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).
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.
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.
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:
Expanding on current efforts to evaluate the role of groundwater dynamics in managing and restoring Chesapeake Bay (USA), the integrated hydrologic model ParFlow-CLM was applied to a 374,976-km2 area encompassing the Chesapeake Bay watershed. The model included a representation of surface water, groundwater and land-surface energy fluxes with spatially variable atmospheric forcing at an hourly time step. The study tackled issues of data availability, access, assembly, and synthesis for estimating hydrogeologic properties in the context of the development of a large-scale model. Hydrogeologic properties from literature and other sources were assembled, processed, and synthesized to derive a conceptual hydrogeologic model consisting of 29 hydrofacies and a three-dimensional hydraulic conductivity field. Evaluation of the ParFlow-CLM model output showed that the constructed model captured seasonal and spatial variability in subsurface storage, surface storage and surface runoff, and produced water-table depths consistent with the topography, meteorological forcing, and hydrogeology. Comparison with well data from the US Geological Survey showed good agreement of model output with observed hydraulic heads for most of the data. Modeled terrestrial water storage changes compared well with GRACE satellite data with a root mean square error of 2.3 cm. Model results showed the dominant contribution of subsurface storage changes (90%) to terrestrial water storage changes in the region.
To learn more about this project and find interactive maps, check out the webpage for “Chessie BIBI” Index for Streams .
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.
The project examined whether stormwater management practices implemented under MS4 permits can lead to measurable differences in stream conditions compared to similar watersheds with few or no stormwater practices and to highly forested reference watersheds.
Copies of Appendix A and Appendix B are available online.
In 2021, Commissioners of the Interstate Commission on the Potomac River Basin (ICPRB) passed a Resolution on Enhancing Water Supply Resilience for the Washington Metropolitan Area. This resolution is the first step in updating the two foundational agreements of the Washington metropolitan area cooperative water supply system: the Low Flow Allocation Agreement (LFAA) of 1978 and the Water Supply Coordination Agreement (WSCA) of 1982. To facilitate such an update the resolution called for the following action items:
To address the third action item, a virtual workshop was held over one-and-a-half days in May 2022, with the explicit purpose of answering the following questions with respect to the Potomac River, which supplies most of the Washington, D. C., metropolitan area drinking water:
The information presented and discussed during the workshop provides input to the LFAA workgroup in the event the group recommends revisiting the current environmental flow-by target used during low flow periods. The question of whether or not to study the flow-by was informally discussed during the workshop but the intent of the workshop was to gather the relevant information, not recommend a course of action.
In accordance with one of the technical recommendations of the Potomac Basin Comprehensive Water Resources Plan’s water use and supplies challenge area, this pamphlet has been produced to document and share high-level results. This pamphlet provides a “report on basin-wise water uses,” and ultimately acts as a first step toward estimating, “projected demands and consumptive demands.”
Uncertainty quantification between simulated and observed water quality simulations needs to be improved. This study generated and evaluated probabilistic hydrologic and water quality predictions in 18 locations across the U.S. using residual-based modeling. A Box-Cox transformation scheme group provided the best predictive uncertainties for all case studies. The tradeoffs in the performance metrics for a single variable predictive uncertainty in a single study watershed were more obvious than those for all hydrologic or water quality cases. Compared to a single realization of simulations, the ensemble average of hydrologic and water quality simulations better represented the predictive uncertainty, especially for large watersheds. This study recommends various opportunities via residual error scheme selection, data monitoring improvement, and hydrologic model enhancement to robust hydrologic and water quality predictive uncertainties. The results could improve the quantification of the predictive uncertainty of hydrologic and water quality simulations and guide probabilistic prediction enhancement.
More information about the paper is available on ScienceDirect.com.
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