By Tracy Robillard, Public Affairs Specialist
When the Corps of Engineers updated the Savannah River Basin drought plan in August 2012, we added a new indicator for determining drought levels: stream flow at the Broad River.
The Broad River is a large, unregulated tributary that flows into the Thurmond reservoir. It begins at the convergence of the Hudson River and Middle Fork on the Franklin and Madison County border in Georgia. It then flows southeastward about 99 miles into the Thurmond reservoir. The Broad River has a drainage area of 1,430 square miles.
Corps water managers chose to add the Broad River as a drought indicator because it provides an accurate representation of natural inflow to the Savannah River Basin. The flows are measured by the U.S. Geological Survey at the Broad River gage near Bell, Ga. The USGS has collected more than 50 years of data at this location. It provides a comprehensive, reliable and historical data set that is valuable in assessing real-time conditions as they compare to historical rates and averages.
Water managers also chose this gage since it’s part of the USGS Hydro-Climatic Data Network (HCDN). This network is a subset of gages where streamflow conditions are minimally affected by human disturbance. Although most streamgages reflect some level of human activity, total water extractions or diversions at HCDN sites are generally less than 5 percent of the mean annual discharge, said Andy Ashley, Savannah District water manager.
By using stream flow, water managers can more accurately assess changes in hydrology before the reservoir levels reflect these changes. If natural inflow is low (10 percent or lower), then water managers can further restrict outflows from the dams and conserve more water in the reservoirs for later use during drought.
To check the real-time stream flow, visit the USGS Water Watch website at http://waterwatch.usgs.gov/. Click the U.S. map for “Current Stream Flow” then click the state of Georgia. The Broad River at Bell gage is located in the north east portion of the state near the Savannah River, about five counties down from the northern Georgia border (see image below). You can also view the Georgia stream gage map directly by clicking here.
Before the 2012 update, the Corps only used reservoir levels as an indicator for drought trigger levels. The updated plan now uses Broad River flows as a secondary indicator of drought during levels 1 and 2.
Drought Level 1 is initiated when either the Hartwell or Thurmond reservoir reaches four feet below full pool (Hartwell: 656 ft-msl; Thurmond: 326 ft-msl). Drought Level 2 begins when the reservoirs decline an additional two feet (Hartwell: 654 ft-msl; Thurmond 324 ft-msl). If stream flows at the Broad River gauge are less than or equal to 10 percent of the historical flow rate (calculated over a 28-day average), we reduce outflows to 4,000 cfs in Level 1 and 3,800 cfs in Level 2. If Broad River flows are higher than the 10-percent historical flow rate, we adjust outflows slightly higher to 4,200 cfs in Level 1 and 4,000 cfs in Level 2. Stream flow is not used as an indicator in level 3.
Keep in mind, the outflow requirements during drought vary depending on the season. We reduce outflows further in the winter months during drought levels 2 and 3. See table below:
When transiting out of a drought, we operate at the specified minimum outflows until the reservoirs rise two feet higher than that trigger level. The basin has remained in drought level 2 since January. Currently, the Hartwell pool has reached beyond two feet above the level 2 trigger; however, the Thurmond pool needs to reach 326 ft-msl before operations can shift to drought level 1. We anticipate reaching drought level 1 in mid-to-late May. We also expect the Hartwell and Thurmond pools to be back in balance around that time.