From the Teton Valley News (Bruce Mason):
The rule of thumb is that 10 inches of snow contains the water equivalent of one inch of water. In other words, if you melted down 10 inches of snow it would produce as much water as if one inch of rain had fallen instead. But like so many other things based on averages, there are wide variations in this rule of thumb and they are very important, making all the difference as the snow melts.
Some snow is very wet with as much as half of the snowflake actually liquid water instead of ice. We have seen plenty of that this winter. With wet snow, 5 inches of snow can produce an inch of water when it melts.
On the other hand, dry snow, our beloved powder, contains very little water with lots of air space between the flakes. It can take 30 inches of very dry, fluffy powder to produce an inch of water.
In a typical winter, we have mixed layers of wet and dry snow on the ground as the spring thaw starts. With a typical mix, the 10 to 1 rule of thumb does pretty well to estimate how much water will come rolling down out of the hills toward us as the snow melts. Depending on conditions, much of it sublimates into the air or soaks down into the water table anyway, so springtime flooding is usually minimal here. Usually.
The thing is, this winter has not been typical. With sparse data to back it up, many weather observers and sportsmen feel that we have had much more wet snow this season than normal. The La Niña watch issued by the NOAA Climate Prediction Center last fall implied that we could be in this situation this spring. Warmer than average temperatures plus higher than average precipitation equals wet snow, and so it seems to have worked out.
How can we get hard data to back up the observations by those in the know about the moisture content of the snow out there? It takes more than measuring the depth; it requires snow water equivalent measurements. Water equivalent measurements are a little more involved than measuring snow depth, but can be much more important to emergency planners trying to prepare for droughts or floods in the spring. In fact in 1997, an unpredicted flash flood in the Fort Collins, Colorado area killed 5 people and did hundreds of millions of dollars of damage. Only 2 inches of rain fell in Fort Collins that day, but no one was measuring southwest of the city where the flood originated and seven times more rain fell.
As a result of the Colorado flood, the importance of widespread precipitation measurement, including the water equivalent of surrounding snow, was realized. Then it was addressed. A network of volunteer observers, called the Community Collaborative Rain, Hail and Snow Network, (CoCoRaHS for short… I guess) was formed. The network began in Colorado, the original Co was for Colorado, not Community, but it has since spread across the US and Canada. Major sponsors of CoCoRaHS include NOAA and the National Science Foundation. Starting next week, when the right equipment arrives from Pocatello, your genial Teton Valley weatherman will be among the CoCoRaHS volunteers in Idaho doing daily snow water equivalent measurements and sending the reports to CoCoRaHS headquarters.