Jump to content






Photo
- - - - -

Water Math


  • Please log in to reply
19 replies to this topic

#1 JohhnyCash

JohhnyCash

    Veteran

  • Premium Member
  • PipPipPipPip
  • 113 posts

Posted 01 July 2015 - 07:06 AM

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



#2 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 01 July 2015 - 07:31 AM

According to City Data, the numbers reported in April 2015 to the State Water Resources Control Board were:

 

138.6 gallons water use per capita per day

population of 63,376

residential use is 60 percent

 

Good news:  Folsom residents lowered per capita water use from 228.6 to 138.6 over 11 months, a 39.37 percent reduction.

 

Bad news:  Folsom residents still use 52.19 percent more than statewide average.

 

I don't have time to compare these to the numbers JC posted and convert seconds into days, gallons into acre feet, etc.  Maybe someone else with more of an affinity for math can do this.

 

The lake level is down to 408 feet, which is 4 feet lower than this time last year.  I don't think it got down to 408 until July 17 last year.  The lake didn't get to 5 mph until after Labor Day last year.  We'll see this year.



#3 nomad

nomad

    Living Legend

  • Premium Member
  • PipPipPipPipPipPipPipPip
  • 2,548 posts

Posted 01 July 2015 - 07:31 AM

What about evaporation?



#4 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 01 July 2015 - 07:40 AM

What about evaporation?

Good question.  The graph does have an EVAP column and it definitely went up the last week, but nowhere near the outflow levels.

 

If you are talking about bird bath evaporation, I'm sure there is some, but it is in the shade and there was quite a bit of humidity yesterday.  Maybe that's why the evaporation level on the graph wasn't as high yesterday even though it was hotter?



#5 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 01 July 2015 - 08:47 AM

If Folsom uses 20,000 acre feet per year of its 34,000 acre feet allotment, then it took almost 11 days from 6/2 to 6/12 (not 2.7 JC's figure) for the lake to go down the 20,000 acre feet that Folsom supposedly uses in a year, with an average outflow of 2,081 cfs and an average inflow of only 855.  The same dates in 2014 show an average outflow of 2,153, which was higher, but the inflow was also considerably higher at 1,119.

 

While 2,500 might be a healthy flow for the American River in a normal year, I find that kind of outflow alarming in a drought year that might continue into next year.

 

The 5,000 gallon a month assumption also is off a little.  The 138.6 figure works out to 4,158.

 

The 13,441 acre feet JC arrives at also wouldn't be correct according to the article where they say Folsom uses 20,000 acre feet a year, down from 25,000 in 2011.  Maybe forgot to add back in business and city and school use?

 

In the same article in March it was said that our allotment was reduced to 27,000 acre feet in 2014 because of it being a drought year.  When So of 50 is built out, water use was estimated at 30,000 acre feet, which is well below 34,000 acre feet we have in water rights, but could be a problem in a drought year if our allotment is reduced.



#6 JohhnyCash

JohhnyCash

    Veteran

  • Premium Member
  • PipPipPipPip
  • 113 posts

Posted 01 July 2015 - 09:41 AM

Correct.  I made no assumptions about business and city use of water or residential common area irrigation.

 

I thought it might be interesting to have a discussion about the water issue in quantitative terms instead of the "Oh my God the lake is empty!" and "Oh my God all the new people are going to steal all our water!"  terms.

 

I light of the more official numbers presented above it looks like Folsom has enough of a water budget but not with much safety margin as might prefer.  The saving graces could be the requirement to further reduce residential and business water usage, which I think is entirely reasonable, and a future re-assignment of the purpose of the lake from flood protection to a water reservior.

 

JC



#7 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 01 July 2015 - 09:54 AM

Correct.  I made no assumptions about business and city use of water or residential common area irrigation.

 

I thought it might be interesting to have a discussion about the water issue in quantitative terms instead of the "Oh my God the lake is empty!" and "Oh my God all the new people are going to steal all our water!"  terms.

 

I light of the more official numbers presented above it looks like Folsom has enough of a water budget but not with much safety margin as might prefer.  The saving graces could be the requirement to further reduce residential and business water usage, which I think is entirely reasonable, and a future re-assignment of the purpose of the lake from flood protection to a water reservior.

 

JC

 

I don't find it an "Oh, my God, the lake is empty!" or about people are going to steal "our" water.  Seems like the same dismissive tone when people warned we were heading for this and predicted all the dead landscaping we see around town.

 

The quantitative terms need to be accurate in light of the realities of increasing demand without increasing supply.  It won't matter what we have on paper if it's not in the lake.  We are talking about creating a permanent drought.

 

The only thing I agree to is the water storage and flood protection needs a second look in order to let the lake fill as much as possible.



#8 Robert Gary

Robert Gary

    Superstar

  • Premium Member
  • PipPipPipPipPipPip
  • 981 posts

Posted 01 July 2015 - 03:26 PM

Remember that water that goes down your sewage is treated and released down stream in the river, adding to the delta flow, and consequently reducing the amount of outflow required of Folsom lake. 

 

If every Folsom resident stopped using water the only saving would be the lawn/sidewalk evaporation. The water down the sewer is, essentially, 100% reused.

 

-Robert



#9 awood

awood

    All Star

  • Premium Member
  • PipPipPipPipPip
  • 468 posts

Posted 01 July 2015 - 03:48 PM

I believe that the point is, IF there were no dam and siphoning of the water off for other uses, Folsom Lake would provide more than enough water for Folsom residents. 

 

Cliff notes version: Folsom residents use <3 days worth of the water that is currently being pumped / siphoned / allowed to leave Folsom lake every day.

 

My take; Politicians and environmental special interests have hand cuffed us to living under unnecessary restrictions on water use by A.) pumping water where nature never intended it to be AND B.) refusing to use available methods for desalination of water to provide water in areas that nature doesn't provide it.  



#10 Robert Gary

Robert Gary

    Superstar

  • Premium Member
  • PipPipPipPipPipPip
  • 981 posts

Posted 01 July 2015 - 03:52 PM

I believe that the point is, IF there were no dam and siphoning of the water off for other uses, Folsom Lake would provide more than enough water for Folsom residents. 

 

 

I'm not following. Before there was the dam there was no lake. The dam was paid for by the federal gov't for the benefit ( flood protection at the time) of the Sacramento valley. 

 

-Robert



#11 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 01 July 2015 - 05:37 PM

 

I'm not following. Before there was the dam there was no lake. The dam was paid for by the federal gov't for the benefit ( flood protection at the time) of the Sacramento valley. 

 

-Robert

 

True.  Before there was a dam, Folsom had, and still has, pre-1914 senior water rights to the South Fork of the American River, which they kept and the Bureau has to figure out how to fulfill that contract along with all the others AND provide flood protection AND be the go-to for keeping salinity out of the Delta.  The problem seems to be with all the building everyone is using more and more of that contracted water and there simply isn't enough to go around with all these dry years.  I mean, has anyone really added it up if everyone took their full allotment what would happen even in a good year?

 

Then you have the State Water Resources Control Board that decides that they can tell senior water rights holders they are being cut off like those farmers in the Delta.

 

I found this interesting read.  It's a letter from Evert Palmer to Ms. Bean at the SWRCB.

 

www.folsom.ca.us/documents/CityofFolsomCommentLetteronProposedRegulatoryFramework_13April2015.pdf.



#12 Silverado

Silverado

    Superstar

  • Photographer
  • 782 posts
  • Gender:Male
  • Location:Pairie Oaks
  • Interests:Photography, ATVs, Jeepin', the Great Outdoors

Posted 01 July 2015 - 06:04 PM

How about a visual aid.  I did some data mining and came up with this graph.  The dark blue is historical data from CDEC showing lake elevation vs. acre feet.  The thin black line is a 5th order curve fit to the historical data, and the light blue shows the extrapolation down to zero.  The elevation is feet above sea level which ends at 300 ft, which looking at historic maps would be about the top of the river channel before the dam was built.  I guess my point is that the data seems reasonable. From what I recall reading the Folsom intake pipes come out of the water at around 100,000 acre feet which looks to be about 340 feet lake level. Seems like if we got down to 320 feet we'd be pretty much done.

 

Folsom%20Lake%20Level.jpg


The inconsistencies are so compounded as to present a seemingly impossible phenomenon. -Mr. Spock

#13 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 01 July 2015 - 06:45 PM

Thanks for the graph, Silverado.  You are right on.  They were calling it dead pool level at 100,000 AF.

On February 6, 2014, it got down to 383.18 feet and 162,617 AF, which I assume is where the light blue starts on that chart???



#14 JohhnyCash

JohhnyCash

    Veteran

  • Premium Member
  • PipPipPipPip
  • 113 posts

Posted 01 July 2015 - 08:31 PM

One has to be extremely careful about projecting curve fits as they are highly unpredictable beyond one or two periods.
Also a projection like this takes no account of the (unknown) shape of the reservoir.

 

It SEEMS ok to make that projection especially because the curve is so smooth, however caution is warranted.

 

JC


 



#15 ducky

ducky

    untitled

  • Premium Member
  • PipPipPipPipPipPipPipPipPipPip
  • 9,115 posts
  • Gender:Female

Posted 02 July 2015 - 07:03 AM

One has to be extremely careful about projecting curve fits as they are highly unpredictable beyond one or two periods.
Also a projection like this takes no account of the (unknown) shape of the reservoir.

 

It SEEMS ok to make that projection especially because the curve is so smooth, however caution is warranted.

 

JC

 

 

 

I don't understand this post.  Are you saying the CDEC chart is wrong and that the lake level didn't actually drop that low in 2014?

Also, you and awood are using that "it takes less than 3 days" figure and I don't think it's accurate.  

You said:

    "It takes 234,212 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."

 

I can't find a spot on the daily graph where the lake has dropped 20,000 AF in less than 3 days.






0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users