Major California reservoirs below 50% capacity as drought wears on

[petronius]

Most of California's major reservoirs are now less than half-full -- or at what officials call a "seriously low" level -- but that's still nowhere near the historic lows set in 1977, the state's driest year on record.

The latest report released Wednesday by the California Department of Water Resources shows 10 of the state's 12 major reservoirs below 50% of their total capacity, with some nearing just 20%.

"They are not historical levels, but they are seriously low," department spokesman Ted Thomas said.

But when all 12 of the major reservoirs are combined, the average is at 60%, Thomas said. That's puts the state in a far better position than it was 37 years ago, when a crippling drought brought the statewide reservoir average down to 41%.

--- Snip ---

http://www.latimes.com/local/lanow/la-me-ln-california-reservoirs-below-capacity-20140723-story.html

Conditions for Major Reservoirs (DWR CDEC)

[antiquie]

1. Big population difference 1977 vs 2014.

1970 19,953,134
1980 23,667,902
1990 29,760,021
2000 33,871,648
2010 37,253,956
2014 38,340,000 est.

I know we are more water-efficient now, especially golf courses, etc. using reclaimed, but still...

[petronius]

2. Excellent point. We may not yet be at the absolute record low point in a

climatological sense, but our vulnerability has certainly changed...

[JayhawkSD]

3. Percentage of capacity does not tell the whole story

"But when all 12 of the major reservoirs are combined, the average is at 60%, Thomas said."

The size of the resevoirs makes a lot of difference, and California's resevoirs vary enormously in size. When you have a tiny little resevoir that is 90% full, and a monstrous big resevoir that is 10% full, they do not average out to give you 50% of your water capacity. That "60% average" statement by Thomas is a bogus and misleading.

If you look at the second link, the biggest resevoirs are at 36%, 37% and 26% full, while down in the southern part of the state Pyramid Lake is at 92% of its teacup-sized capacity. If you add up the total capacities and the total contents, the total percentage of water stored in those "12 major resevoirs" is only 36% of the total capacity.

Edit: I don't know where that water department twit gets that 60%, because if you average the averages you only get 40% full. Still higher than the 35% real number, but nowhere near the 60% that he apparently pulled out of his ass.

[petronius]

4. Nice catch (x 2)! Perhaps he's referring to the average for the date rather than

the capacity; averaging those averages is ~55%. Still not 60%, though. (I'm embarrassed that I missed that; I often have students make that 'unweighted average of the averages' mistake when arguing about grades. )

With your point and antiquie's point above, this adds up to a disappointingly overly-optimistic assessment of the situation...

[JayhawkSD]

6. Actually, I get 40%

If I add all of those averages together and divide by twelve, I get 40%

Not that it really matters, because he’s actually saying something to the effect of “my apple is better than your orange” because the reservoir capacity and population are both just a little bit different now than they were 37 years ago. The population in 1977 was in the close vicinity of 20 million and is about 38 million today, so it has grown something like 90% in the past 37 years. The reservoir capacity has grown from 4300 km3 in 1977 to about 6000 km3 today, or about 40% growth in the same period. These are pretty rough numbers, but they aren't far off.

So, to recap, we have 36% of a capacity which has grown 40% to serve a population which has grown 90% but we are in better shape now than we were when we had 41% of capacity back then. Brilliant.

[petronius]

7. That's how I read it - I think I'll go out and celebrate this good new by

hosing the dust off the sidewalk...

[antiquie]

5. What about the Diamond Lake Reservoir?

It was built to see SoCal through emergencies so I don't think it is included in the figures. When built, it was to keep SoCal from taking too much from the north when the state is dry. I wonder what triggers use of its reserves.