess up this calculation and have not been considered. BTW, the other way to look at it is gross water usage, which ignores differences in residential use, and non-residential uses by simply dividing total population by total water usage. This leaves you with about 315 gallons per capita per day (which can be thought of your water use at home and at work, school and everywhere else you go.
So, back to your question about Folsom's measly 34,000 acre feet (of which we currently only use 23,000). The city has 27,000 acre feet of "pre-1914" water rights that are guaranteed by the USBR. The problem is that of the 214,000 acre feet in the lake, a significant percentage of it is below the level of the intake pipes that the city (and San Juan) use. So, it they can't get to it, they can't deliver it, guarantee or not.
