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Water Supply Outlook

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Water Supply Outlook – September, 2016

The water supply outlooks are published by ICPRB’s  Section for Cooperative Water Supply Operations  (CO-OP) staff on a monthly basis between April and October of each year. The September report states that despite a few weeks of continuous dry weather in the Potomac Basin, there is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2016 summer and fall seasons. Read More…

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Water Supply Outlook – July, 2016

Precipitation levels have been above normal and are expected to continue at that level. There is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2016 summer and fall seasons. Read more…

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Water Supply Outlook – June, 2016

After the majority of the Mid-Atlantic area received higher-than-average rainfall during the month of May, there is a below normal probability of releases from the Washington metropolitan area reservoirs for the upcoming season. Read more in the Water Supply Outlook.

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April Water Supply Outlook Available

Interstate Commission on the Potomac River Basin provides a monthly Water Supply Outlook and Status from April to October of each year. Based on careful data analyses, the report provides information on the possibility of a water release from local reservoirs. Read the April Report… 

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September Water Supply Outlook

Summary/Conclusions:

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.

Learn More

 

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Metropolitan Washington Drinking Water Supply Adequate to 2035

The existing water supplies that serve more than four million residents can adequately meet demands through the Year 2035, according to a new study by the Interstate Commission on the Potomac River Basin (ICPRB). Drought conditions as severe as the worst drought on record will likely require some form of mandatory use restrictions, however.

Looking further to 2040, the current supply system could experience considerable stress requiring stronger use restrictions, and some reservoir resources may be exhausted. During a severe 2040 drought there also is a small probability that the flow of the Potomac River could drop slightly below the environmental flow guideline of 100 million gallons per day (mgd) as measured at Little Falls Dam.

The study also assessed possible effects of climate change on the system. The impact on water supply varies dramatically depending on the change in stream flow that may result from changes in precipitation and temperature. Results from this study indicate that in the event of a severe drought with 2040 forecasted demands, the following range of potential impacts on the system could be expected.

  • If summer flows fall by 10 percent or more: During a severe drought, most system reservoirs would be drained and on some days the system would be unable to meet water supply demands and the 100 mgd environmental flow-by at Little Falls.
  • If summer flows change by 0 to +10 percent: the moderate increase in flows would not be enough to prevent some water use restrictions from occurring during a severe drought; some reservoir storage could be seriously depleted.
  • If summer flows rise by 20 percent or more: a substantial increase in flows would increase metropolitan area water supplies sufficiently to allow the current metropolitan area system to meet forecasted 2040 demands.

The study makes several recommendations, including,

  • The region’s water suppliers should continue their efforts to identify and evaluate potential new water supply storage facilities and conduct an evaluation of the relative benefits of new storage facilities, non-structural changes in operations, and other options. Enhance flow forecasting abilities to allow more precise operation and avoidance of shortages.
  • Further develop ICPRB’s database and model of Potomac basin water withdrawals and consumptive use to provide a sound foundation for basin-wide water supply planning and for a planned basin-wide comprehensive plan.

The study is performed every five years by the ICPRB Section for Water Supply Operations on the Potomac River (CO-OP). The CO-OP works with the metropolitan area water suppliers to coordinate normally independent water supply operations during droughts, and studies the water supply system to improve operations and plan for its future reliability.

You can download the complete study on the ICPRB website.

The ICPRB CO-OP works closely with the Washington metropolitan area water utilities, Fairfax Water, the Washington Suburban Sanitary Commission, and the U.S. Army Corps of Engineers Washington Aqueduct Division, which collectively provide the bulk of potable water to the region’s residents. The suppliers usually operate independently, but work together with CO-OP during drought conditions to assure that all demands are met equitably. The CO-OP also provides important services, including annual drought exercises an analyses of potential changes to the existing system.

Water Supply Outlook

August Water Supply Outlook

Summary/Conclusions:

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 July has been scattered and variable, with a basin averaged precipitation 0.5 inches below normal. Precipitation is expected to be near or above normal in the next month. Dry weather conditions are still possible later this summer. 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 1 to 4 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 weighting for current conditions. The conditional probability of 1 to 4 percent compares to a historical probability of 7 to 15 percent and is considered the more reliable indicator.

Learn More

Water Supply Outlook

May Water Supply Outlook

Summary/Conclusions:

There is 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. According to MARFC’s Water Resource Outlook for the southern portion of the Middle Atlantic there has been enough precipitation to offset any long-term dry weather conditions in the Potomac basin. At present, there is sufficient flow in the Potomac River to meet Washington metropolitan area’s water demands without augmentation from upstream reservoirs. In the event that low-flow conditions do develop, the metro area is well-protected from a water supply shortage because of carefully designed drought-contingency plans.

ICPRB’s Low Flow Outlook:

There is a 6 to 11 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 6 to 11 percent compares to a historical probability of 8 to 15 percent and is considered the more reliable indicator.

Learn more about the Water Supply Outlook.

Water Supply Outlook

April Water Supply Outlook

(Download PDF)

Summary/Conclusions:
There is 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. According to MARFC’s Water Resource Outlook for the southern portion of the Middle Atlantic there has been enough precipitation to offset some of the developing dry weather conditions in the Potomac basin. If precipitation continues to be below normal, however, degradation in the outlook may occur. 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 metro area is well protected from a water supply shortage because of carefully designed drought-contingency plans.

ICPRB’s Low Flow Outlook:
There is a 5 to 10 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 5 to 10 percent compares to a historical probability of 8 to 15 percent and is considered the more reliable indicator.

Learn more about the Water Supply Outlook.