by Peter K. Weiskel and Phillip J. Zarriello, USGS, MA-RI District, Northborough, MA

The 162 mi² Sudbury River Basin, in the western portion of the Boston metropolitan area, has experienced rapid changes in land-use, water use, and population over the past 15 years. From 1990 to 2000, town populations in the basin increased an average of 21 percent. The Towns of Hopkinton and Westborough, located in the southwestern headwaters of the basin (fig.1), were among the fastest growing towns in the State over this period (45 and 27 percent increases in population, respectively). Although some of the larger basin communities, such as Framingham and Marlborough, are served partially or completely by imported water from the Massachusetts Water Resources Authority (MWRA), the most rapid growth is occurring in communities that withdraw water exclusively from the Sudbury Basin.

Increased withdrawals from the shallow, valley-fill aquifers of the basin have begun to affect the flow of the Sudbury River mainstem and its tributaries, especially in the summer when flows are naturally at their lowest. The Massachusetts Department of Conservation and Recreation (MDCR) considers the Sudbury to be a "stressed basin", where water use practices have the potential to impact aquatic habitat (V. Gartland, written commun., 2002). MDCR also manages several reservoirs in the basin for purposes of recreation, flood control, and water supply, and the Massachusetts Department of Environmental Protection (MDEP) regulates over 30 public-supply wells in the basin (fig. 1). In spite of the large number of public-supply wells in the basin, the Metropolitan Area Planning Council (MAPC) has shown that the Towns of Hopkinton, Ashland, Wayland, and Sudbury will likely not have sufficient water supplies by 2025 to meet demand (Flint, 2001). Finally, there is concern that low-flow conditions may begin to affect aquatic habitat in the Great Meadows National Wildlife Refuge, which is managed by the U.S. Fish & Wildlife Service (USFWS) and contains one of the largest riparian wetland ecosystems in Massachusetts.

To improve the understanding and management of water resources in the Sudbury River Basin, the U.S. Geological Survey proposes to conduct an investigation, in cooperation with MDCR, MAPC and other governmental and non-governmental stakeholders. The purpose of the investigation is to evaluate streamflow conditions and the effects of existing and proposed practices regarding reservoir management, water withdrawals, and wastewater return flows. The model will help MDCR, the basin communities, and other stakeholders to better manage the ground and surface waters of the basin, consistent with needs and constraints of water supply, waste-water discharge, recreation, flood control, and aquatic biota. The USGS has extensive experience simulating the response of Massachusetts river basins to withdrawals, discharges, diversions, and land-use change (Zarriello and Ries, 2000; Zarriello, 2002a; 2002b; DeSimone and others, 2002; DeSimone and others, in prep.). The specific objectives of this study are to:

(1) Construct a simulation model that accurately represents present-day rainfall-runoff and water-storage behavior in the Sudbury Basin, using existing hydrologic, surficial-geologic, land-use, and water-use information.

(2) Use the calibrated model to simulate the effects of alternative water-management, land-use, and potential drought scenarios on streamflows and storage in the basin.

Objective 1. Model construction and calibration. The USGS proposes to use the Hydrologic Systems Model--Fortran (HSPF) to simulate the hydrology of the basin. This model incorporates surficial-geologic, water-storage, land-use, and water-use information (withdrawals, discharges, and inter-basin transfers), and is driven by hourly meteorological data. The long-term USGS stream gage on the Sudbury River at Saxonville (106 mi² drainage area, about two-thirds of the basin area; see fig. 1 for location) will serve as the main point for model calibration. Partial record sites will be established near the basin mouth and at several main tributaries to calibrate other areas of the model. Continuous daily-discharge records will be developed for the model calibration period (expected to span about 5 years) from the relation of instantaneous flow measurements at the partial record site to nearby stream gaging stations (see, for example, DeSimone and others, 2002 and DeSimone and others, in prep.).

A study of water use in the Sudbury, Assabet, and Concord Basins, now being conducted by USGS in cooperation with MDCR and MDEP, is expected to provide most of the water use information needed for model simulations (Barlow and others, written commun., 2003). However, these data will need to be transformed from a monthly time step to a daily time step to be consistent with the HSPF model. To accomplish this task daily records will be obtained from suppliers able to share this information.

Operational practices and constraints of the major lakes in the tributary Sudbury River basin--Whitehall Reservoir in Hopkinton (601 acres), Lake Cochituate in Framingham, Natick, and Wayland (594 acres), and the Sudbury Reservoirs in Marlborough and Southborough (1,292 acres)--will be incorporated into the HSPF model by creating a "special action" to simulate dam operations. Operational aspects will include maintenance of optimal seasonal levels for (1) recreation, (2) flood control, and (3) streamflow augmentation during low flow periods. Operational constraints and other model features will be developed in cooperation with the MDCR and other regional, State and Federal agencies responsible for water resources in the basin (e.g., the Massachusetts Department of Environmental Protection [MDEP] and the MWRA).

Objective 2. Model scenarios. When the HSPF model is developed and calibrated, a select number of management scenarios will be simulated. Basin communities and non-governmental stakeholder organizations will be asked to participate the selection process. A Technical Advisory Committee will be established to provide USGS with input concerning potential model scenarios. The effects of the following stressors on basin water resources could be assessed:

• changes in land-use/land-cover (for example, "build-out" to the limit of current zoning).
• changes in the location, rate, and timing of ground- and surface-water withdrawals.
• changes in the location, rate, and timing of wastewater discharge.
• changes in the transfer of water or wastewater into or out of the basin.
• water conservation practices.
• stormwater management practices.
• reservoir management practices.
• potential drought scenarios.

The effects of these scenarios on streamflow will be evaluated from long-term (1960 to recent) simulation results, by analyzing changes in the duration and frequency of high and low streamflows at selected points in the basin (see, for example, Zarriello and Ries, 2000). The effects of selected scenarios on reservoir storage and the flooding duration (hydroperiod) in major wetland reaches, including portions of the Great Meadows National Wildlife Refuge, could also be assessed.

The study results will be published in a USGS Water-Resources Investigations Report. A Fact Sheet will also be published to summarize results for the general public. The models generated by the project will be made available in electronic form to the funding agencies, the basin communities, and the interested public.

The proposed study is directly relevant to the needs of the cooperating agencies and the mission of the USGS. It will have the following benefits:

• The study will advance basic understanding of how regional hydrologic systems in New England respond to rapid land-use and water-use change (suburban sprawl).
• The study will advance a key science priority of both the USGS Director and the Eastern Region Director: availability of water for human and ecological needs.
• The study will extend the capabilities of the HSPF watershed model by creating a special action to address dam operations for multiple reservoirs.
• The study will provide useful information and simulation tools to non-governmental organizations and government agencies at the local, regional, State, and Federal levels--each of whom are responsible for different aspects of the basin's water resources and have potentially conflicting interests.
• Data collected by the study will be incorporated into national data bases, and modeling results of the study will have transfer value to other rapidly developing regions of the Nation attempting to balance multiple demands upon water resources.