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2025 Washington Metropolitan Area Water Supply Study: Demand and Resource Availability Forecast for the Year 2050

Authors: S.N. AHMED, K. BENCALA, S. NUMMER, C.L. SCHULTZ, AND A. SECK | Pub: # ICP-596 | Published in December 2025

This is the eighth in a series of studies by the Interstate Commission on the Potomac River Basin (ICPRB) Section for Cooperative Water Supply Operations on the Potomac (CO-OP) to evaluate the adequacy of the Washington, DC, metropolitan area (WMA) water supplies to meet expected water demands over the coming decades. The WMA has a unique, cooperative water supply system that was established more than 40 years ago by a set of regional agreements signed by the Fairfax County Water Authority (Fairfax Water), the Washington Suburban Sanitary Commission (WSSC Water), the Washington Aqueduct Division of the U.S. Army Corps of Engineers (Washington Aqueduct), the District of Columbia, and CO-OP. One of the requirements of the regional agreements is that every five years, such an evaluation be conducted. The current study’s planning horizon is 2050. It provides decision-makers with the following:

Forecasts of water demands for the WMA, 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 meteorological conditions and upstream consumptive use of water on system resources
An evaluation of the ability of existing and planned system resources to meet the forecasted demands

A fact sheet is available with a summary of the report’s findings >>>

Stream Health in the Chesapeake Bay Watershed, 2018 – 2023 Update

Authors: C. Buchanan, R. D. Jepsen, and E. Young. | Pub: # ICP25-1 | Published in December 2025

Non-tidal stream health in the Chesapeake Bay watershed is evaluated every six years to accommodate rotational sampling schedules of the major stream monitoring programs. A 2024 Data Call brought in stream macroinvertebrate data and related water quality and habitat data collected by eighteen monitoring programs in the watershed between 2018 and 2023, the fourth 6-year interval. The percentage of stream miles with healthy macroinvertebrate communities increased an estimated 2,026 miles in this interval, for a 1.4% improvement over the previous interval. High flows in 2018 and 2019 and the COVID19 pandemic in 2020 disrupted monitoring activities, and stream evaluations in the interval are based on less data. Since 2000 – 2005, the first 6-year interval, stream miles with healthy macroinvertebrate communities have increased 10.3% overall, and an estimated two-thirds (66.7%) of stream miles in the watershed are now thought to be healthy. Improvement is unevenly distributed in the watershed. Stream health appears to be improving in eight more rural western and southern bioregions; it is not improving in two central, heavily populated bioregions, and is degrading in two eastern, highly agricultural bioregions. Additional data and new criteria for separating streams from rivers have slightly changed the percentages of healthy streams for previous intervals but not the overall findings. The average rate of improvement is 3.4% over a 6-year interval, which suggests the revised Stream Health goal can be achieved in 2024 – 2029, the next 6-year interval, as ongoing efforts to restore and protect the Bay watershed continue.

Learn more about Chessie BIBI >>>

Is Hot Drought a Risk in the US Mid-Atlantic? A Potomac Basin Case Study

Authors: C. L. Schultz, A. Seck, S. N. Ahmed Published in June 2025

This paper was published in the Journal of the American Water Resources Association (Volume 61, Issue 3, June 2025)

Interannual variability of streamflow will increase under a future climate, but at the regional scale, there is uncertainty regarding changes in drought severity, and in particular, changes in extreme hydrological drought that could necessitate new water supply infrastructure. This is due to the wide range of regional projections for precipitation and the challenge of estimating statistics in a nonstationary climate. We assess changes in annual streamflow in the Potomac River Basin using a nonparametric approach based on a climate response function and the K-nearest neighbor method, which is relied on to construct time series of sufficient length to compute extreme quantile values. Our results indicate that future Potomac River flows will be impacted by “hot drought”, that is, increasing drought severity caused by rising temperatures coupled with natural variability in precipitation. Average precipitation is projected to increase in the Potomac basin by 9%–12% in the period 2039–2069 and by 11%–16% by 2070–2099. Average streamflow increases more modestly, by 4%–7% in 2039–2069 and by 2 to 9% in 2070–2099, whereas annual flows in an extreme drought year decrease by 3 to 26% in 2039–2069 and by 2%–49% in 2070–2099, assuming a medium sensitivity of flow to temperature. Our approach can provide multi-model consensus inputs for water supply planning models to support decision-making regarding new infrastructure.

DOI number: https://onlinelibrary.wiley.com/doi/10.1111/1752-1688.70031

Evaluating time-lagged relationships between groundwater storage and river discharge using GRACE-based data: insights from the Potomac Basin

Authors: AJ Villaruel, Alimatou Seck and Cherie Schultz Published in July 2025

This paper was published in the Environmental Research Communications (7 075003)

This study evaluates the utility of a recently available GRACE-based groundwater drought index (GDI) in supporting regional water supply management, with application to the Potomac River Basin, in the U.S. Middle Atlantic region. As the primary drinking water source for the Washington Metropolitan Area (WMA), effective management of the Potomac River’s resources is critical, especially in the context of climate change, with the expected increase in severity and frequency of extreme events. Our analysis integrates 22 years of data, including GRACE-based groundwater storage (GWS) index estimates, river discharge (Q) measurements, and meteorological records, to investigate trends and predictive relationships between past GWS, as determined by the GRACE-based drought index, and streamflow. Seasonal Mann-Kendall trend analyses consistently identified severe declining trends in groundwater storage (GWS), as well as moderate declines in minimum streamflow and well water levels over the past 22 years. Granger Causality (GC) tests revealed significant time lags of 49 weeks to 22 months at weekly and monthly scales, respectively depending on a region’s hydrogeomorphic characteristics. Vector Autoregressive (VAR) Models and Forecast Error Variance Decomposition (FEVD) highlighted the variable contributions of precipitation and temperature to the GWS-Q relationship, revealing a strong autoregressive component of Q, but also reveal that GWS plays an important role, and this role increases with time. These findings underscore the interconnectedness of groundwater and surface water systems and the importance of integrated predictive models to enhance water management strategies. Incorporating GRACE-based seasonal groundwater forecasts into drought preparedness tools could bolster efforts to mitigate regional climate change impacts and improve the resilience of water resources in the Potomac River Basin. While practical use of native GRACE data has been challenging for local, small-scale applications, this study demonstrates the utility of the GRACE-based GDI in forecasting low flows and informing regional water resource management decisions during droughts.

DOI: 10.1088/2515-7620/ade36f

An Inventory of Potomac Basin Entities with a Role in Sustainable Water Resources Management

Authors: Heidi Moltz Published in March 2025

The Potomac Basin Comprehensive Water Resources Plan (2018) outlines a shared vision of sustainable water resource management in the basin. Responding to a Plan recommendation, the Interstate Commission on the Potomac River Basin (ICPRB) created an inventory of organizations with a role in sustainable water resources management in the basin in 2022. The inventory was the product of an iterative process initiated by ICPRB staff with input from the plan’s Advisory Committee.

Potomac Basin Trends in Water Use

Authors: Heidi Moltz, Andrea Nagel, and Stephanie Nummer-Fantozz Published in March 2024

This pamphlet summarizes projected reported water withdrawals and consumptive use (CU) in the Potomac basin. This work, along with the results
presented in the Potomac Basin Reported Water Use pamphlet and Wallace et al. (2023), addresses the technical recommendation, “Report on Basin-Wide Water Uses, Projected Demands, and Consumptive Demands” in the Potomac Basin Comprehensive Water Resources Plan (Recommendation 3.3.2 A).

Evaluation of reported and unreported water uses in various sectors of the Potomac basin for the year 2017

Authors: Carlington W. Wallace, Heidi L. N. Moltz, Andrea Nagel, Stephanie Nummer, Karin R. Bencala | Pub: # PRC 24_2 | Published in July 2024

Water resource planners and managers in the Mid-Atlantic United States typically determine the sufficiency of water supplies to meet demand by comparing (1) water use as reported to the state by individual water users to (2) metrics of water availability calculated from observed water monitoring networks. This paper focuses on determining whether this means of measuring water use is sufficient for proactive and sustainable management of water resources. The Potomac basin study area illustrates the point that, while state-reported water use databases typically cover the largest individual water users, unreported water uses can cumulatively comprise a substantial portion of the overall water use. If left unaccounted for, the system is vulnerable to human demand exceeding supplies, with attendant detrimental effects to aquatic habitats and organisms, especially given the exacerbating effects of climate change on the variability of water supplies. Planners and managers are therefore encouraged to consider the full spectrum of water uses, regardless of state reporting requirements.

2023 CO-OP Drought Operations

Authors: S. Ahmed, C. Davis, H. Moltz, S. Nummer, C. Schultz, and A. Seck | Pub: # ICP24-1 | Published in November 2024

The Washington, DC, metropolitan area experienced unusually dry conditions in the summer and fall of 2023, and flows in the Potomac River fell to levels requiring the Interstate Commission on the Potomac River Basin (ICPRB) Section for Cooperative Water Supply Operations on the Potomac (CO-OP) to conduct drought response activities in support of the major regional water suppliers: Fairfax Water, Loudoun Water, WSSC Water, and the Washington Aqueduct, a Division of the U.S. Army Corps of Engineers. This report provides a brief summary of these activities and of related issues and discussions that arose. It also documents the take-aways of a Post-Drought Operations Review meeting that took place on November 3, 2023, and a subsequent meeting of the CO-OP Operations Committee on November 17.

Stream Biological Health in the Chesapeake Bay Watershed

Authors: Claire Buchanan, Rikke Jepson, and Michael E. Mallonee | Pub: # ICP23-1 | Published in February 2023

To learn more about this project and find interactive maps, check out the webpage for “Chessie BIBI” Index for Streams .

Executive Report

This report offers a numeric value for the 2008 Baseline referenced in the 2014 Chesapeake Agreement’s stream health goal as well as evidence of a net improving trend in stream health in the Chesapeake watershed. The report demonstrates a process for tracking progress in achieving the stream health goal to “improve health and function of ten percent of stream miles above the 2008 baseline.” The bioregion, family-level version of the Chesapeake Basin-wide Index of Biotic Integrity, or “Chessie BIBI,” is used to quantify stream health. The index is calculated from macroinvertebrate data collected by state, federal, county, and volunteer monitoring programs with kick net methods and was developed specifically for 1st – 4th order streams in the Chesapeake watershed (Smith et al. 2017). The 2008 Baseline is the 2006 – 2011 period because it encompasses all sampling schedules of the watershed’s state monitoring programs, most of which employ rotational sampling.

Gaps in the monitoring data’s spatial and temporal coverage make it difficult to directly estimate percentages of healthy streams in the pre-baseline (2000 – 2005), baseline, and subsequent “first interval” (2012 – 2017) periods. Statistical analyses indicate approximately 61.7% (~89,317 miles) of non-tidal stream miles likely supported healthy macroinvertebrate communities in the baseline period. The percentage increased to 67.8% (~98,049 miles) in the first interval. Despite this roughly 6% net improvement, some areas of the watershed show degrading trends. The net improving trend, however, suggests the collective impact of multiple environmental stressors on streams may be slowly lessening in many parts of the Chesapeake watershed. Identifying which factors are responsible for the net improvement would be speculative at this point, although long-term efforts to conserve forests, preserve and restore riparian corridors and wetlands, mitigate acid rain and mine drainage, slow stormwater runoff, and reduce nutrients and sediment loads have all likely contributed. Metrics for a variety of environmental stressors are currently being explored and will help future investigations of stream macroinvertebrate responses to those stressors. They can help explain why the current trend is happening.

The purpose of this report is to present the monitoring-based results and provide CBP with a process for tracking progress in achieving the Chesapeake watershed’s stream health goal. The process differs in some respects from those of the state agencies who use the data differently and for state regulatory purposes. We fully expect the Chessie BIBI results will also differ from state results at times, even though the underlying raw data are the same. The Chessie BIBI can be used for inter-jurisdictional, watershed-based planning and evaluation.

2022 Washington Metropolitan Area Drought Exercise

Authors: S. Ahmed, C. Davis, H. Moltz, S. Nummer, C. Schultz, and L. Vawter | Pub: # ICP23-3 | Published in May 2023

The Washington, D.C., metropolitan area (WMA) relies on the Potomac River for over three-quarters of its water supply. The area’s three major water suppliers (“CO-OP suppliers”), Fairfax County Water Authority (Fairfax Water), Washington Suburban Sanitary Commission (WSSC Water), and Washington Aqueduct (a Division of the U.S. Army Corps of Engineers) participate in a cooperative system of water supply planning and management. This participation includes joint funding of water supply storage in reservoirs located upstream of the suppliers’ Potomac River intakes and coordinated operations during droughts.

During times of drought, the Section for Cooperative Water Supply Operations on the Potomac (CO-OP) of the Interstate Commission on the Potomac River Basin (ICPRB) plays a crucial role in coordinating water supply operations. By coordinating withdrawals from the Potomac River, Patuxent, and Occoquan reservoirs, the CO-OP staff help ensure that water resources are being utilized efficiently and effectively for the benefit of the system. When the forecasted flow in the river is not sufficient to meet expected demands, the CO-OP staff make requests for releases from upstream reservoirs. These demands include the water supply needs of the WMA and an environmental flow-by of 100 million gallons per day (MGD), or 155 cubic feet per second (cfs), on the Potomac River below the Little Falls Dam near Washington, D.C.

The ICPRB CO-OP section conducts an annual drought exercise to maintain readiness for drought conditions. These exercises serve as a platform for CO-OP staff to evaluate and discuss water management strategies with relevant stakeholders, prior to a real drought scenario. The activities aid in training CO-OP staff on regional agreements, tools, and decision-making processes. Moreover, they offer participants the chance to refine their communication processes and enhance organizational efficiency.

This report describes activities conducted during the 2022 Drought Exercise. The virtual training took place on Tuesday, Wednesday, and Thursday, November 15-17, from 7:30 A.M. to 2:00 P.M. Communications during the exercise were via telephone, email, and Microsoft Teams, and all operations were “simulated.” Stakeholders received twice-daily email reports on “actual” precipitation, river flow, water withdrawals, and “simulated” operations and reservoir storages. This year’s exercise included the following elements:

A regional Drought Coordination Technical Committee (DCTC) conference call to discuss potential water use restrictions associated with the Metropolitan Washington Council of Governments (MWCOG) “Warning” stage,
Communication with Washington Aqueduct on the Low Flow Allocation Agreement (LFAA) thresholds, and
Data collection and operational forecasts through CO-OP’s Data Portal and daily flow forecast tool to determine the need for “simulated” releases from Little Seneca and North Branch reservoirs (Jennings Randolph and Savage).