Land and water resource programs are increasingly being based on the concept of the “watershed” as the basic unit for analysis, planning and management activities. This is a clear acknowledgement that hydrologic processes do not recognize political boundaries. A watershed defines the extent of land surface from which water drains to a specific point (i.e., “pour” point) in the landscape. Since there are an infinite number of points, there are an infinite number of corresponding watersheds that could be delineated. Fortunately, the branching, hierarchical pattern of the hydrographic network for any geographic area (where tributary channels join to form progressively larger channels downstream from headwater reaches to the eventual outlet) provides a guide that enables the landscape to be systematically subdivided into standard sets of hydrologic units of comparable size. Starting with a major river basin like that of the Connecticut River, sets of progressively smaller subdivisions can be defined, based on judicious selection of pour points located along the drainage network, so that all the smaller units fit perfectly within the boundaries of the all the larger units. This relationship between each part and the whole is referred to as a “nested hierarchy.” The USGS and the Natural Resources Conservation Service (NRCS) have defined such a nested hierarchy of hydrologic units for the entire nation. Each level within the hierarchy is associated with a numeric identification code that has a specific number of digits. In New Hampshire the largest unit is the river basin with its 4-digit identifier. Each successive level of subdivision adds a suffix of two more digits to the base 4-digit code (creating the standard hydrologic unit code or HUC), defining in increasing detail 6-digit, 8-digit, 10-digit and 12-digit hydrologic units.
The New Hampshire Watershed Boundary Dataset (NHWBD) is a geographic information systems (GIS) dataset comprised of a nested hierarchy of drainage divides interpreted from 1:24,000-scale topographic contours as presented on 7.5-minute USGS quadrangle maps. These features have been delineated and digitized by NRCS and subject to a thorough review by NHGS. Because topography determines the location of both drainage channels and drainage divides, the NHWBD must be consistently aligned with the drainage lines as represented in the New Hampshire implementation of the National Hydrography Dataset (NHHD). In its role as the official steward of the NHHD, the NHGS also has stewardship responsibilities for maintaining and enhancing the NHWBD. Vertical integration of these two keystone datasets is a critical measure of data quality and utility.
Users of the NHWBD should be aware that the hydrologic units delineated for the most upstream reaches at any given level in the drainage hierarchy (i.e., HUC8 vs. HUC10) are the only ones that correspond to true watersheds as defined above (i.e., they encompass the entire upstream drainage area). All other hydrologic units define just the area of the land surface that drains directly to the length of stream between a specific pour point and the next pour point upstream from it. The true watershed for any given pour point, therefore, is a combination of all the upstream units at the same level in the drainage hierarchy.
Accuracy of the boundaries is constrained by the accuracy inherent in the original USGS topographic contours. Areas of uncertainty remain where flow directions cannot be unambiguously determined from the map contours. Field checks are likely required in some instances. NHGS welcomes reports of inaccuracies or other data quality issues as the best source of information for targeting revisions and corrections to the NHWRB.
A long term goal of NHGS is to enhance the accuracy of the topographic data that is the basis for delineating watersheds.
1) Working with USGS to complete statewide coverage of gridded elevation values at a 10-meter spacing (known to GIS users as 10-meter digital elevation models or DEMs), derived from existing contour maps.
2) Promoting investment in the latest airborne laser scanning or Light Detection and Ranging (LiDAR) remote sensing technology to acquire a new generation of high-resolution land surface elevation data capable of supporting 1-meter or 2-meter DEMs.
While anticipating that LiDAR data will be more widely available in the not-too-distant future, NHGS has been working to take maximum advantage of the new 10-meter DEMs as part of a project to identify stressed watersheds in conjunction with the State’s Water Resources Plan initiative. This effort has involved an intensive subdivision of the existing 12-digit hydrologic units into small catchments associated with individual stream segments or reaches at the 1:24,000-scale. The resulting catchment dataset depicts the hydrologic landscape of New Hampshire with an unprecedented level of detail, creating other opportunities for more rigorous analysis of the state’s land and water resources, such as the Stressed Basins Project.
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