What is a Runoff Coefficient?

The runoff coefficient is the fraction of rpecipitation that runs off into streams and lakes. For a typical forrested watershed, it is about 10 to 20 percent.

To find the runoff coefficient, one must divide the amount of water from the storm in the stream or lake by the volume of preciptation that happened in a storm.

Near the datalogger site on April 29th, 2008. A high porportion of the rain from the storm flowed into the Nequasset Stream.

Finding the Runoff Coefficient

To find the runoff coefficient, we had to convert stage (depth of water) to discharge (volume of water) at the site. In order to do this, we had to figure out the correlation between the stage at the culverts, for which a discharge rating curve had been formulated, and the logger site where we took data continuously during the storm.

Because our depth had been taken every 10 seconds by the datalogger, we now had the volume of water that flowed through the stream every 10 seconds during the storm. After subtracting the base flow (the groundwater that was already flowing through the stream, which we calculated to be of a depth of about 1.15 cubic feet per second) from overall flow, we were left with only the stormflow.

Groundwater flow in Nequasset Stream prior to storm.

The stormflow discharge (volume) is dependant on the stage (depth) at the datalogger site. Measurements were taken from during the April 29, 2008 storm.

By adding up each 10 second interval, we were able to calculate the cumulative stormflow at 88,908,235 cubic feet.

Next, we had to calculate the total amount of rain that fell during the storm. By multiplying the amount of rain per square inch (4.32 in) by the area of the watershed (9.906 miles), we found the total amount of rain that had fallen during the storm. The total amount of rain was 98,958, 562.5500 cubic feet.

The stormflow (88,908,235 cubic feet) divided by the amount of rain (98, 958,562.56 cubic feet) equaled 0.89844, or 89.4 percent.

Conclusion

For a forested waterhsed, 89.4 percent is very high. Other factors and assumptions must be taken into account, however.

Firstly, rain does not fall evenly during a storm event. Though our rain gauge showed that 4.3 inches of rain fell during the storm, Brunswick recieved 6.05 inches, and Boothbay got just under five inches of rain. With five inches of rain, the runoff coefficient would be 77.26 percent; with six inches, it would be 64.39 percent.

It does make sense, though, that the runoff coefficient would be higher than normal due to the climate and geography of the region. For one thing, the storm was very large, and the rain probably saturated the water table very quickly, increasing the amount that ran off into the stream. Also, the storm occured in the spring before the trees had leaves yet, which lessened the amount of rain that they absorbed. The geology of the region also contributed. Due to much bedrock and wetlands, the water table had minimal storage space, leading to increased runoff.

Assumptions

The amount of storm flow was based on a rating curve from upstream, which may have not been exact. Also, as mentioned previously, less precipitation may have fallen in the area of the rain gauge than in other parts of the watershed.

Wetlands in the Nequasset watershed on April 29th, 2008. Wetlands are where the water table is already at the surface level, and contribute to a high runoff coefficient.