What is the Outlook?
The water supply outlooks are published by ICPRB's Cooperative Water Supply Operations Section (CO-OP) staff on a monthly basis between April and October of each year. They are meant to provide an update on the possibility of water supply releases from the area's reservoirs based on long-term precipitation data, flows, and other information for the Potomac basin. The next Water Supply Outlook will be available in April 2015.
Water Supply Outlook for October 2014
There is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2014 fall season. Generally, the use of Jennings Randolph and Little Seneca reservoirs are triggered by low flows brought about by a combination of low summer precipitation and low groundwater levels. It is typical that the probability of releases decreases around this time of year, when evaporative loss and drinking water demands begin to decline due to lower temperatures. The Potomac basin has not had much precipitation for the month of September, however most flows are near normal due to precipitation that occurred in the last couple of days. Groundwater levels are generally near normal with both above and below normal levels scattered throughout the area. Precipitation from a tropical system is not expected in the near term. Daily monitoring of Point of Rocks and Little Falls flows began on September 17 and will continue to prepare for the possibility that more serious drought conditions develop in the upcoming weeks. At present, there is sufficient flow in the Potomac River to meet the Washington metropolitan area’s water demands without augmentation from upstream reservoirs. In the event that low-flow conditions do develop, the metropolitan area is well-protected from a water supply shortage because of carefully designed drought-contingency plans.
ICPRB’s Low Flow Outlook:
There is a less than 1 to 3 percent conditional probability that natural 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. Note that natural flow is defined as observed flow at the Little Falls gage plus total Washington metropolitan Potomac withdrawals, with an adjustment made to remove the effect of North Branch reservoir releases on stream flow.
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 record without weighting for current conditions. The conditional probability of less than 1 to 3 percent compares to a historical probability of 3 to 5 percent and is considered the more reliable indicator.
Graphical Updates of River Flow and Drought Conditions:
Recent precipitation and forecasts: