[orlp]

> On the one hand, a huge dam reservoir does increase the level of water evaporation relative to an undammed river by increasing the amount of water surface area.

That depends entirely on the depth of the river and the depth of the reservoir. If the average depth of the reservoir is deeper than the average depth of a river there is less surface area.

[spectraldrift]

I had the same initial thought as well, but I realized this caveat would only be true if dams just made rivers deeper. But dams flood valleys and they make the water body much wider, not just deeper. A river that was 300 feet wide becomes a lake that’s miles wide. The width increase (10-100x) completely overwhelms the depth increase (maybe 3-10x), so surface area increases substantially.

Given that Earth averages towards smooth, the rate of width increase overwhelms the rate of depth increase when dams fill.

The only exceptions might be narrow canyon dams with nowhere to spread laterally, or dams on already-wide rivers. But those are rare.

[orlp]

> The width increase (10-100x) completely overwhelms the depth increase (maybe 3-10x), so surface area increases substantially.

No it doesn't.

The only thing that matters (in this oversimplified calculation which only takes into account surface area) is average depth of the freshwater while it is on land. If the reservoir is on average deeper than the rivers the freshwater otherwise would be flowing in, there will be less evaporation per liter of freshwater available for use.

Now a dam also increases the total amount of freshwater that's kept on the land in a steady state situation compared to if the water flowed free into the sea. It would be absurd to count this as "extra evaporation" when this extra freshwater otherwise would've simply be lost when it would flow into the sea instead of being kept in the reservoir.

[spectraldrift]

You're confusing surface area with surface area to volume ratio.

Furthermore the fact that dams increase evaporation though this mechanism is an easily verifiable scientific fact. https://riverresourcehub.org/resources/how-dams-affect-water...

[bombcar]

It’s more complicated than even that, I suspect, as a rapidly flowing river with rapids and spray may evaporate more water than a slow meandering river that flows smoothly.