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

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

January 22, 2026

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

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

January 22, 2026

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

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

December 5, 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 >>>

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2023 CO-OP Drought Operations

November 22, 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.

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2022 Washington Metropolitan Area Drought Exercise

May 24, 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).
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2020 Washington Metropolitan Area Drought Exercise

March 4, 2021

This report describes activities conducted during the 2020 drought exercise. The exercise was virtual, and took place on Monday, Tuesday, and Wednesday, November 16-18, from 7:30 AM to 4:00 PM.
Communications during the exercise were via telephone, email, and Microsoft Teams Meeting, and all
operations were “simulated.” Twice daily email reports were sent out to stakeholders reporting on current flow and demand conditions and on simulated operations. The exercise included two special events:

  • An actual test release from Little Seneca Reservoir, which was conducted over an approximately
    12-hour period, beginning at 10:00 AM on Tuesday, November 17.
  • A webinar by Hazen & Sawyer on the use of the Potomac OASIS model to provide probabilistic
    information on future streamflows and reservoir storage levels. A PDF of the webinar on forecast informed reservoir operations is available.

Learn more about previous drought exercises and the ICPRB’s Section for Cooperative Water Supply Operations on the Potomac on the Drought Monitoring and Operations page.

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2020 Washington Metropolitan Area Water Supply Reliability Study: Demand and Resource Availability Forecast for the Year 2050

December 4, 2020

Every five years since 1990, ICPRB’s Section for Cooperative Water Supply Operations on the Potomac (CO-OP) has conducted a water demand and resource availability forecast for the Washington, D.C., metropolitan area. These studies assess whether or not the current water supply system will be able to meet the needs of the region 20 or more years in the future.

Learn more about these reports on the CO-OP Long-Term Planning page.

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Planning Assistance to States: Jennings Randolph Lake Scoping Study Phase II Report

August 24, 2020

The watershed of the North Branch Potomac River experienced severe environmental degradation and flooding in the 20th century. A dam across the river mainstem was completed in 1982, creating Jennings Randolph Lake. The lake and dam are operated by the U. S. Army Corps of Engineers for four authorized purposes: control floods, dilute downstream pollution, supply drinking to Washington DC during droughts, and provide recreation. Water quality in the North Branch watershed has improved considerably since the dam was built due to many factors, including regulatory enforcement, mine runoff mitigation, wastewater treatment, infrastructure improvements, forest regrowth and the abatement of acid rain (see ICPRB report 19-4). This pilot study was done to determine if an update of the 1997 Reservoir Regulation manual is appropriate at this time. The report reviews and evaluates each of the authorized purposes in terms of their original management goals and objectives, current relevance, and future application.

A copy of the report is available here.

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