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ICPRB outlook:
The water supply outlooks are published during the spring through fall seasons. The next water supply outlook will be published in the month of April 2010.
October 2009 Outlook
Summary/conclusions:
The probability of releases this summer and fall from the Washington metropolitan area’s back-up water supply reservoirs is below normal. Despite decreased precipitation, probabilities for releases have decreased as we move later into the year, when evaporative loss and drinking water demands decline. Generally, the use of Jennings Randolph and Little Seneca reservoirs is triggered by low flows brought about by a combination of low summer rainfall, low precipitation in the prior 12 months, and low groundwater levels. Rainfall in September was below normal for most of the basin. Groundwater well levels remain normal to above normal throughout most of the basin. Two of the groundwater well levels remained below normal levels. Stream flows at Point of Rocks and Little Falls began declining in early September - flows at Point of Rocks dropped below the low flow monitoring threshold of 2,000 cfs (1,292 mg). When this occurs, daily monitoring and reporting of Potomac withdrawals by the CO-OP water suppliers and ICPRB are conducted.
ICPRB outlook:
There is a less than one- to two-percent conditional probability that Potomac flow will drop below 600- to 700-million gallons per day (MGD) at Little Falls through December 31 of this year; at these flow levels, water supply releases from Jennings Randolph and Little Seneca Reservoirs may occur. Releases occur when predicted flow is less than demand plus a required flow-by: demand ranges from 400 to 700 MGD during the summer months and the minimum flow-by at Little Falls is 100 MGD.
The conditional probability is estimated by analyzing the historical stream flow records and giving consideration to recent stream flow values, precipitation totals for the prior 12 months, current groundwater levels, and the current Palmer Drought Index. Past years in which watershed conditions most closely resemble current conditions are weighted more heavily in the determination of conditional probability. The historical, or unconditional, probability is based on an analysis of the historical stream flow record without weighting for current conditions. The conditional probability of less than one- to two-percent compares to a historical probability of one- to four-percent and is considered the more reliable indicator.
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Graphical Updates of River Flow and Reservoir Storage
Recent Potomac flow:
2009 Flow at Little Falls
2002 Flow at Little Falls
1966 Flow at Little Falls
Stage at Edwards Ferry
USGS Potomac real-time gages
Drought Status:
National Weather Service Potomac Basin Monitor
Daily Updates
Recent precipitation and forecasts:
Monthly Precipitation in the Potomac Basin
Precipitation maps from the MARFC
Quantitative Precipitation Forecast, MARFC 1-2 days
Quantitative Precipitation Forecast, NWS 1-5 days
Groundwater conditions:
Basin groundwater levels
Archived Water Supply Outlooks:
Contact
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for archived outlooks
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