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 n

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.