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Virginia Stream Flows and Tropical Depression Fay

What a difference two days and a tropical depression can make in the highly variable world of stream flows.

On August 26-28, 2008, the remnants of Tropical Storm Fay brought several inches of rain to many parts of Virginia and neighboring states.  Rainfall amounts in Virginia and other southeastern states between August 22 and August 28 are shown in the following figure, taken from the National Oceanic and Atmospheric Agency’s (NOAA) Southeast Regional Climate Center Web site.  The color legend to the right indicates the number of inches of rainfall in areas of corresponding color on the map.

Taken from http://www.sercc.com/climateinfo/precip_maps, 9/1/08.

This rainfall led to a rapid change in stream flows in much of Virginia; in fact, it changed several Virginia streams from a record daily low flow to a record daily high flow.

The U.S. Geological Survey (USGS) provides (online at http://waterdata.usgs.gov/va/nwis/rt) maps and graphs of current stream flow at about 190 gaging stations in Virginia.  The stations record stream flow (and other information) every 15 to 60 minutes.  USGS compares these readings to historical flows for a given day for the gaging station’s period of record (each station has at least 30 years of records) and classifies the readings as follows:
·record low flow;
·equal to or less than the 10th percentile (the lowest 10 percent of flows);
·10th to 24th percentile;
·25th and 74th percentile (the middle 50 percent of flows);
·75th to 89th percentile;
·equal to or greater than the 90th percentile (the upper 10 percent of flows);
·record high flow.
           
USGS then plots the stations on a map with colored dots corresponding to the flow categories, as shown in the following map from August 26, 7:30 a.m. EDT.  As you can see from the this map, most streams being monitored in Virginia were at record low flows or were in the lowest 10 percent of flows on August 26.

 

Taken from http://water.usgs.gov/waterwatch/?m=real&r=va, 8/26/08.

Color coding is as follows:

 

By August 28 at 3:30 p.m. EDT, the statewide graph (reproduced below) showed streams mostly at record highs or in the upper 10 percent of flows.  Note that three stations on the New River (within the box in the graph below) changed from record low flows on August 26 to record high flows on August 28.  The photo below shows the New River near one of these stations a few hours after its record high.

Taken from http://water.usgs.gov/waterwatch/?m=real&r=va, 8/28/08.

 

New River at Radford, Va., approximately 7 p.m. on August 28, 2008.  The New River at the Radford gage was at a record daily low in the morning of August 26 and a record daily high on in the afternoon of August 28. Photo by Alan Raflo.

           
The final graph below tells the larger story of stream flows in Virginia throughout Summer 2008.  Constructed from data taken weekly from the USGS stream flow maps, the graph shows the record lows (dark blue line), lowest 10 percent flows (purple line), highest 10 percent flows (light blue line), and record highs (yellow line) from May 21, 2008 (far left) to September 1, 2008 (far right).  As the purple line shows, the number of streams in the lowest 10 percent of flows increased from less than 10 on May 21 to nearly 70 just before Fay’s rains arrived on August 26, after which the number fell precipitously.  By September 1, most stations had flows within the middle ranges (very few highs or lows).

           
If dry weather returns, a lack of available groundwater recharge (a deficit that has been developing since at least the drought in 2007) could lead to stream flows dropping significantly once more.  Alternatively, passage of other tropical storm systems could help overcome the groundwater deficit and reduce the occurrence of historic low flows.

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