[threepointer]

This is a workbook that I developed over the years. It shows the update from the prior update to the current update (today). The reservoir levels are updated and compared to capacity and average in the table.
As far as the spectrum goes: Blue is Excellent, Dark Green is Very Good, Light Green is Good......all the way down to Orange which is very bad. The small table on the upper right is the color coding legend.

Interesting to note in the small images on the right.  The proportionate share of capacity by region of these reservoirs across the state is:  North 55.7%, South 41.4%, South 2.9%.  There are another approximately 175 additional reservoirs around the state.  In total they have about 1/2 of the capacity of the 48 reservoirs on this table.  I assume that they are distributed in the same manner as the 48 reservoirs.  This helps place in context the water situation in Southern California where the majority of Californians live.

I should note that I posted this earlier with another image hosting site (Imgbox).  I was alerted by a Cardboard poster that my post included images of "very busty young ladies in very tight tops" below the post.  These may have been inserted by the image hosting site.  I am using a different image site this time(ImbBB).  If you are looking for very busty young ladies in very tight tops, try Google.

[img][/img]

[threepointer]

This is a workbook that I developed over the years. It shows the update from the prior update to the current update (today). The reservoir levels are updated and compared to capacity and average in the table.
As far as the spectrum goes: Blue is Excellent, Dark Green is Very Good, Light Green is Good......all the way down to Orange which is very bad. The small table on the upper right is the color coding legend.

Interesting to note in the small images on the right.  The proportionate share of capacity by region of these reservoirs across the state is:  North 55.7%, South 41.4%, South 2.9%.  There are another approximately 175 additional reservoirs around the state.  In total they have about 1/2 of the capacity of the 48 reservoirs on this table.  I assume that they are distributed in the same manner as the 48 reservoirs.  This helps place in context the water situation in Southern California where the majority of Californians live.

I should note that I posted this earlier with another image hosting site (Imgbox).  I was alerted by a Cardboard poster that my post included images of "very busty young ladies in very tight tops" below the post.  These may have been inserted by the image hosting site.  I am using a different image site this time(ImbBB).  If you are looking for very busty young ladies in very tight tops, try Google.

[img][/img]

[Mick]

After March's storms, California's reservoirs are at 116% of the historical amount through this time of year.

How California Reservoir Levels Stand After Storms

Question for threepointer and any hydrologist/technologist familiar with desalinization. The general idea is to create water farms far from shore (at least five miles) so as not to disturb the local, near-shore environment. The water is cleaner that far out and would require less energy because it requires considerably less water to produce a like amount of fresh water.

Does this appear valid to you? A substantially unique approach to desalinization at greater efficiency, lower cost, fewer harmful by-products? Does it pass the smell test?

New desalination technology being tested in California could lower costs of tapping seawater

[Mick]

After March's storms, California's reservoirs are at 116% of the historical amount through this time of year.

How California Reservoir Levels Stand After Storms

Question for threepointer and any hydrologist/technologist familiar with desalinization. The general idea is to create water farms far from shore (at least five miles) so as not to disturb the local, near-shore environment. The water is cleaner that far out and would require less energy because it requires considerably less water to produce a like amount of fresh water.

Does this appear valid to you? A substantially unique approach to desalinization at greater efficiency, lower cost, fewer harmful by-products? Does it pass the smell test?

New desalination technology being tested in California could lower costs of tapping seawater

[BostonCard]

California evidently has a water trifecta:

https://phys.org/news/2025-04-years-cali...dium=email

The near-average snowpack has given the state a third straight year of ample water supplies in the mountains—something that hasn't happened in a quarter of a century.

BC

[BostonCard]

California evidently has a water trifecta:

https://phys.org/news/2025-04-years-cali...dium=email

The near-average snowpack has given the state a third straight year of ample water supplies in the mountains—something that hasn't happened in a quarter of a century.

BC

[Mick]

The Colorado River's underwater basin has been seriously depleted. It has lost an amount equivalent to Lake Mead (U. S. largest reservoir) since 2003.

The Colorado River is running low. The picture looks even worse underground, study says.

---

https://cdec.water.ca.gov/

If you access the link, go to the daily statewide reservoir report.

The "water year" runs from October 1 through the following September 30, so this is the eight-month update after the new year. Reservoir capacity updated for May 28, 2025 (using threepointer's scale). Listed with % of Average to date, and the % change from two months ago:

Large (1 MM+ AF):
Oroville, 99% (+30%) / 121% / (-3%) 
Shasta, 92% (+15%) / 110% / (-20%)
Pine Flat, 85% (39%) / 124% / (-8%)
San Luis, 67% (-2%) / 94% / (-15%)
New Melones, 81% (+4%) / 129% / (-10%)
Trinity, 92% (+17) / 117% / (-7%) 
Don Pedro, 93% (+24%) / 117% / (-16%)  
Lake McClure, 92% (+29%) / 139% / (-5%)  
Berryessa, 95% (+2%) / 115% / (-11%) 
Almanor, 83% (+10%) / 110% / (-7%) 

Medium (300k - 999k AF, where available)
Folsom, 94% (+57%) / 117% / (+28%) 
New Bullards, 98% (+21%) / 111% / (-12%)
Diamond Valley, 96% (-1%) / 127% / (-9%)
Millerton Lake, 89% (+45%) / 115% / (+32%)
Isabella, 61% (+34%) / 130% / (+22%) 
Lake San Antonio, 71% (0%) / 121% (-32%)
Lake Tahoe/Tahoe City, 90% (+25%) / 180% / (-26%) 
Nacimiento, 60% (+8%) / 99% / (-38%) 
New Hogan, 66% (+14%) / 121% / (-21%)
Castaic, 95% (+19%) / 109% / (+10%) 
Clear Lake, 100% (+22%) / 129% / (-73%)

SF Bay Reservoirs (and of local interest, where available)
Hetch Hetchy, 97% (+30%) / 120% / (+17%)
Calaveras, 79% (+1%) / 149% / (-33%)
Del Valle, 52% (+18%) / 100% / (+10%)
Lexington Reservoir, 66% (+22%) / 99% / (-24%)
San Antonio Reservoir, 71% (-27%) / 121% / (+5%)
Crystal Springs Reservoir, 65% (-9%) / 86% / (-18%)

[Mick]

The Colorado River's underwater basin has been seriously depleted. It has lost an amount equivalent to Lake Mead (U. S. largest reservoir) since 2003.

The Colorado River is running low. The picture looks even worse underground, study says.

---

https://cdec.water.ca.gov/

If you access the link, go to the daily statewide reservoir report.

The "water year" runs from October 1 through the following September 30, so this is the eight-month update after the new year. Reservoir capacity updated for May 28, 2025 (using threepointer's scale). Listed with % of Average to date, and the % change from two months ago:

Large (1 MM+ AF):
Oroville, 99% (+30%) / 121% / (-3%) 
Shasta, 92% (+15%) / 110% / (-20%)
Pine Flat, 85% (39%) / 124% / (-8%)
San Luis, 67% (-2%) / 94% / (-15%)
New Melones, 81% (+4%) / 129% / (-10%)
Trinity, 92% (+17) / 117% / (-7%) 
Don Pedro, 93% (+24%) / 117% / (-16%)  
Lake McClure, 92% (+29%) / 139% / (-5%)  
Berryessa, 95% (+2%) / 115% / (-11%) 
Almanor, 83% (+10%) / 110% / (-7%) 

Medium (300k - 999k AF, where available)
Folsom, 94% (+57%) / 117% / (+28%) 
New Bullards, 98% (+21%) / 111% / (-12%)
Diamond Valley, 96% (-1%) / 127% / (-9%)
Millerton Lake, 89% (+45%) / 115% / (+32%)
Isabella, 61% (+34%) / 130% / (+22%) 
Lake San Antonio, 71% (0%) / 121% (-32%)
Lake Tahoe/Tahoe City, 90% (+25%) / 180% / (-26%) 
Nacimiento, 60% (+8%) / 99% / (-38%) 
New Hogan, 66% (+14%) / 121% / (-21%)
Castaic, 95% (+19%) / 109% / (+10%) 
Clear Lake, 100% (+22%) / 129% / (-73%)

SF Bay Reservoirs (and of local interest, where available)
Hetch Hetchy, 97% (+30%) / 120% / (+17%)
Calaveras, 79% (+1%) / 149% / (-33%)
Del Valle, 52% (+18%) / 100% / (+10%)
Lexington Reservoir, 66% (+22%) / 99% / (-24%)
San Antonio Reservoir, 71% (-27%) / 121% / (+5%)
Crystal Springs Reservoir, 65% (-9%) / 86% / (-18%)

[Mick]

We're doing okay in CA with water. Other states, particularly those depending on Colorado River water, not so much. Groundwater in the region has vanished equivalent to the amount of Lake Mead, 27.8 million acre-feet.

A Lake Mead's worth of water has vanished from the ground. Could Las Vegas suffer? | Science & Technology News

[Mick]

We're doing okay in CA with water. Other states, particularly those depending on Colorado River water, not so much. Groundwater in the region has vanished equivalent to the amount of Lake Mead, 27.8 million acre-feet.

A Lake Mead's worth of water has vanished from the ground. Could Las Vegas suffer? | Science & Technology News

[Mick]

Clear Lake under stress. Cyanotoxins found in 56% of homes in the area. Needs a dozen projects, cost over $15 mms. to restore the lake.

California's oldest lake is in crisis

[Mick]

Clear Lake under stress. Cyanotoxins found in 56% of homes in the area. Needs a dozen projects, cost over $15 mms. to restore the lake.

California's oldest lake is in crisis

[GK3]

Clear Lake under stress. Cyanotoxins found in 56% of homes in the area. Needs a dozen projects, cost over $15 mms. to restore the lake.

California's oldest lake is in crisis

This is, I am afraid, only the tip of the iceberg. I takes political will and MONEY to fix these problems and both seem in short supply now days.  The Clear Lake issue is indeed a problem, but but I think the Colorado River issue is far worse. Something like 40 million people depend on the Colorado River for water.  If the aquifer remains depleted and the river flow remains low, I do not see how this can all work out well. We have kicked the can down the road for years and it does not seem that the anyone except those that live there care.

[GK3]

Clear Lake under stress. Cyanotoxins found in 56% of homes in the area. Needs a dozen projects, cost over $15 mms. to restore the lake.

California's oldest lake is in crisis

This is, I am afraid, only the tip of the iceberg. I takes political will and MONEY to fix these problems and both seem in short supply now days.  The Clear Lake issue is indeed a problem, but but I think the Colorado River issue is far worse. Something like 40 million people depend on the Colorado River for water.  If the aquifer remains depleted and the river flow remains low, I do not see how this can all work out well. We have kicked the can down the road for years and it does not seem that the anyone except those that live there care.

[cctop]

Now I'm glad I didn't buy that attractively-priced home on Clear Lake twenty years ago.

In related water news, there were good numbers this year for groundwater recharge:

SF Chronicle: California groundwater levels see another bump -- but longterm trend still grim

[cctop]

Now I'm glad I didn't buy that attractively-priced home on Clear Lake twenty years ago.

In related water news, there were good numbers this year for groundwater recharge:

SF Chronicle: California groundwater levels see another bump -- but longterm trend still grim

[Leftcoast]

Paywalled.  If you have time can you give us a quick summary?  No problem if busy!

Now I'm glad I didn't buy that attractively-priced home on Clear Lake twenty years ago.

In related water news, there were good numbers this year for groundwater recharge:

SF Chronicle: California groundwater levels see another bump -- but longterm trend still grim

[Leftcoast]

Paywalled.  If you have time can you give us a quick summary?  No problem if busy!

Now I'm glad I didn't buy that attractively-priced home on Clear Lake twenty years ago.

In related water news, there were good numbers this year for groundwater recharge:

SF Chronicle: California groundwater levels see another bump -- but longterm trend still grim

[Mick]

high points:

1) Second straight year groundwater wasn't drawn down.
2) Aquifers gained 2.2 million acre-feet of water across dozens of groundwater basins tracked by the state.
3) That's about half of what can be held in CA's largest reservoir, Shasta.
4) Ground had been sinking most in the agricultural heartland, the San Joaquin Valley and in Tulare Lake area.
5) Groundwater regulation was introduced just 10 years ago.
6) Groundwater accounts for 40% of CA's normal consumption, but 60% in dry years.
7) CA water agencies collectively put 1.9 million acre feet back in aquifer recharge between 2023 and 2024. The year prior, 4.1 million acre feet were returned by CA to aquifers (2022/3) which resulted in a net 8.7 million acre feet returned.
8) Controversial project is the Delta Conveyance Project, $20 billion, 45 mile tunnel used to send North CA water to South CA. It is proposed, needed, and bitterly opposed.

[Mick]

high points:

1) Second straight year groundwater wasn't drawn down.
2) Aquifers gained 2.2 million acre-feet of water across dozens of groundwater basins tracked by the state.
3) That's about half of what can be held in CA's largest reservoir, Shasta.
4) Ground had been sinking most in the agricultural heartland, the San Joaquin Valley and in Tulare Lake area.
5) Groundwater regulation was introduced just 10 years ago.
6) Groundwater accounts for 40% of CA's normal consumption, but 60% in dry years.
7) CA water agencies collectively put 1.9 million acre feet back in aquifer recharge between 2023 and 2024. The year prior, 4.1 million acre feet were returned by CA to aquifers (2022/3) which resulted in a net 8.7 million acre feet returned.
8) Controversial project is the Delta Conveyance Project, $20 billion, 45 mile tunnel used to send North CA water to South CA. It is proposed, needed, and bitterly opposed.

[cctop]

Regarding point #4, if you search for "San Joaquin valley sinking" there are a lot of interesting articles, and the first result is an article from the Stanford Report last year.

My understanding is that it's like we're living on a sponge that compresses as we remove water from it, and that some amount of the compression is permanent as the soil compacts.

In case you've never seen it, here's a famous photo that purports to show the amount of subsidence at the maximum spot in the valley.

[cctop]

Regarding point #4, if you search for "San Joaquin valley sinking" there are a lot of interesting articles, and the first result is an article from the Stanford Report last year.

My understanding is that it's like we're living on a sponge that compresses as we remove water from it, and that some amount of the compression is permanent as the soil compacts.

In case you've never seen it, here's a famous photo that purports to show the amount of subsidence at the maximum spot in the valley.

[M T]

Another way to think of it is that we live on layers of granular material (think sand) that tends to settle/compress over time.  Water (or other incompressible fluid) in that material, trapped in pockets, will perhaps change how fast it compresses. 

It seems to me that underground "rivers" or reservoirs where the water can flow in (and, thus, can flow out) shouldn't change that compression rate much since, under pressure, the water will flow out.  Of course, the pressure it takes the fluid to flow through the material will contribute to a difference in pressure of the fluid in different regions.

[M T]

Another way to think of it is that we live on layers of granular material (think sand) that tends to settle/compress over time.  Water (or other incompressible fluid) in that material, trapped in pockets, will perhaps change how fast it compresses. 

It seems to me that underground "rivers" or reservoirs where the water can flow in (and, thus, can flow out) shouldn't change that compression rate much since, under pressure, the water will flow out.  Of course, the pressure it takes the fluid to flow through the material will contribute to a difference in pressure of the fluid in different regions.

[Leftcoast]

high points:

1) Second straight year …..

Thanks for the summary, Mick

[Leftcoast]

high points:

1) Second straight year …..

Thanks for the summary, Mick

[Mick]

A Yolo County judge rejected a lawsuit brought by the Sierra Club, Center for Biological Diversity and other groups who claimed that the Sites Reservoir environmental impact report was inadequate and needed to be redone.

The $4.5 billion Sites Reservoir will be 70 miles NW of Sacto, and will provide water to 500k acres of Central Valley farmlands and 24 million people, including residents of Santa Clara County, parts of the East Bay and Los Angeles. It would store water diverted from the Sacto River in wet years.

The project was supported by Pres Biden, Gov Newsom, Farm organizations, organized labor and 20+ water agencies. It would be the largest new reservoir in California (8th largest in the state) since 1979.

https://www.mercurynews.com/2024/06/05/c...al-groups/

The $4.5 billion Sites Reservoir is now the $6.2 billion Sites Reservoir.
Massive California reservoir to cost billions more than expected

First major reservoir newly constructed in CA since 1979. 38% increase in cost attributed to "inflation." Construction to begin next year, finished by 2033. 13 miles long, 4 miles wide.

[Mick]

A Yolo County judge rejected a lawsuit brought by the Sierra Club, Center for Biological Diversity and other groups who claimed that the Sites Reservoir environmental impact report was inadequate and needed to be redone.

The $4.5 billion Sites Reservoir will be 70 miles NW of Sacto, and will provide water to 500k acres of Central Valley farmlands and 24 million people, including residents of Santa Clara County, parts of the East Bay and Los Angeles. It would store water diverted from the Sacto River in wet years.

The project was supported by Pres Biden, Gov Newsom, Farm organizations, organized labor and 20+ water agencies. It would be the largest new reservoir in California (8th largest in the state) since 1979.

https://www.mercurynews.com/2024/06/05/c...al-groups/

The $4.5 billion Sites Reservoir is now the $6.2 billion Sites Reservoir.
Massive California reservoir to cost billions more than expected

First major reservoir newly constructed in CA since 1979. 38% increase in cost attributed to "inflation." Construction to begin next year, finished by 2033. 13 miles long, 4 miles wide.

[Maple Leaf]

https://sjvwater.org/boswell-silent-and-...85c59b2ed0

On a related issue regarding California Water situation the farming company J.G Boswell is planning to continue with the pumping of underground water to the extent that it could drop the Tulare Lake Bed by another ten (10) feet and the small town of Corcoran by a similar amount.

[Maple Leaf]

https://sjvwater.org/boswell-silent-and-...85c59b2ed0

On a related issue regarding California Water situation the farming company J.G Boswell is planning to continue with the pumping of underground water to the extent that it could drop the Tulare Lake Bed by another ten (10) feet and the small town of Corcoran by a similar amount.

[M T]

https://sjvwater.org/boswell-silent-and-...85c59b2ed0

On a related issue regarding California Water situation the farming company J.G Boswell is planning to continue with the pumping of underground water to the extent that it could drop the Tulare Lake Bed by another ten (10) feet and the small town of Corcoran by a similar amount.

Hmmm …. I thought satellites can measure land elevation quite accurately. If not, then maybe drones can.  I don’t understand why they need that company to provide the data on the land elevation.

[M T]

https://sjvwater.org/boswell-silent-and-...85c59b2ed0

On a related issue regarding California Water situation the farming company J.G Boswell is planning to continue with the pumping of underground water to the extent that it could drop the Tulare Lake Bed by another ten (10) feet and the small town of Corcoran by a similar amount.

Hmmm …. I thought satellites can measure land elevation quite accurately. If not, then maybe drones can.  I don’t understand why they need that company to provide the data on the land elevation.

[Leftcoast]

For those who live farther away, this has been a welcome cool Summer in much of California and the Pacific Northwest.

This late July day is an example, Bay Area cities are showing highs in the 60s and 70s while the Central Valley (Fresno, Modesto, Sacramento) sees highs in the 80s.  We're being spoiled by unseasonably moderate temperatures.  I wonder how much this slows reservoir evaporation and field water usage?

[Leftcoast]

For those who live farther away, this has been a welcome cool Summer in much of California and the Pacific Northwest.

This late July day is an example, Bay Area cities are showing highs in the 60s and 70s while the Central Valley (Fresno, Modesto, Sacramento) sees highs in the 80s.  We're being spoiled by unseasonably moderate temperatures.  I wonder how much this slows reservoir evaporation and field water usage?

[Mick]

For those who live farther away, this has been a welcome cool Summer in much of California and the Pacific Northwest.

This late July day is an example, Bay Area cities are showing highs in the 60s and 70s while the Central Valley (Fresno, Modesto, Sacramento) sees highs in the 80s.  We're being spoiled by unseasonably moderate temperatures.  I wonder how much this slows reservoir evaporation and field water usage?

I asked premium chatGPT that question. It depends. High humidity reduced evaporation rate. High winds accelerate evaporation. Reply was that a 19 degree increase in the temperature roughly doubles the evaporation rate. In an outdoor water reservoir:

At 15°C (59°F), evaporation might be ~2 mm/day.
At 30°C (86°F), it might rise to ~5–6 mm/day—more than double, depending on humidity and wind.

Evaporation is not linear, it spikes at high temperature. Evaporation depends (of course) on surface area. Here's the impact on the five largest CA reservoirs:

Estimated Impact on California's Largest Reservoirs
Reservoir Surface Area (acres) Current Evap. Loss (inches/year) Estimated Increase (inches/year) New Est. Evap. Loss (acre-feet/year)
Shasta ~29,500 ~70 ~133 ~326,500
Oroville ~15,800 ~70 ~133 ~174,500
Trinity ~17,200 ~65 ~124 ~177,700
New Melones ~12,500 ~65 ~124 ~129,200
San Luis ~12,700 ~80 ~152 ~160,900

So a 19 degree increase will result in evaporation rates increasing by 90% to 110%, with additional water loss of from 480,000 to 600,000 acre-feet annually in those five reservoirs. An acre foot is 326,000 gallons, so 600k acre feet is 195 billion gallons, enough water for around 4 million households.

[Mick]

For those who live farther away, this has been a welcome cool Summer in much of California and the Pacific Northwest.

This late July day is an example, Bay Area cities are showing highs in the 60s and 70s while the Central Valley (Fresno, Modesto, Sacramento) sees highs in the 80s.  We're being spoiled by unseasonably moderate temperatures.  I wonder how much this slows reservoir evaporation and field water usage?

I asked premium chatGPT that question. It depends. High humidity reduced evaporation rate. High winds accelerate evaporation. Reply was that a 19 degree increase in the temperature roughly doubles the evaporation rate. In an outdoor water reservoir:

At 15°C (59°F), evaporation might be ~2 mm/day.
At 30°C (86°F), it might rise to ~5–6 mm/day—more than double, depending on humidity and wind.

Evaporation is not linear, it spikes at high temperature. Evaporation depends (of course) on surface area. Here's the impact on the five largest CA reservoirs:

Estimated Impact on California's Largest Reservoirs
Reservoir Surface Area (acres) Current Evap. Loss (inches/year) Estimated Increase (inches/year) New Est. Evap. Loss (acre-feet/year)
Shasta ~29,500 ~70 ~133 ~326,500
Oroville ~15,800 ~70 ~133 ~174,500
Trinity ~17,200 ~65 ~124 ~177,700
New Melones ~12,500 ~65 ~124 ~129,200
San Luis ~12,700 ~80 ~152 ~160,900

So a 19 degree increase will result in evaporation rates increasing by 90% to 110%, with additional water loss of from 480,000 to 600,000 acre-feet annually in those five reservoirs. An acre foot is 326,000 gallons, so 600k acre feet is 195 billion gallons, enough water for around 4 million households.

[Leftcoast]

Thanks, Mick.  Useful.

[Leftcoast]

Thanks, Mick.  Useful.

[M T]

For those who live farther away, this has been a welcome cool Summer in much of California and the Pacific Northwest.

This late July day is an example, Bay Area cities are showing highs in the 60s and 70s while the Central Valley (Fresno, Modesto, Sacramento) sees highs in the 80s.  We're being spoiled by unseasonably moderate temperatures.  I wonder how much this slows reservoir evaporation and field water usage?

I asked premium chatGPT that question. It depends. High humidity reduced evaporation rate. High winds accelerate evaporation. Reply was that a 19 degree increase in the temperature roughly doubles the evaporation rate. In an outdoor water reservoir:

At 15°C (59°F), evaporation might be ~2 mm/day.
At 30°C (86°F), it might rise to ~5–6 mm/day—more than double, depending on humidity and wind.

Evaporation is not linear, it spikes at high temperature. Evaporation depends (of course) on surface area. Here's the impact on the five largest CA reservoirs:

Estimated Impact on California's Largest Reservoirs
Reservoir Surface Area (acres) Current Evap. Loss (inches/year) Estimated Increase (inches/year) New Est. Evap. Loss (acre-feet/year)
Shasta ~29,500 ~70 ~133 ~326,500
Oroville ~15,800 ~70 ~133 ~174,500
Trinity ~17,200 ~65 ~124 ~177,700
New Melones ~12,500 ~65 ~124 ~129,200
San Luis ~12,700 ~80 ~152 ~160,900

So a 19 degree increase will result in evaporation rates increasing by 90% to 110%, with additional water loss of from 480,000 to 600,000 acre-feet annually in those five reservoirs. An acre foot is 326,000 gallons, so 600k acre feet is 195 billion gallons, enough water for around 4 million households.

Why 19? Why not 119 degree increase?  Without some basis in reality, the numbers can be made anything.  And, obviously, the starting point matters.   Regarding the "estimated impact":  What is impacting?   Some non-realistic scenario?   GI,GO.

In addition to the air temperature and wind speed (both of which vary throughout the day), the water temperature matters (which probably doesn't vary much in a day).  Other factors are going to be involved (surface area (less in summer/fall; more in winter), solar radiation, water flow or turnover, etc)

Here's an available study of evaporation of water into the atmosphere in California. That's not reservoir evaporation though, but all evaporation.  Figure 6 and 7 are interesting.  Precipitation is the primary factor, but evaporation is buffered.  Total evaporation is (if I read it correctly) 160.8 cubic kilometers, or about 130.4 million acre-feet, per year.

This article indicates (if I read it correctly) is that the current evaporation losses from reservoirs in California is 1.728 x 10^9  cubic meters (ie, 1.728 cubic kilometers), or about 1,400,000 acre-feet, per year.

I'm not sure whether I understand that 1% of annual evaporation is from the reservoirs.  My gut reaction is that is high.

An interesting, related 2019 article
Those 96 Million Black Balls in LA's Reservoir Are Not Just There to Save Water

[M T]

For those who live farther away, this has been a welcome cool Summer in much of California and the Pacific Northwest.

This late July day is an example, Bay Area cities are showing highs in the 60s and 70s while the Central Valley (Fresno, Modesto, Sacramento) sees highs in the 80s.  We're being spoiled by unseasonably moderate temperatures.  I wonder how much this slows reservoir evaporation and field water usage?

I asked premium chatGPT that question. It depends. High humidity reduced evaporation rate. High winds accelerate evaporation. Reply was that a 19 degree increase in the temperature roughly doubles the evaporation rate. In an outdoor water reservoir:

At 15°C (59°F), evaporation might be ~2 mm/day.
At 30°C (86°F), it might rise to ~5–6 mm/day—more than double, depending on humidity and wind.

Evaporation is not linear, it spikes at high temperature. Evaporation depends (of course) on surface area. Here's the impact on the five largest CA reservoirs:

Estimated Impact on California's Largest Reservoirs
Reservoir Surface Area (acres) Current Evap. Loss (inches/year) Estimated Increase (inches/year) New Est. Evap. Loss (acre-feet/year)
Shasta ~29,500 ~70 ~133 ~326,500
Oroville ~15,800 ~70 ~133 ~174,500
Trinity ~17,200 ~65 ~124 ~177,700
New Melones ~12,500 ~65 ~124 ~129,200
San Luis ~12,700 ~80 ~152 ~160,900

So a 19 degree increase will result in evaporation rates increasing by 90% to 110%, with additional water loss of from 480,000 to 600,000 acre-feet annually in those five reservoirs. An acre foot is 326,000 gallons, so 600k acre feet is 195 billion gallons, enough water for around 4 million households.

Why 19? Why not 119 degree increase?  Without some basis in reality, the numbers can be made anything.  And, obviously, the starting point matters.   Regarding the "estimated impact":  What is impacting?   Some non-realistic scenario?   GI,GO.

In addition to the air temperature and wind speed (both of which vary throughout the day), the water temperature matters (which probably doesn't vary much in a day).  Other factors are going to be involved (surface area (less in summer/fall; more in winter), solar radiation, water flow or turnover, etc)

Here's an available study of evaporation of water into the atmosphere in California. That's not reservoir evaporation though, but all evaporation.  Figure 6 and 7 are interesting.  Precipitation is the primary factor, but evaporation is buffered.  Total evaporation is (if I read it correctly) 160.8 cubic kilometers, or about 130.4 million acre-feet, per year.

This article indicates (if I read it correctly) is that the current evaporation losses from reservoirs in California is 1.728 x 10^9  cubic meters (ie, 1.728 cubic kilometers), or about 1,400,000 acre-feet, per year.

I'm not sure whether I understand that 1% of annual evaporation is from the reservoirs.  My gut reaction is that is high.

An interesting, related 2019 article
Those 96 Million Black Balls in LA's Reservoir Are Not Just There to Save Water