What is snow water equivalent (SWE)?

Snow water equivalent (SWE) is how much water is locked up in the snowpack. Melt the snow in place and measure how deep the water would stand, and that depth in inches is the SWE. A snowpack with 20 inches of SWE will yield about 20 inches of water when it melts. It is the single most useful number for judging the coming spring and summer water supply across the West.

Why depth is the wrong number

People instinctively reach for snow depth, but depth alone can mislead you badly. Fresh powder can be 90 percent air. A late-season snowpack that has settled and refrozen is dense and heavy. Two feet of dry January powder and two feet of wet April snow hold completely different amounts of water. SWE cuts through that by measuring the water content directly, which is why every water-supply forecast is built on SWE and not on how tall the snow is.

The shape of a snow year

Western mountain snowpack follows a predictable arc. It starts building in October, deepens through the winter storms, and normally reaches its peak around the first of April. Then the melt takes over: through late spring and early summer the snowpack releases its water into the rivers and reservoirs, and by early summer most of it has melted out. That slow release is the whole point. The snowpack is a frozen reservoir that hands its water to the real reservoirs over months, long after the storms have stopped.

Worked example: reading a snow year

The chart above tracks Colorado's statewide snowpack for the current water year against the 1991-2020 median for each date. Read the two lines together. The median line is the normal arc, building to an early-April peak and melting out by summer. The current-year line is this season against that normal: where it sits above the median the state banked more water than usual, where it sits below it banked less. Once both lines fall to the bottom the snow has melted out, which is why in summer a snowpack page reports the season's peak rather than a live number.

Why it matters downstream

A big snowpack is next summer's water in the bank, and a thin one is a warning that arrives months early. The snowpack fills the rivers this site tracks and refills the reservoirs behind them. SWE does not translate one-for-one into runoff, because dry soils drink the first melt, dust darkens the snow and speeds it up, and some of it sublimates straight into the air. So SWE is the starting point for a runoff forecast, not the forecast itself. This site reports the snowpack as measured and does not forecast the runoff.