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 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 meet 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 total net water withdrawals to estimated summer stream flow.
To facilitate this analysis, NHGS subdivided the landscape into thousands of discrete geographic units (an average of 0.5 square miles in size – the area of the entire state of New Hampshire is approximately 9,350 square miles) using a GIS and digital elevation data to automate the process. These landscape units, referred to as “catchments”, define the land area that drains to the surface water network between confluences (points where one stream or river joins another) or to any pond or lake that is at least 5 acres in size.
Confluence-to-confluence and waterbody catchments
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 upstream contributing area. In both cases, water availability was estimated by solving a regression equation for natural stream flow during summer low flow (i.e., seasonally stressed) conditions (for details, see Flynn, R.H., 2003, Development of regression equations to estimate flow durations and low-flow-frequency statistics in New Hampshire streams: U.S. Geological Survey Water-Resources Investigations Report 02-4298, 66 p. [REPORT AVAILABLE ONLINE ]). Stream flow during this season is generally considered to be supplied by groundwater that discharges into the stream channel. Furthermore, this flow of water approximates the amount of groundwater recharge that occurs in the upstream watershed on a yearly basis.
The value for total net water withdrawals that serves as the numerator in the WBI ratio was 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 and estimates of withdrawals and returns by unregistered water systems (a combination of community and non-community water supplies) provided by the US Geological Survey (USGS). USGS also developed estimates for total water withdrawals by households with private wells and returns by households with on-site septic systems. Estimates were provided for each census block in the state based on population and assumed per capita water use values. NHGS then partitioned these estimated amounts among catchments by taking into account landscape factors that determine what portions of each catchment were likely to have areas of residential development that are not served by a public water supply and/or connected to a sewer system. If withdrawals exceed returns, then the WBI has a positive value; if returns exceed withdrawals, the WBI is negative.
The final WBI values were sorted in descending order and ranked accordingly to identify the highest and lowest values, respectively. To aid interpretation, the ranking was performed for each of the state’s 8-digit hydrologic cataloging units 
(these are watersheds corresponding to major surface water features, such as the Contoocook River, Upper Connecticut River, etc.). In other words, WBI serves to highlight those catchments that are likely to be “most stressed” and “least stressed” from a water quantity perspective, relative to all the catchments that make up each of the larger 8-digit watersheds.
The “most” (red) and “least” (blue) stressed catchments
within the Piscataqua River Watershed are highlighted
The analysis also included a statewide screening of the percent of impervious land cover per each catchment. This utilized the National Land Cover Dataset from 2001 as the data source so results reflect conditions that existed at that time. Results are also limited by 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 it documented effects 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 the 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 of the 8-digit watersheds will be posted on the NHGS home page for viewing and download when available.
Contact Rick Chormann at (603) 271-1975 or frederick.chormann@des.nh.gov for additional information.
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