[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.