Daily Potomac flow and demand update (Tuesday, 2023-09-19)

Situational Awareness (Tuesday, 2023-09-19)
Potomac River flows are holding relatively steady due to recent rainfall. According to the Middle Atlantic River Forecast Center, no rain is expected in the basin over the next 3 days. The week-long outlook for the basin calls for an average accumulation of approximately 0.9 inches of rain.

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 multi-sensor precipitation estimates)
Yesterday's area-weighted average basin precipitation: 0.14 (inches)
Past 3-day cumulative area-weighted average basin precipitation: 0.9 (inches)
Past 7-day cumulative area-weighted average basin precipitation: 1.09 (inches)

Daily flows:
Little Falls gage flow 2023-09-18: 763 MGD (1180 cfs)
Little Falls gage flow 2023-09-19: 737 MGD (est., based on recently available real-time data) (1140 cfs)
Note: Gage flow at Little Falls is measured after water supply withdrawals.
Point of Rocks flow 2023-09-18: 1021 MGD (1580 cfs)
Point of Rocks flow 2023-09-19: 1163 MGD (est., based on recently available real-time data) (1800 cfs)

Yesterday's estimated daily flows:
Great Falls estimated flow 2023-09-18: 763 MGD (1181 cfs)
Note: Estimated as gage flow at Little Falls plus Little Falls water supply withdrawal.

Yesterday's Washington metropolitan area Potomac River withdrawals and discharges (2023-09-18):
Fairfax Water Corbalis withdrawal - Potomac: 102 MGD
WSSC Water Potomac withdrawal: 129 MGD
Washington Aqueduct withdrawal - Great Falls: 144 MGD
Washington Aqueduct withdrawal - Little Falls: 0 MGD
Loudoun Water withdrawal: 5 MGD
Loudoun Water Broad Run discharge: 6 MGD
Total Potomac withdrawal: 380 MGD
Total net Potomac withdrawal: 374 MGD

Loudoun Water Potomac River (PR) flow values for drought operations protocol (based on yesterday's flows):
QPR: 1580 cfs
QPR, obs: 1580 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 – see https://www.weather.gov/erh/mmefs_marfc?id=PORM2&model=NAEFS.

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.