[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