Reservoir water balance

A reservoir is an accounting problem: inflows minus outflows equals the change in storage. Every drought is that sum turning negative for long enough.

Percentage full: Relative to each reservoir’s own capacity. A reservoir at 40% may hold far more water than another at 90%, so percentages compare a reservoir with its own past, not with other reservoirs.
Drought sequence: Low rainfall reduces runoff first, then storage, then — much later — groundwater. A dry summer alone rarely empties a reservoir; two dry winters can.
Compensation flow: A licensed release that continues during drought. It reduces available supply but protects the river below the dam.

Text description

A reservoir with inflows on the left and outflows on the right. Inflows are catchment runoff, direct rainfall on the reservoir surface, and any transferred water. Outflows are supply abstraction, compensation flow released to the river below, evaporation from the surface, and spill when the reservoir is full. The difference between total inflow and total outflow is the change in stored volume.