The elephant in the room that I'm surprised wasn't mentioned is that there's no spare water in the Colorado. The Colorado River Compact allocated more than 100% of the current flow of the Colorado River to the different states (and a separate agreement adds up Mexico as well too), and it's at the point where a "good" winter snowpack means that the water level doesn't fall instead of providing meaningful recharge. The snow in 2016-2017 meant a return to 2014's low, but 2014 itself saw a miserable 20-foot fall in lake levels that there's no prospect for reversing. So where will the water to pump into the lake come from?
First, the water is lost - because evaporation over the large area of the reservoir only increases with its area, which increases with its level
Second, the scheme decreases flow, and that means level of the downstream. This means negative effects on ecosystems downstream.
Third, large hydroelectric dams require subordinate downstream reservoirs to tackle exactly the second problem - keep the downstream flow of the system (of the two+ dams) at least at some acceptable level to hide the variations in the main dam's
outflow which are due to daily/weekly/monthly/yearly fluctuations in inflow and power requirements.
Fourth - hydroelectric turbines require some amount of backpressure to operate. If the backpressure (that is the level of water downstream of the dam) gets too low, they can't operate unless risking damage. (that's actually the original reason behind the subordinate downstream dams)
So the project just does not make any sense. Where are they going to get the water?
Can you spell this out in more detail? I would have thought that in the near future, it would just time shift flow from daytime (when solar production is high) to nighttime, but perhaps my mental model is over simplified?
The most obvious explanation is that for an unit of power (not energy, mind) one has to spend a unit of flow (which is unit of volume per unit of time) falling over a unit of height (which the dam is for).
Now if you pump that unit of flow back up, total downstream loses that same unit of flow.
Second order effect is that when you pump water back up into the reservoir, it is evaporating at an increased rate.
Determining the evoparation increase is not trivial, since the reservoir surface area increases much faster than linear compared to its level at the dam. If they didn't, the dam would've been that much higher, because evaporative losses of the future reservoir are a major factor in the design of the dam.
I'm only asking about the first order effect (your second point, not your first). My question was: if you're letting that water back out every night, don't you get the unit of flow back then? It's delayed by up to 12 hours, but is that a really big effect?
You have to keep enough water somewhere downstream of the dam for the night so that you have something to pump up during the day. If you don't, there's either not enough water to pump up, or severe level fluctuations. If you do, it requires another dam, evapo losses, etc, etc.
