Depth tells you almost nothing
Snow water equivalent is the depth multiplied by the density divided by 62.4, because 62.4 pounds per cubic foot is what liquid water weighs. That single division is the whole conversion, and it is the number that separates a nuisance from a flood.
Twenty two inches of settled snow at 15 pounds per cubic foot is 24 percent water, which is 5.29 inches of water equivalent. The same 22 inches of dry powder at 3 pounds per cubic foot is 4.8 percent water, or 1.06 inches. The same 22 inches of old wet pack at 30 is 10.58 inches. One measurement of depth, three answers spanning a factor of ten, and no way to tell them apart by looking at the stake.
| Snow | lb per cu ft | Water by volume | 22 in holds |
|---|---|---|---|
| Fresh dry powder | 3 | 4.8% | 1.06 in of water |
| New snow near freezing | 6 | 9.6% | 2.12 in |
| Settled snow | 15 | 24.0% | 5.29 in |
| Packed or plowed | 22 | 35.3% | 7.76 in |
| Wet heavy snow | 30 | 48.1% | 10.58 in |
Measuring density on site is not hard and it beats picking from a table. Push a straight-sided container of known volume down through the pack, weigh what comes out, and divide the weight in pounds by the volume in cubic feet. A one gallon container is 0.1337 cubic feet, so the snow in it weighing 2 pounds is a density near 15.
Three numbers that always work
One inch of water over one square foot weighs 5.2 pounds. That is 62.4 divided by 12, and it is the constant behind every snow load figure ever quoted. One inch of water over one square foot is also 0.6234 gallons. And one acre inch is 27,154 gallons. Those three take you from any snow measurement to a weight, a volume or a flow without needing anything else.
Worked on the default: 5,000 square feet under 5.29 inches of water equivalent is 5,000 times 5.29 divided by 12, which is 2,203 cubic feet, times 7.4805 gallons per cubic foot, which is 16,483 gallons. Cross-check it by mass: 5,000 square feet at 22 inches deep is 9,167 cubic feet of snow, times 15 pounds is 137,500 pounds, divided by 8.343 pounds per gallon is 16,481 gallons. The two routes agree, which is the point of checking.
Rain on snow is the case that hurts
A cold snowpack absorbs rain. A ripe one, already at freezing and already holding liquid water in its pores, does not: the rain runs through and arrives at the ground on top of whatever was melting anyway. Underneath, the ground is frozen, so the infiltration that would normally take a inch of rain is not available. The result is that a modest rainfall on an old snowpack produces runoff that a much larger summer storm would not.
On the default numbers, an inch of rain on the pack contributes 2,182 gallons of runoff on top of 1,091 gallons of melt — the rain is twice the melt on that day. The runoff share field is where the frozen ground shows up, and setting it to 90 or 95 percent for a February thaw is not pessimistic.
What this page is not
It is not a peak flow model and it does not produce a design storm. Melt is spread evenly across a day here, which real melt is not: it concentrates in the afternoon, and a drainage structure sees a peak several times the daily average. If you need a peak flow for sizing anything, the curve number runoff calculator handles the rainfall side of that properly and the culvert and swale calculator takes it to a channel. Storage volume for water going into the ground is the dry well sizing calculator, and how quickly a basin lets it back out is the basin drawdown calculator.
Where the snow came from, if you plowed it into a pile, is the snow stacking space calculator — a pile is a concentrated version of this same water, released from one spot instead of spread over a lot. And the weight side of the same arithmetic, on a roof rather than on the ground, is the roof snow load calculator.
Questions people ask
How do I convert snow depth to inches of water?
Multiply the depth by the snow density in pounds per cubic foot and divide by 62.4. Settled snow near 15 pounds per cubic foot is 24 percent water, so 22 inches of it holds 5.29 inches of water. There is no fixed ratio: the old ten-to-one rule describes fresh snow around 6 pounds per cubic foot and is wrong by a factor of two or more for anything that has been on the ground a while.
How many gallons of water are in a foot of snow?
Over one square foot, a foot of settled snow at 15 pounds per cubic foot holds 15 pounds of water, which is 1.8 gallons. Over 1,000 square feet that is 1,798 gallons. Powder at 3 pounds per cubic foot gives a fifth of that and wet snow at 30 gives double it, so the density is the whole answer and the depth is only a multiplier.
What melt rate should I use?
Your own, from local experience or local data. Melt depends on sun, wind, air temperature and rain, and it varies by a factor of several between a grey thaw and a sunny one with warm wind. The field is an input rather than a constant for that reason. The useful way to run it is twice: once at the fastest thaw you have seen and once at a slow one, and read the two flows.
Why is a thaw with rain so much worse than either alone?
Because a ripe snowpack cannot store the rain and frozen ground cannot absorb it. The pack is already at freezing with liquid water in it, so rain passes through and adds to the melt at the surface below. Meanwhile the infiltration that would normally take an inch of rain is unavailable. On the default numbers the rain contributes twice as much runoff as the melt does on that day.
Will my sump pump keep up with the melt?
This page will not answer that, and it should not. It reports the daily volume from your own figures and, if you enter a pump rating, states one as a share of the other with both numbers labelled as yours. Real melt is not evenly spread over 24 hours, the head a pump works against changes its actual flow, and how much of a site drains to one pit is a question about the site. Someone who can see the property has to answer it.