There is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2015 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. The MARFC’s Water Resource Outlook for the southern portion of the Middle Atlantic reports that precipitation in the month of August has been below normal, with a basin averaged precipitation 1.9 inches below normal. Precipitation is expected to be below normal to above normal in the next month. Low precipitation and river flow caused COOP to initiate daily monitoring and reporting of Potomac River flows and withdrawals on August 31. COOP will continue to prepare for the possibility that more serious drought conditions could 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 wellprotected from a water supply shortage because of carefully designed drought-contingency plans.
ICPRB’s Low Flow Outlook:
There is a 3 to 12 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. 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 conditional probability of 3 to 12 percent compares to a historical probability of 7 to 14 percent and is considered the more reliable indicator.
Note: Natural flow at Little Falls was slightly below the 1200 MGD on September 1. The WSO’s probabilistic model uses a monthly time step and relies on data from the previous months to compute conditional probabilities for the coming months. Results shown in the last column of the table below reflect conditions as of August 31.