Situational Awareness (Wednesday, 2024-07-17)
Stream flows throughout the Potomac basin are continuing to decline. Flows in some major Potomac River tributaries have fallen below historic minimums for this day of the year – notably in the South Branch Potomac River near Springfield, WV, and the Shenandoah River at Millville, WV. According to the Middle Atlantic River Forecast Center predictions, 0.4 inches of rain is expected in the basin over the next day and 0.6 inches over the next 3 days.
Special Conditions
Yesterday staff from the USGS MD-DE-DC Water Science Center took field measurements at the Point of Rocks gage and subsequently revised the stage-discharge relationship, resulting in a lowering of reported flows.
Diversions of water by a National Park Service contractor are occurring at Dam 4 on Tuesday, July 16, and at Dam 5 on Wednesday, July 17, to allow diving inspections of the upstream sides of these historic structures. These diversions are creating considerable intra-daily downstream flow variability in the river. This has caused the temporary increase in flow now visible at the Point of Rocks gage, but flow at Point of Rocks is expected to fall to a new low level later today. These fluctuations will be followed by more tomorrow and will affect flows at Little Falls.
Recent basin-wide average precipitation (above Little Falls):
(based on CO-OP's Low Flow Forecast System analysis of Middle Atlantic River Forecast Center (MARFC) gridded multisensor precipitation estimates)
Yesterday's area-weighted average basin precipitation: 0.07 (inches)
Past 3-day cumulative area-weighted average basin precipitation: 0.13 (inches)
Past 7-day cumulative area-weighted average basin precipitation: 0.39 (inches)
Daily flows:
Little Falls gage flow 2024-07-16: 989 MGD (1530 cfs)
Little Falls gage flow 2024-07-17: 866 MGD (est., based on recently available real-time data) (1340 cfs)
Note: Gage flow at Little Falls is measured after water supply withdrawals.
Point of Rocks flow 2024-07-16: 918 MGD (1420 cfs)
Point of Rocks flow 2024-07-17: 1099 MGD (est., based on recently available real-time data) (1700 cfs)
Yesterday's estimated daily flows:
Great Falls estimated flow 2024-07-16: 1151 MGD (1780 cfs)
Note: Estimated flow below Great Falls intake is flow at Little Falls plus Little Falls water supply withdrawal.
Yesterday's Washington metropolitan area Potomac River withdrawals and discharges (2024-07-16):
Fairfax Water Corbalis withdrawal - Potomac: 131 MGD
WSSC Water Potomac withdrawal: 144 MGD – PROVISIONAL VALUE BASED ON PARTIAL DATA
Washington Aqueduct withdrawal - Great Falls: 0 MGD
Washington Aqueduct withdrawal - Little Falls: 162 MGD
Loudoun Water withdrawal: 14 MGD
Loudoun Water Broad Run discharge: 4 MGD
Total Potomac withdrawal: 306 MGD
Total net Potomac withdrawal: 446 MGD – PROVISIONAL VALUE
Loudoun Water Potomac River (PR) flow values for drought operations protocol (based on yesterday's flows):
QPR: 1420 cfs
QPR, obs: 1420 cfs
QPR, WS: 0 cfs
Note to Loudoun Water: See NWS ensemble flow forecasts (HEFS, GEFS, NAEFS) for Point of Rocks flow for the next 7 days – https://www.weather.gov/erh/mmefs_marfc?id=PORM2&model=NAEFS. See US Army Corps of Engineers Potomac River Basin page for observed releases from Savage and Jennings Randolph reservoirs - https://www.nab-wc.usace.army.mil/nab/poto.html; See US Army Corps of Engineers North Branch Potomac River and Savage River page for scheduled releases - https://www.nab-wc.usace.army.mil/nab/northBranch.html.
Note: Potomac River flow at Point of Rocks has dropped below the low-flow threshold of 2,000 cfs. When this occurs, per the Water Supply Coordination Agreement of 1982, CO-OP begins daily monitoring and reporting of Potomac flows and withdrawals on behalf of the CO-OP suppliers, Fairfax Water, Washington Aqueduct, and WSSC Water. Suppliers can review data at CO-OP's Data Portal at www.icprbcoop.org. Reports formatted in approved templates should be submitted by email to coop@icprb.org by 8:00 am for automated processing. Published monitoring reports are at http://icprbcoop.org/products/monitoring-reports.