Water Supply Outlook and Status

Interstate Commission on the Potomac River Basin

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, October 2019

This is the final report of the 2019 season. The next Water Supply Outlook will be released in spring 2020.


There is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2019 fall season. 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. 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. Potomac basin streamflows are near or below normal, influenced by much below normal precipitation and near normal groundwater levels. Almost no precipitation (0.9 inch basin average) occurred in the Potomac basin for the month of September. The majority of areas in the Potomac basin have been designated as abnormally dry (D0), which means that the basin is showing dryness but not yet in drought. However, areas in Southern Maryland and Northern Virginia have been designated as being in moderate drought (D1). If below average rainfall continues (as currently expected in the short term) then further degradation in conditions is expected to occur. 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 23 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 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 one to four 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-by. 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 one to four percent conditional probability compares to the three to five percent historical probability and is considered the more reliable indicator.

Drought Status:

Recent precipitation and forecasts:

Groundwater Conditions:

Water Supply Outlook Archive:

2019: April | May | June | July | August | September | October

2018: April | May | June | July | August | September | October

2017: April | May | June | July | August | September | October 

2016: April | May | June | July | August | September | October

2015: April | May | June | July | August | September | October

2014: October

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