Stage-storage curve vs. rating curve: what's the difference?
Both terms show up in the same engineering binder for the same dam, both get called "the curve" by people who work the asset every day, and both have "stage" somewhere in the name. That's about where the resemblance ends. If you're trying to back out a storage number from public gauge data, mixing these two up will give you a result that looks plausible and is wrong by a wide margin.
The stage-storage curve: how much water is in the bathtub
A stage-storage curve, sometimes called an area-capacity curve, elevation-storage curve, or just "the capacity table," answers one question: at a given water-surface elevation, how much volume is sitting behind the dam? It's built from a bathymetric or topographic survey of the basin, integrating the cross-sectional area of the reservoir bed at each elevation increment from the streambed up to full pool. The output is a lookup table or smooth curve: feed in a stage reading, get back acre-feet or cubic meters.
This curve is a physical fact about the shape of the basin, not a measurement of anything moving. It doesn't change week to week. It does drift over decades as sediment settles out of the inflow and fills in the lower pool, which is why a capacity table from a dam's original 1960s design documents can overstate live storage by a meaningful margin today. Utilities resurvey occasionally, but not on any schedule traders can count on, and the updated numbers rarely make it into a public filing.
The rating curve: how fast water is moving past a point
A rating curve, or stage-discharge curve, answers a different question entirely: at a given stage reading at a specific cross-section, how much flow is passing that point right now? It's built from direct streamflow gaugings, where field crews measure actual discharge across a range of river stages and fit a curve through the points. You'll see rating curves for the river upstream of the reservoir (inflow), for the tailrace below the powerhouse (outflow), and sometimes for a spillway gate separately from the main channel.
A rating curve tells you a rate. A stage-storage curve tells you a volume. One is a tap, the other is a tank. Confusing them means confusing how fast water is draining with how much is left in the pool, and those two numbers move on completely different clocks, especially during a drawdown when inflow has dropped off but the pool is still being pulled down by release schedules set weeks earlier.
Why this trips people up at an actual dam
Part of the confusion is that both curves get derived from the same stage gauge. A single pressure transducer or staff gauge on the pool reports one number, and depending on which table you run that number through, you get either a volume or, at a different structure, a flow rate. Dam operations staff move fluently between the two because they need both for day-to-day control. Analysts coming from the trading side, pulling numbers from a disclosure or a public hydro database, don't always get told which table produced the figure in front of them, and a volume mislabeled as a flow (or the reverse) is an easy error to carry into a position.
The practical problem for anyone estimating storage across a basin isn't really the rating curve at all, since discharge data is comparatively easy to find for most monitored rivers. The hard part is the stage-storage curve itself: the specific, current, sediment-adjusted capacity table for that reservoir, which the dam owner holds and doesn't always publish, let alone update. Without it, a stage reading alone tells you almost nothing about how much energy is banked behind the structure.
That's the gap a water-surface-area approach sidesteps. Reservoir Monitoring takes the pool's surface extent from satellite imagery and runs it through each reservoir's own area-capacity curve to get a storage estimate on a weekly cadence, without waiting on a utility to release an updated table. If you've been stuck triangulating storage from stage gauges and old PDFs, it's worth a look at how that series gets built.