Water Budgets

Problem

John Wilcox ©Boston Herald

Water resources are being increasingly stressed throughout Massachusetts by urbanization.  Urban growth produces more impervious surfaces, greater water withdrawals, and more movement of water and wastewater far from their sources of origin.  A water budget allows for an accurate assessment of the combined urban impact.  The approach uses a balance-sheet to account for all the water that enters or leaves a watershed and estimates the impact on watershed’s natural streamflow.

Objective

CRWA and ESS Group were selected by the Executive Office of Environmental Affairs (EOEA) to perform water budgets for small subbasins in all communities in the Commonwealth.  The water budgets are designed to assess the impact of current water use on local groundwater and streamflow.  The assessment is intended to provide a framework for long-term water resources planning and protection of essential aquatic ecosystems.

This statewide project will produces water budgets for all the 2,200 subbasins (small watersheds) in Massachusetts.  This project, which commenced in November 2005 and will continue until June 2008, dovetails well with CRWA’s flow trading efforts.  Statewide maps of subbasin impact will aid in prioritizing restoration efforts and could form the basis for initiating a trading program using water banking.

Approach

Our ground-breaking work in water budgets began in the Charles River watershed but is applicable in other watersheds as well.  The approach evolved from a seasonal analysis of a larger watershed to a monthly budget for all small subbasins about five square miles in area.  The current approach focuses on the impacts on baseflow or low flow.

The water budget includes water withdrawals, wastewater discharges, import and export of water via pipe networks, evaporative losses from irrigation, and lost recharge from impervious surfaces (roads, parking lots, buildings).  Return flow from septic tanks and infiltration of groundwater into sewer pipes are implicitly included.

Because streamflow is seasonal and small impacts can have a large relative impact during low flow periods, the net water gain or deficit is compared against an estimate of the natural streamflow for each subbasin.  The median of the monthly average natural flow is considered a yardstick of streamflow that is protective for fish, so a statistical estimate of this value is used as the “natural” streamflow for determining the relative impact.

The results are graphically depicted by mapping the spatial and seasonal variation of the impacts for all subbasins.

Prior Work

CRWA developed this water budget methodology in project for the Town of Blackstone to help them prioritize recharge sites.  The Town has water resources impacts from large water withdrawals, a newly-developed sewer system, and localized impervious areas.  The water budget identified the Lower Mill River and the Quick River as the most stressed subbasins in the town, primarily because the public water wells are in, or near, their sub-watersheds.

The impacts of impervious surfaces and sewers were greatest in the spring since high groundwater levels aids infiltration into sewer pipes and stormwater runoff from impervious surfaces is not absorbed by adjacent wet soils.  The impacts of pumping and irrigation peaked in the summer.  Streamflow impacts were greater in the summer when streamflows are naturally low.  With more development, there will be more withdrawals, irrigation losses, impervious areas, and sewered areas that will further reduce streamflows in the town.

Current and Future Work

The Stony Brook pilot project is the first analysis in EOEA statewide water budgets effort.  A computer program was developed by CRWA/ESS to automated the water budget analysis and reporting.  This program synthesizes all available data down to the subbasin level, calculates subbasin impacts, maps the results, and creates a summary reports for watershed and all associated towns.  This pilot project will be completed by the end of February, 2007.

The automated water budgets program will be refined and applied to all subbasin in Massachusetts.  The final product will include a simple “what if” tool to evaluate various water management or remediation options.