This is a summary of the ICPRB report published September 2020:
The Washington, DC, metropolitan area (WMA) with over three
million workers many of whom serve or support the federal government is also home to almost five million residents.
The region’s water suppliers have an important responsibility beyond supplying
the needs or the residents: to provide 24/7 water that ensures the federal
government, including Congress, the Pentagon, and key agencies can function.
The water suppliers share the Potomac River as the major
regional water resource, and so 35 years ago and came together to form the
Interstate Commission on the Potomac River Basin (ICPRB) and a cooperative
agreement (Co-Op) of funding and using
the water resources available regionally. One of the requirements of the
agreement is that every five years a study be conducted to evaluate whether
available resources will be able to meet forecasted water demands. The seventh
in the series of such studies has just been released. This time the ICPRB found that if droughts
become much more severe as predicted in the climate forecast, even with the
addition of the Vulcan Quarry, Milston Quarry, Travilah Quarry and Luck Stone
Quarry B as reservoirs (adding over 13 billion gallons of water storage) and
using water restrictions and demand management the WMA water supply may be
unable to meet combined water supply needs and the environmental flow-by at
Little Falls. In other words according to the forecast by 2050 we run out of
water for periods under more than one third of the forecast scenarios. It is to
be noted that without the addition of the 7.8 billion gallon Travilah Quarry as
additional storage the WMA water systems experience failures by 2040 in the
forecasts.
The new ICPRB study forecasts of water demands for the WMA through,
2050, taking into account projected demographic and societal changes that may
affect future water use, forecasts of
water availability, considering the potential impact of changes in climate and
upstream water use on system resources, and an evaluation of the ability of current and
planned system resources to meet the forecasted demands. Using various scenarios
the current study also assesses the
effectiveness of several options for enhancing the current regional water
supply system that were recommended in a special study conducted in 2017 (Schultz
et al., 2017). This special study evaluated and compared the ability of 10
proposed changes and additions to the WMA water supply system to meet the
challenges of growing regional demand for water with a supply that does not
grow and the potential impacts of climate change when the region expects to
have more intense wet years and longer droughts.
The Potomac River supplies, on average, just over three
quarters of the WMA’s surface water needs. The rest of the water supply comes
from Occoquan River, the Patuxent River and regional groundwater supplies an
estimated 27% of end use demand. The Co-Op members provided the funding for
three upstream reservoirs: Jennings Randolph, Little Seneca, and Savage. Water
in these reservoirs is released during drought to augment natural river flow.
In addition, Fairfax Water and WSSC Water rely daily on reservoirs outside of
the drainage area of the Potomac River, on the Occoquan River (7.85 billion
gallons) and the Patuxent River (10.4 billion gallons), respectively. Two
additional reservoirs are planned to be in place within the next 20 years:
Loudoun Water’s Milestone Reservoir(1.25 billion gallons) , scheduled for completion in 2024, and
Fairfax Water’s Vulcan Quarry Phase 1 (1.7 billion gallons), planned to be in
place by 2040 to augment their Occoquan supply.
Due to continuing improvements in efficiencies of household
water fixtures and appliances and consumer behavior, water use in the WMA has
remained remarkably steady for almost three decades despite continuing
population growth. Water demand averaged 453 million gallons per day (MGD) for
the CO-OP suppliers during the most recent period for which data is available
(2014-2018). This does not count the water use from groundwater nor the water
use by smaller water supply utilities that have their own water supply. Forecasts
of average annual water demand were developed using average per person and household
use and a forecast that population in the WMA in 2050 will be 6.1 million, a
27% increase from 2018 levels. The ICPRB projects that average annual water demand will increase to 501 MGD (10%) by
2040 and to 528 MGD (16%) by 2050. The estimated uncertainties (one standard
error) in 2040 and 2050 are ±9.7% and ±10.4%, respectively.
While the demand for water increases, the climate projections
indicate that the mid-Atlantic states, on average, are becoming and will
continue to get “wetter.” Climate scientists also warn; however, that extreme conditions, that is, floods and
droughts, will become more severe. Our
water infrastructure will have to include more water storage to meet a larger
demand during longer droughts.
The ICPRB used nine scenarios to represent ranges of
uncertainties in the impact of climate change on water availability in the
Potomac basin and in future WMA water demand. For each scenario, Potomac
Reservoir and River Simulation Model (PRRISM) simulations were done in four different configurations of the WMA
system: a system with current and planned resources, and a system that has been
enhanced with operation controls using
water restrictions, the current system
with operating controls and the Travilah Quarry and finally the current system
with operation controls the Travilah Quarry and Luck Stone Quarry B added
storage.
On average, precipitation in the Potomac River watershed in
2040 and 2050 is projected to increase by 8% and 10%, respectively, and
temperature is projected to increase by 2.16 °C (3.9 °F) and 2.5 °C (4.5 °F),
respectively. There is tremendous uncertainty about how climate change will
affect streamflows. This study relies on a simple climate response function,
based on a least squares multiple regression analysis, to predict mean annual
natural Potomac River flow from mean annual precipitation, mean annual
temperature, and the previous year’s mean flow.
Four summary statistics are used as key performance metrics are
listed in the colored boxes below:
- Percent years with no Potomac flow deficits: the percentage of years in the simulation period in which flow in the Potomac River at Little Falls is above 100 MGD (the Little Falls flow-by) on every day of the year, that is, in which combined WMA Potomac water supply needs and the environmental flow-by at Little Falls is always met.
- Percent years with emergency restrictions: the percentage of years over the simulation period in which emergency water use restrictions are implemented on one or more days of the year. In this study, emergency restrictions are assumed to be implemented when combined water supply storage in Jennings Randolph and Little Seneca reservoirs is below 5% of the combined capacity.
- Maximum 1-day Potomac flow deficit (MGD): the maximum shortfall in meeting combined WMA Potomac water supply needs and the Little Falls environmental flow-by on any single day of the simulation period.
- Minimum Travilah Quarry storage (BG): the minimum storage in Travilah experienced over the course of the simulation period.
The last statistic, minimum Travilah Quarry storage, is of interest because of the dual role that Travilah is expected to play in the WMA water supply system: as a backup supply in case of an emergency spill and as a resource to mitigate drought. Reductions in Travilah storage during drought reduce or eliminate this reservoir’s ability to serve as a backup supply in case of a spill. Results of the study indicate that if droughts become much more severe as climate models forecast, the WMA system may be unable to meet combined water supply needs and the environmental flow-by at Little Falls even if all of the recommended options of the 2017 alternatives study are implemented, including Travilah Quarry and Luck Stone Quarry B.
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