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Back to League of Women Voters DC Voter, March 1999




To the reader: The full text of this report, as well as the text of this executive summary, may be accessed on the world wide web at A limited number of copies of both the report and the summary are also available from Bob Perry, Falls Church League, tel. 703-534-5421.

The Washington, D.C. region has expanded dramatically in population and geographic area in the past few decades. This report addresses the availability of water to meet the long run needs of the region as it continues to grow. The supply of water varies both seasonally and from year to year, while demand has grown steadily with the growth of the region's population. Projections by the Interstate Commission on the Potomac River Basin (ICPRB) indicate that demand could exceed supply early in the 21st century in the event of severe drought. This report explains how this could occur and discusses several strategies which may enable the region to avoid water shortages in case of drought.


In 1997, the region's water system supplied an average of 468 million gallons per day (mgd). Over 96% of this was provided by three major suppliers; Fairfax County Water Authority serving most of northern Virginia, the Washington Aqueduct serving the District of Columbia, Arlington, and Falls Church, and the Washington Suburban Sanitary Commission serving most of the Maryland suburbs. In addition, smaller water companies serve a few other jurisdictions across the region.

About 75% of the region's water comes from the Potomac River, and the remainder from the Patuxent and Occoquan Rivers. A severe drought in 1966, when flows on the Potomac briefly dropped to below today's production levels, led the region to begin collaborating on drought management. In 1978, the Low Flow Allocation Agreement was signed, specifying how water will be allocated among the three major suppliers in case of drought. In the 1 980s, two dams were completed whose reservoirs store water for release into the Potomac in case of drought. The agreement and the dams were expected to ensure that the region could handle a severe drought without excessively inconveniencing its population.

Demand and Supply Projections

The region's three major water companies are mandated to assess the supply and demand for water every 5 years. The 1995 study, prepared by the ICPRB, provides the information on demand and supply on which much of this report is based.

Demand projections

The ICPRB uses a relatively simple model to project water demand. It considers three types of water users; single family homes, multifamily buildings, and employees. The study estimates "water use factors," coefficients indicating the average quantity of water consumed by each type of user in each jurisdiction. Demand for water is calculated as the number of users in each category multiplied by the water use factor for that category and jurisdiction. The projections of the number of users are based on population projections provided by the local jurisdictions to the Metropolitan Washington Council of Governments (COG). The ICPRB model assumes that the water use factors will remain constant throughout the period of concern; that is, consumption per user will not change in the future.

In addition to the water known to be consumed by households or employees, some water is not accounted for. This unaccounted for water is attributable to leaks, faulty water meters, pipe maintenance, and fire fighting. The ICPRB estimates unaccounted water from historic records. Factoring in unaccounted- for water, the total water demand for each year in the future is estimated as single family use + multifamily use + employee use + unaccounted retail water + unaccounted wholesale water.

Supply projections

Water supply projections are based on historical records of both the free-flowing Potomac River and the inflow and outflow for the reservoirs which currently serve the system. They also take into account an "environmental flow-by" at Little Falls Dam of 100 (mad) to protect aquatic life. To determine when supply might exceed demand, the ICRPB simulated a recurrence of the so-called "drought of record" which occurred in 1930-31, whose probability in any given year is I to 2 per cent. It is important to bear in mind that although this probability is fairly low, such a drought could occur at any time.

Comparison of supply and demand

ICPRB projections indicate that under a reocurrence of the drought of record, demand would exceed supply in 2035. That is, by 2035 population would be high enough to exhaust the water supply in the reservoirs; before that year, existing water storage systems could meet water demand even if the worst drought on record were to reoccur. This projection assumes that water in the Savage River Reservoir, which is designated for diluting polluted water rather than for consumption, would be made available in case of drought. Without that source, the reservoirs could be depleted by 2025.

In practice, the adequacy of water supply would be constrained well before existing storage systems are depleted. Experience with water shortages is limited; however in the past one water supplier made plans to impose water use restrictions if storage dropped to 40% of reservoir capacity. The ICPRB projections indicate that under the drought of record, storage levels could be reduced to 40% as soon as 2015 with the Savage River Reservoir and 2005 without it. Thus, although an actual deficit in the supply does not occur in the projections until 2025 or 2035, heightened concern and the imposition of water use restrictions could occur much sooner. These projections require us to address the problem now.

Strategies to Prevent Water Shortages

The present study identifies several strategies for responding to the prospect of future water shortages. These are conservation, identification of new water sources, reducing waste in the water distribution system, managing growth in the metropolitan area, and reclaiming treated waste water.


Water conservation involves a combination of measures to reduce water consumption or slow its growth relative to projected needs. Water prices can be structured to create a financial incentive for households, manufacturers, and commercial enterprises to reduce consumption. In commercial and industrial establishments, process changes can offer cost-effective reductions in water demand.

Regulatory policy can require technologies to reduce water use; for example, building codes often require low-flow toilets and shower heads in new construction. All of these measures are accompanied by education efforts to encourage households and businesses to reduce water use. Demand reduction may also include use restrictions when extreme conditions occur. Rather than build a supply system adequate to allow lawn-watering during droughts, for example, a community can consciously choose to sacrifice lawns when a drought occurs and invest resources elsewhere.

New sources

Potential new sources of water include surface or ground water from within the Potomac basin and transfer of water from outside the basin. One way to increase water supply during droughts would be to construct one or more additional reservoirs on currently free-flowing reaches of the Potomac and its tributaries. Although this option has been routine in the past, today it would likely face intense opposition and practical difficulties in the Washington area. There has been widespread development both in the valleys that would be flooded and along potential shorelines in much of the basin. In more remote areas, there would be intense opposition to impounding free-flowing streams in scenic areas. Ground water might be obtained from wells in the coastal plain of southern Maryland. Previous studies have indicated that these might offer a maximum yield of 100 mad, but this would be available under drought conditions only and not as a sustainable supply. The transfer of water from outside the Potomac basin, a strategy commonly used in the western United States, would face enormous difficulties and seems unlikely.

Reducing unaccounted-for water

The water utilities consider unaccounted-for water to be the difference between their production or purchase and the quantity paid for by final consumers. Some of these losses are attributable to unmetered uses, such as fire fighting, flushing pipes during routine maintenance, and use in public buildings and parks. Others are due to to illegal hookups or inaccurate meters. Still others are due to actual physical losses from leaky pipes and similar problems. Total unaccounted-for water for the three main suppliers in the Washington area ranges from 10 to 28%. In contrast, the American Water Works Association considers a 10% loss to be acceptable in a well-run system. The incentive to reduce these losses depends in part on whether the cost of doing so is less than the cost of producing additional water. Both the actual losses and the means of detecting and reporting them differ among the utilities. Therefore, detailed studies of unaccounted-for water leading to a concerted reduction effort may be justified.

Growth management

Growth management could limit the increase in regional population and hence the demand for water. Environmentalists will recognize in this approach the principle of limiting growth to the natural carrying capacity of the area. Sentiment for limiting growth fluctuates over time; right now the tools for doing so are more readily available in Maryland than in Virginia. Nationwide, there is a movement to concentrate new development in higher density mixed-use centers where infrastructure, including water and sewer lines, already exists. This results in substantial savings in water use through smaller lawns to be watered, smaller houses, fewer miles of pipes that may be susceptible to leaks, and less water used during flushing. While growth controls may be desirable, they are not a likely strategy for reducing water demand, since they go far beyond the relatively narrow infrastructure concerns of water supply.

Reuse of waste water

The recycling and reuse of treated waste water is common throughout the country where surface water is the raw water source. This option is already in use in the Washington, D.C. area at the Upper Occoquan Sewage Authority (UOSA) treatment plant in Centreville, Virginia. The Fairfax County Water Authority withdraws raw water from the Occoquan downstream from the UOSA plant, treats it by conventional methods, and converts it into potable water delivered to its customers. The reuse of effluent from the Blue Plains treatment plant on the Potomac estuary downstream from Washington may offer a similar opportunity. As much as 200 to 300 mad might be pumped upstream and discharged into the Potomac above the intake used to supply Washington. This would ensure a supply of water to the Washington Aqueduct and permit higher withdrawals from the river by other users. The major concern is with this strategy is clearly its health implications. Technical experts feel that the resulting water can be potable; however, the treatment required may make it costly.


Drinking water shortages may confront the Washington metropolitan area early in the 21st century. This is not a problem which can be put off while the region's jurisdictions cope with more immediate problems. While this report does not identify clear choices among the options for preventing water shortages, we can make a number of useful observations:

  • Water conservation offers both short- and long-term ways for reducing water use. These should be considered irrespective of our approach to water supply.
  • Exploiting new water sources through construction of new reservoirs or building large-scale ground-water extraction facilities may face serious difficulties due to increased population in the areas affected and large-scale land-use changes in recent years.
  • Reducing waste in the distribution systems is a financially viable way to save water where losses are great and the cost of producing additional water exceeds that of finding and fixing leaks.
  • Growth management could lead to reduced demand for water in the region; however, the scope of growth management efforts goes far beyond the issues considered in this report.
  • Reclamation of waste water from the Blue Plains plant is a technical option which warrants additional evaluation with current data.

More thorough technical and financial analyses of all the options are needed in order to make informed decisions about how to address projected water shortages. Concurrently, there must be broad public discussion of the region's water supply issues involving all interested stakeholders. As we approach the 21st century, public policy questions like this one are no longer the purview of water systems engineers alone; they must be addressed by everyone who has a stake in the outcome.

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