Rainfall in Watersheds

Last night we had a nice rain, something that has been all too rare over the last month. The automated weather station at Lake Bemidji State Park shows that the rain started just before 0200 and continued until about 0500 with an average rate of roughly 1 cm per hour; the peak rate occurred at about 0400 when it briefly reached 1.8 cm hour. If, for the sake of discussion we assume that measurements from this site were typical over a larger area we can make some interesting calculations about the total volume of water falling within a given watershed. For example, I live in the the Lake Marquette watershed which, according to the USGS Interactive Watershed webpage, has an area of about 6.89 square miles. Of this total about 8.5% is lake surface and an additional 15.9% is wetland for a total storage area of 24.4%. So, if three centimeters of rain fell over this area last night, how much water fell in our basin overnight?

First, we need to think about units -- we could report the total in gallons, cubic feet, acre feet, cubic meters -- how do we decide? Gallons, acres, and feet are of course familiar but they are also cumbersome so it is simplest to do our calculations using standard SI units and to report our total rainfall volume in cubic meters. So, if we use a handy web-based conversion program we can quickly convert 6.89 square miles to 17,845,018 square meters which can then be multiplied by 0.03 meters (3 cm) to yield 535,350 cubic meters of water. Turns out that this is roughly 1/27th of the estimated 14,742,995 cubic meter volume of Lake Marquette in a single and rather unremarkable rainfall event (more on esitmating lake volumes later)!

The Lake Marquette watershed also receives water from the Schoolcraft River and associated upstream watersheds with a total area of an additional 164.4 square miles (425,794,045 square meters or 42,579 hectares). Three centimeters of rain over this area would yield an additional 12,773,821 cubic meters of water so that just over an inch of rain falling over the Schoolcraft watershed produces an amount of water roughly equivalent to the entire volume of Lake Marquette! Needless to say, I'll see noticeable increases in water level over the next few days. The lag time to changes in water level depends on the amount of storage area in the basin and for Lake Marquette there is typical a 4-7 day delay between a rainfall event and peak water level. I'll write more about variability of discharge rate from watersheds in a later post. /dps