Water resources in the state of New Hampshire have always been seen as plentiful and virtually inexhaustible. However, increasing development pressures, especially in the southeastern portion of the state, have begun to challenge this perception. Before taking action, resource planners and managers first need to identify those watersheds that are likely to be experiencing the most “stress” due to water withdrawals. The “Stressed Basins Project” has been undertaken by the NH Geological Survey (NHGS) as an initial, systematic screening of water demand versus availability across the entire state.
Current levels of stress on existing water resources need to be assessed as part of any planning process designed to evaluate future demands while minimizing unacceptable environmental impacts. The Stressed Basins Project serves this purpose in the context of the state’s Water Resources Plan Process. NHGS was able to take full advantage of its role as steward of several key datasets, including registered water use and surface water hydrography, to develop a “water balance index” (WBI) as an indicator of hydrologic stress. The WBI is calculated as the ratio of net water withdrawals to estimated summer stream flow, Equation 1.
|Water Withdrawals – Water Returns|
|Equation 1: Water Balance Index =|
|Summer Stream Low Flow|
To facilitate this analysis, NHGS subdivided the landscape into thousands of discrete geographic units (an average of 0.5 square mile in size) using a geographic information system (GIS) to automate the process. These landscape units, referred to as “catchments”, define the land area that drains to specific segments of the surface water network. Catchments were delineated either from stream confluences (points where one stream or river joins another) or to the outlet of any pond or lake that is at least 5 acres in size (see Figure 1). The groundwater from each catchment is also assumed to drain directly to the surface water network.
WBI values were calculated in two different ways for each catchment, one representing stress solely within the local limits of the catchment and another representing the cumulative effect of stresses within the local catchment and its entire upstream drainage area. Based on Equation 1, if withdrawals exceed returns, then the WBI has a positive value; if returns exceed withdrawals, the WBI is negative. In both cases, streamflow was estimated by solving a regression equation for natural stream discharge during summer low flow (i.e., seasonally stressed) conditions (for details, see Flynn, R.H., 2003 [Report available online]. Stream flow during this season is generally considered to be supplied predominantly by groundwater that discharges into the stream channel. Furthermore, this flow of water approximates the average rate of groundwater recharge that occurs in the upstream watershed on a yearly basis. Therefore, these stream flow estimates represent a highly significant line item in the overall “water budget” for a watershed.
Total net water withdrawals were estimated by combining water use data from a number of sources for the year 2005. These include registered water withdrawals and returns as reported to NHGS by facilities that use more than 20,000 gallons per day (gpd). The U.S. Geological Survey (USGS) provided estimates of withdrawals and returns by unregistered water systems (a combination of community and non-community water supplies). USGS also developed estimates for total water withdrawals by households with private wells and water returns by households with on-site septic systems. These estimates were provided for each census block in the state based on population and assumed per capita water use values. NHGS then reapportioned these census block-based estimates by taking into account factors that determine where areas of residential development that are not served by a public water supply and/or are not connected to a sewer system (i.e., have their own water wells and/or septic systems) are most likely to be located within each catchment.
To analyze the final WBI values, catchments having an aggregate withdrawal greater than or equal to 20,000 gpd were sorted by their WBI values and ranked from the highest to the lowest values. The 20,000 gpd threshold was used to exclude catchments with relatively minor amounts of water use or zero water use from being ranked. Also, because comparable data on water use were not available for analysis in catchments outside of New Hampshire, areas draining into the Connecticut River from Vermont did not contribute to the calculated WBI values for downstream catchments. This was also true of the upper portions of the Androscoggin River in Maine, the lower portion of the Merrimack River in Massachusetts and the Maine portion of the Piscataqua River. To aid interpretation, the ranking was performed by individual study area. In the case of the Piscataqua\Coastal watershed and the Saco River watershed, the study area corresponded to the state’s 8-digit hydrologic cataloging units (01060002 and 01060003, respectively). The other study areas encompass the Androscoggin River basin, Merrimack River basin, Saco River basin and Connecticut River basin. Based on the ranking, the most stressed catchments were interpreted to be those catchments that were in the top 20 percent of all ranked values and had a WBI value of 50 percent or greater. With respect to the second criterion, these WBI values indicated that withdrawals were equivalent to more than half of the summer low flow estimate for the stream that drains the catchment. An example of the final values, from the Piscataqua River basin, is shown in Figure 2. The population of catchments ranked in the top 20% but having WBI values less that 50 percent of summer low flow were classified as “moderately stressed.”
The analysis also included a statewide screening of the percent of impervious land cover per catchment using the National Land Cover Dataset from 2001 as the data source. Therefore, results reflect conditions that existed in 2001. Results are also subject to inaccuracies that are inherent in the process of classifying land cover characteristics based on satellite imagery. Nonetheless, percent imperviousness represents a significant watershed characteristic due to its documented effect on both surface water quality and the water balance (i.e., increased stormwater runoff and reduced groundwater recharge). The approach developed by NHGS for calculating percent imperviousness and WBI can be readily modified to incorporate other watershed characteristics of interest.
GIS data created for the project will ultimately be available for download from GRANIT, the New Hampshire GIS data clearinghouse. Maps of each study area are available through the following links for viewing and download:
- Piscataqua River Basin [85MB]
- Merrimack River Basin [84MB]
- Saco River Basin [83MB]
- Androscoggin River Basin [86MB]
- North [86MB], Central [83MB], Southern [85MB] Connecticut River Basin
Contact Rick Chormann at (603) 271-1975 or [email protected] for additional information.
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