You not only need variable volume reservoirs on both ends, but you also want to have very high drop, because liars from seepage and evaporation scale with volum

You not only need variable volume reservoirs on both ends, but you also want to have very high drop, because liars from seepage and evaporation scale with volume but not with height. Low-drop pumped storage might be useful for very short term storage, but usually you'd want something that is efficient over a wider range of usages.