January 31, 2008 issue
Understanding Percentile
A percentile is a value on a scale of 100 that indicates the percent of a distribution that is equal to or below it. For example, on the map of daily streamflow conditions, a river discharge at the 90th percentile is equal to or greater than 90 percent of the discharge values recorded on this day of the year during all years that measurements have been made. In general,
• A percentile greater than 75 is considered above normal.
• A percentile between 25 and 75 is considered normal.
• A percentile less than 25 is considered below normal.
Both the Watauga River and the South Fork New River are well below normal.
Understanding Discharge
Discharge is the volume of water that passes a given location within a given period of time and is usually expressed in cubic feet per second.
Understanding Cubic Feet Per Second
Cubic feet per second is a rate of the flow, such as in streams and rivers. It is equal to a volume of water one foot high and one foot wide flowing a distance of one foot in one second. One cfs is equal to 7.48 gallons of water flowing each second. As an example, if your car's gas tank is 2 feet by 1 foot by 1 foot (2 cubic feet), then gas flowing at a rate of 1 cubic foot/second would fill the tank in two seconds.
These graphs from the U.S. Geological Survey shows that the median daily discharge rate on the Watauga River for the past 67 years has been relatively constant at approximately 150 to 170 cubic feet per second, with few major variations up or down. Measured discharge since December 2007, however, shows significant decreases below the median, well under 100 cubic feet per second, with a significant drop on January 25.
According to the USGS, the long-term median flow on the Watauga on January 27 for the past 67 years is 172 cubic feet per second. On January 27, 2008 it was 59 cubic feet per second. The Watauga River is currently ranked in the less than 10th percentile.
The South Fork New River graph is even more dramatic, with only two measurements exceeding the median daily statistic. All other measurements from the end of December until the present show alarmingly low discharge rates, with one measurement in mid-January dipping below 100 cubic feet per second.
Long-term median flow on the South Fork New River on January 27 for the past 80 years is 402 cubic feet per second. On January 27, 2008 it was 216 cubic feet per second. The South Fork New River is currently ranked in the 10th to 24th percentile
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