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.
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.