Washington DC metropolitan area water supply demand forecast and resource availability analysis
A study by the Interstate Commission on the Potomac River Basin.

Introduction

The forecasting of demand is critical to those involved in water resources planning. These forecasts help managers assess the adequacy of the present resources to meet future demands. Since the time required is lengthy to build new resources or implement demand management strategies, forecasts of future demands help managers and municipalities to plan for the future.

The study was conducted in two parts. The first study element provides an estimate of the Washington metropolitan area (WMA) water supply demands in the year 2020. The second major study element shows how the current system of rivers and reservoirs functions while meeting estimated future demands. The main focus of the study is to assess the ability of the regional water resources to meet the water supply needs of the WMA population as it continues to increase.

Background

The majority (approximately 90 percent) of the WMA’s population relies on water furnished by three agencies:

• The Washington Aqueduct Division of the U.S. Army Corps of Engineers (Aqueduct) serving the District of Columbia and portions of Virginia.
• The Fairfax County Water Authority (FCWA) serving parts of northern Virginia.
• The Washington Suburban Sanitary Commission (WSSC) serving the Maryland suburbs.

These agencies supply treated water either directly to customers or through wholesale suppliers. The three major water suppliers cooperate on water supply operations in the Potomac, essentially operating as one entity in sharing water across the Potomac, Patuxent and Occoquan basins in periods of low flow. This cooperative work is coordinated by a special section of ICPRB, called the "Section for Cooperative Water Supply Operations" (CO-OP).

The study applies to the Washington metropolitan area CO-OP member water suppliers and their wholesale customers. The wholesale customers of the CO-OP member water suppliers include the Loudoun County Sanitation Authority, the Prince William County Service Authority, the Virginia American Water Company, the Vienna Department of Public Works, the District of Columbia Water and Sewer Authority, the Arlington County Department of Public Works, and the Falls Church Department of Public Works.

The natural flow in the Potomac River supplies approximately 75 percent of the water supply withdrawals in the WMA, with the remainder supplied by FCWA’s Occoquan Reservoir and WSSC’s Patuxent reservoirs. The Potomac is the sole source of supply for the Aqueduct. All three suppliers contribute to the cost of construction and operation of two reservoirs (Jennings Randolph and Little Seneca) in the Potomac River basin which are used for low flow augmentation.

The Low Flow Allocation Agreement (LFAA), signed by the United States, Maryland, Virginia, the District of Columbia, the Washington Suburban Sanitary Commission, and the Fairfax County Water Authority, requires that "In April 1990 and in April of each fifth year thereafter ... the Aqueduct, the Authority, the Commission and the District shall review and evaluate the adequacy of the then available water supplies to meet the water demands in the Washington Metropolitan Area which may then be expected to occur during the succeeding twenty year period." At their meeting of April 28, 1999, the parties to the LFAA directed the ICPRB’s CO-OP Section to conduct the required review and evaluation of demands and supplies. This report is the third of three such reports prepared by ICPRB.

Demand projection

The estimate of future demands is based on three types of water uses, namely single family household use, multi-family (apartment) water use, and employee water use. Projections of numbers of households and employees were based on the Metropolitan Washington Council of Governments (MWCOG) employment and household projections, which were collaboratively developed by MWCOG and local government planners and demographers. Coefficients were developed for each jurisdiction in the WMA to describe average daily water use by each type of water user. Demand estimates were developed by multiplying estimates of the number of each type of water user and the coefficients describing average water use for each jurisdiction.

Estimates of future demand developed for this study take into account several factors that can affect future demand. Per household water use was assumed to be lower in the future than it is today as a result of the Energy Policy Act of 1992. Seasonal water use patterns were used to convert the forecast of annual average demand into summertime peak use estimates. High estimates of future growth were used as an alternative basis on which to predict future demand. An estimate was made of unaccounted/unmetered water use for each jurisdiction. The impacts of potential climate change on future demand were examined.

Resource analysis

The resource analysis examines the existing water system’s ability to meet future demands. The operation of the water resource system is modeled so that the Occoquan and Patuxent reservoirs are used sustainably, and Jennings Randolph and Little Seneca reservoirs are used to augment low flows in the Potomac River.

As part of the resource analysis developed in this study, several factors that can affect future resources were examined. Emergency demand reduction strategies, i.e., voluntary and mandatory restrictions were modeled. (Voluntary restrictions were assumed when combined Jennings Randolph and Little Seneca reservoir storage was less than 60 percent full, and mandatory restrictions were assumed when either reservoir storage was less than 25 percent full.) The effects of siltation on reservoir storage over time were incorporated into the analysis. Increasing return flows from wastewater treatment plants upstream of the Potomac water supply intakes and Occoquan Reservoir were included. The current recommended environmental flow rate for Little Falls was included. The efficiency of a Jennings Randolph release was investigated and included. (Not all water released from Jennings Randolph is used as water supply due to the inability to make perfect forecasts of demand and river-flow 9 days ahead of time, which is the travel time of a Jennings Randolph release to the water supply intakes.) The effects of climate change on resources were investigated. The effects of upstream consumptive demand on historical streamflow resources were included.

Results

Under the most likely population growth scenario, demands will increase by approximately 100 mgd for the CO-OP utilities from a current average annual water use of 480 mgd to 579 mgd, an increase of 21 percent. The high growth scenario results in an increase of annual water use from 480 mgd to 606 mgd, an increase of 126 mgd or 26 percent.

Although the MWCOG population forecast was for the year 2020, the forecast was extended to the year 2040 by assuming a continuation of similar rates of growth. This extension allowed for a broader analysis of when the water resource system might be stressed. It should be noted that the population forecast (and corresponding demand forecast) beyond the 2020 horizon is a rough approximation.

A range of demand forecasts was compared with the available resources. Assuming a repeat of the drought of record, the following results were obtained:

• Using the most likely growth scenario, current resources met 2020 levels of demand with about 20 percent reserve storage in the Potomac reservoirs.

• Using the most likely growth scenario, current resources met 2030 levels of demand with about 10 percent reserve storage in the Potomac reservoirs.

• Using the high growth scenario, current resources met 2020 levels of demand with about 15 percent reserve storage in the Potomac reservoirs.

• Using the high growth scenario, current resources met 2030 levels of demand with about 1 percent reserve storage in the Potomac reservoirs.

Additional results were obtained from an investigation of model sensitivity analyses:

• Storage in the Potomac reservoirs was nearly depleted given the most likely forecast of 2020 demands and a reduction in streamflow resources of ten percent.

• The potential effects of climate change on resources were investigated but were not explicitly included because there was a lack of any clear climate change result for this region’s resources.

• If climate change were to occur, demands could increase and streamflow resources could decrease relative to historical conditions. Resource sensitivity analysis indicates that given a reduction in historical streamflow of 5 percent and a 9.5 percent increase in June through September demands, the system of reservoirs could meet most likely 2020 demands but reserve storage would be nearly depleted.

• The average annual demand for the WMA is forecast to increase by approximately 100 mgd for the most likely scenario and approximately 126 mgd for the high growth scenario in the year 2020. Sensitivity analysis shows that the current system of reservoirs would be able to meet an increase in average annual demand of up to 150 mgd under a repeat of 1930-1931 flow conditions.

Conclusions

Two demand forecasts (most likely and high growth scenarios) were compared with the available resources. Assuming a repetition of the drought of record the following conclusions can be made:

• The current system of resources is adequate to meet the most likely and high growth estimates of 2020 demands even if the worst drought of record was to be repeated.

• Storage in the Potomac reservoirs was depleted given demands in excess of the high growth forecasts for 2030.

• Reserve storage in the Potomac reservoirs was sensitive to small reductions in the historical streamflow data.

• Climate change may have an impact on resources that would change the study results, especially given the sensitivity of Potomac reservoir storage to changes in historical streamflow data. Uncertainty in the current state of knowledge of future climate change precludes an acceptable forecast of what the effect on resources might be.

• Because of the current uncertainty and magnitude of impact of the potential effect of climate change on resources, future demand and resource studies might consider: 1) an examination of how extreme droughts might be influenced by potential climate change, and 2) a stochastic analysis to quantify the risks of experiencing a drought that is more extreme than historical observed droughts.

• A change in the minimum environmental flow rate might affect the results of the resource availability analysis.