Flood Math in Boulder, Colorado

Flood Math in Boulder, Colorado


At Boulder's climate station, 14.6 inches of rain fell between Monday, September 9, and early Friday morning, September 13, 2013. Most of that rain (more than 9 inches) fell on September 12th.

That’s a lot, right? Or is it?

You may be thinking this rain is nothing unusual if you are reading this while sitting on the side of Mount Waialeale, on the island of Kauai in Hawaii. And it would be nothing special to you because you are in one of the rainiest places on Earth where more than 450 inches fall each year on average.

Alternatively, if you are in Death Valley National Park in California, a place that gets less than 2.0 inches of rain in a typical year, you may be thinking that this amount of rain is extraordinary.

What might seem like a lot of rain for one area is actually a drop in the bucket for another. That’s because different regions have different climates and they get different amounts of rain.

To know if Boulder’s rainfall this week has been unusual, you’d have to know what’s usual. The usual, or average, rainfall is described as a part of climate. Boulder, Colorado, on average gets about 1.6 inches of rain during the month of September. So having 14.6 inches of rain during less than a week in September - that’s a lot.

Understanding your region’s climate can help you know if the weather is weird. And this week in Colorado the weather has been very weird.

What’s an inch of water?

Rain gauge.
Credit: CoCoRaHS

Now, because I have had quite a lot of time to consider rainfall totals this week, I have been thinking about inches of water. Water is measured in inches because that’s the way rain gauges measure water.  (If you’d like to monitor your own rain gauge as part of a national network, check out CoCoRaHS.)  But inches of a liquid are a strange thing to visualize. We rarely tell people that they need to drink eight inches of milk each day or that pasta should be cooked in six inches of boiling water. How much water is an inch?

I suggest we do some flood math to relate inches of water to something we can visualize, like a gallon jug.

If 14.6 inches of rain fell over the 25 square miles of Boulder this week, and one inch of rain over one square mile equals 17,400,400 gallons of water, how many gallons of water fell on Boulder?

Now, before I give away the answer, let me just admit that rainfall is always more complicated than this. It does not fall evenly over an area. Take a look at the maps on the CoCoRaHS website and you will see from the rainfall totals how patchy rain can be. So this calculation is a generalization, but it’s a good start at envisioning the amount of water.  

Did you do the calculation? Did you multiply the number of inches of water by the number of gallons in an inch and then multiply the number of square miles of the area? When you are done, check your answer below the flood photo.

Boulder Creek on September 13, 2013, with the Gilbert White Memorial Flood Marker showing the water levels of 50-, 100-, and 500-year floods.
Credit: Lisa Gardiner/UCAR

(Answer: 6,351 million gallons of water!)