How much rain fell during Tropical Storm Irene? Seems like an easy question, but it’s not.

The National Weather Service relies on volunteers to collect rainfall, and given the variance in rainfall and the finite number of volunteers, there are bound to be gaps in the data record.

For the current issue of the Adirondack Explorer, Nancy Bernstein created a rainfall map based on the Weather Service’s own maps. It shows that more than seven inches of rain fell in Keene, Jay, and Au Sable Forks. But how much more? The Explorer’s publisher, Tom Woodman, measured eleven inches at his home in Keene.

The map indicates that six to seven inches fell in Keene Valley. But a worker at Johns Brook Lodge, a few miles away, collected thirteen inches in a bucket. Admittedly, a bucket might not be the most accurate rain gauge, but if we assume it provided an overestimate, we must also take into account that the bucket was not deployed for the entirety of the storm.

And there were no rain gauges on top of the High Peaks, where some of the heaviest rain might have fallen.

These uncertainties notwithstanding, we thought it would be interesting to calculate how much water Irene dumped in the Ausable River watershed, where most of the flooding occurred. The number we came up with (it appears in the new issue of the Explorer) was sixty-two billion gallons—enough to fill Stillwater Reservoir one and a half times.

For our calculation, we assumed that an average of seven inches of rain fell in the watershed. Given the data available, that seemed like a reasonable—and conservative—estimate.

The rest is basic arithmetic. Each inch of rain amounts to twenty-seven thousand gallons of water per acre. The Ausable River watershed encompasses 512 square miles, or 327,680 acres. And so we have:

• 27,000 gallons per acre x 7 = 189,000 gallons per acre.

• 189,000 gallons per acre x 327,680 acres = 61,931,520,000 gallons.

As mentioned, sixty-two billion gallons exceeds the capacity of the nine-mile-long Stillwater Reservoir. By way of another comparison, it’s estimated that Lake George, the biggest lake wholly within the Adirondack Park, contains 550 billion gallons of water.

Once all this water funneled down the valleys of the mountains into brooks and then rivers, it created enormous destruction. We’ve all seen the pictures of damaged houses, cratered highways, piles of trees, and fields of mud. Numbers provide another way for understanding the power of moving water unleashed by Irene.

At its peak, the Ausable’s flow rate was at least fifty thousand cubic feet per second, according to Andy Nash, a National Weather Service meteorologist in Burlington. Normally, its flow rate is three hundred to five hundred cubic feet per second. So during Irene, the river’s flow was more than a hundred times greater than normal.

Nash pointed out that the Mississippi River, as it passes St. Louis, has a flow rate of one hundred thousand cubic feet per second. During the peak of Irene, then, the Ausable was equivalent to half the Mississippi.

The latest Explorer includes thirteen pages devoted to Irene. The following stories are available online:

The Big Rain is an overview of the storm.

Shaking off the storm is about the resilience of businesses and residents who were hit hardest by Irene.

New slide guide describes hikes up five of the slides created by Irene.

In the wake of the flood is Tom Woodman’s poignant account of the damage to the library in Upper Jay.

Rainfall map by Nancy Bernstein.

Phil Brown is the editor of the Adirondack Explorer newsmagazine.