Water Use and Balance

How do you know when water withdrawals or other activities such as land developement are affecting streamflow and water quality? One useful tool is a simple "water balance," an analysis showing how much water is entering and leaving a drainage basin (aka watershed). The analysis can be done on large watersheds like the Assabet's or on smaller sub-basins, like Cold Harbor Brook's. (Another tool is the more complex groundwater model.)

But first . . . where does our drinking water come from?
Let's take a look at the communities in the Assabet watershed. Seven of the Assabet's 20 communities -- Acton, Concord, Hudson, Marlborough, Maynard, Northborough and Westborough -- get all or part of their public water supply from groundwater and surface water sources in the Assabet watershed. (Note that not everyone in these communities has municipal water; some residents and businesses have private wells or buy water from private water suppliers.) The table below shows the percent of the public water supply in these towns coming from the Assabet River watershed in 2002.

Withdrawals for Public Water Supply (2002)*
Community Total 2002 reported water use (MGY**) Water withdrawn from Assabet watershed (MGY) Percent of public water supply from Assabet watershed
Acton 679.7 679.7 100%
Concord 861.0 209.9 24%
Hudson 877.1 778.8 89%
Marlborough 1,818.8 215.2 12%
Maynard 349.6 349.6 100%
Northborough 344.8 0.0*** 0%***
Westborough 803.7 509.8 63%
* Public Water Supply Annual Statistical Reports 2002. Massachusetts DEP
** MGY = million gallons per year
** 100% of Northborough's water came from MWRA in 2002

Seasonal water use:
During the summer we use more (in some cases much more) water - mostly for lawn watering and other outdoor uses such as car washing. Data from the 2002 water-use reports shows that average daily water use in the seven communites increased 12-49% during the summer. Increased summertime water use can reduce streamflows just when the streams are naturally at their lowest and the habitat for fish is at its most vulnerable.

The table below shows average daily water use during the summer (June-August) compared with the rest of the year (Jan.-May and Sept.-Dec).

Average Daily Summer Water Use (2002)
Community Average daily water use Jan-May & Sept.-Dec. '02 (MGD*) Average daily summertime '02 water use (June-Aug.) Percent increase (summertime vs. other nine months)
Acton 1.73 2.26 30%
Concord 2.10 3.12 49%
Hudson 2.26 2.82 25%
Marlborough 4.51 6.38 41%
Maynard 0.95 1.08 14%
Northborough 0.86 1.21 40%
Westborough 2.12 2.45 15%
*MGD = million gallons per day

Water balance in the Assabet
Surprisingly, the Assabet watershed as a whole is a net importer of water because Hudson, Westborough, and Maynard withdraw water from wells located in the Sudbury Watershed, Shrewsbury withdraws water from the Blackstone Watershed and both Marlborough and Northborough purchase some of their water from the MWRA. An estimated 9.6 million gallons of water per day enters the Assabet Watershed from outside the natural drainage area. But very little of this water recharges aquifers within the Assabet watershed. Nearly all of it becomes wastewater that is piped to the wastewater treatment plants, treated, and discharged directly into the Assabet River - bypassing the small streams entirely. An estimated 6.4 mgd is withdrawn from groundwater and surface water sources in the Assabet watershed.

The consulting firm Earth Tech prepared a water balance in 2002 for the six Assabet River Consortium communities (Shrewsbury, Westborough, Marlborough, Northborough, Hudson, and Maynard) as part of their Comprehensive Wastewater Facilities Planning work. This analysis shows that some drainage areas (or "sub-basins") in the Assabet watershed are hydrologically stressed, that is, they are net exporters of groundwater. Whether a sub-basin is gaining or losing water may be significant for streams because exporting sub-basins, i.e. areas where more groundwater is pumped out of the ground than is replaced or "recharged," are at risk for reduced summer time stream flows because the aquifers are, in effect, being mined.

This "mining" of groundwater could also reduce the amount of water that can be withdrawn from these aquifers by communities over the long term. The Consortium's water balance shows that in 1999, nearly half of the Assabet sub-basins (9 of 22) within Consortium communities were in "deficit" because of water withdrawals (see map of the sub-basins in deficit). This means that more water was withdrawn from these sub-basins and discharged to the Assabet River as wastewater effluent than was returned to the sub-basins as groundwater recharge. Together the communities withdrew an estimated 6.4 MGD from the Assabet watershed in 1999 and recharged only 2.3 MGD to groundwater. The balance, 4.1 MGD, was discharged to the river as wastewater or lost through evapotranspiration.

Predictions for 2025
The water balance study based on 1999 data predicts that in 2025, assuming that the projected water demands in the Consortium communities are met, nearly all of the sub-basins (18 of 22) would have significant "deficits" caused by water withdrawals. Under this scenario, nearly 100% of the water withdrawn from the Assabet sub-basins would be exported from the watershed. These deficits could reduce the yield of local water sources (particularly groundwater sources), leading to local water shortages. This would also further reduce baseflows and dilution of wastewater effluent in the Assabet River, and decrease minimum flows in tributary streams.

However, since 1999, communities have significantly reduced their per capita water consumption, and some are increasing their water imports from the MWRA's Quabbin Reservoir. New permits issued under the Water Management Act as revised by the SWMI process are expected to increase the sustainability of withdrawals. Development has continued with more and more impervious cover, but with better stormwater management methods and requirements. And lastly, climate disruption is bringing more intense precipitation (resulting in less recharge) and hotter summers.

Additional information about this water balance may be found in Chapter 7 of the Assabet River Consortium's Phase II CWMP/EIR summary document dated, May 2002. Summary water balance spreadsheets prepared by Earth Tech for the Consortium below (in pdf format):

Non-municipal Water Withdrawals 1999 & 2025 7.66 KB
1999 Annual Water Use by Community 10.29 KB
1999 Annual Water Use by Subbasin 10.05 KB
1999 Summer Water Use by Community9.44 KB
1999 Summer Water Use by Subbasin10.21 KB
2025 Annual Water Use by Community 9.68 KB
2025 Annual Water Use by Subbasin 12.07 KB
2025 Summer Water Use by Community 9.11 KB
2025 Summer Water Use by Subbasin 9.57 KB
Water Balance vs. August Median Flow by Stream 6.8 KB