Water Supply Outlook and Status

Wide=angle view of a very full Potomac river at Great Falls during winter. Bare trees are seen on both sides of the river.

What is the Outlook?

The water supply outlook is published by ICPRB’s Section for Cooperative Water Supply Operations (CO-OP) staff on a monthly basis between April and October of each year. It provides 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.

Download: Water Supply Outlook for September 2018

Water Supply Outlook, September 2018

Summary/Conclusions:

There is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2018 summer and fall seasons. Generally, the use of Jennings Randolph and Little Seneca reservoirs is triggered by low flows brought about by a combination of low summer precipitation and low groundwater levels. Recent precipitation has left the majority of the Potomac basin with 1 to over 3 inches above average rainfall for the month. Current (August 31) streamflow data from the U.S. Geological Survey shows that stream flows are above or much above normal. Groundwater levels are mostly much above normal. According to the Middle Atlantic River Forecast Center, the outlook for water resources and water supplies is good in the Potomac Basin. 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 Washington 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 minimal (<1 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 environmental flow. Drinking water 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 weighing for current conditions. The minimal (<1 percent) conditional probability compares to the 7 to 14 percent historical probability and is considered the more reliable indicator.

USGS Potomac real-time gages

Drought Status:

Recent precipitation and forecasts:

Groundwater Conditions:

Basin groundwater levels

Water Supply Outlook Archive:

2014: October

Please contact us if you would like to be notified when new Water Supply Outlooks are posted.