In the early 1990s, the Environmental Protection Agency (EPA) produced a preliminary release of Reach File Version 3, known as RF3-Alpha, using the 1988 1:100,00-scale DLG hydrography file. RF3-Alpha represented the third in a series of hydrologic databases, each containing a greater level of detail, which provide a national surface water drainage network and an associated addressing scheme for linking water-related data to the network. The addressing scheme is based upon unique identifiers for each individual stretch of surface water or "reach". These reach codes play the same role as street names in street addresses, providing the framework for linking water resource data associated with monitoring sites, permitted dischargers, drinking water supplies, use-impaired waters, fish consumption advisories, etc.
In 1993, the EPA approached the USGS to discuss the possibility of integrating RF3-Alpha with the latest DLG hydrography file to produce a more applications-oriented, maintainable product that has become known as the National Hydrography Dataset (NHD). While initially based on 1:100,000-scale data, the NHD is designed to incorporate -- and encourage the development of -- higher-resolution data required by many users.
The USGS National Elevation Dataset (NED) has been developed by merging the highest-resolution, best-quality elevation data available across the United States. NED is the result of the maturation of the USGS effort to provide 1:24,000-scale Digital Elevation Model (DEM) data for the conterminous US and 1:63,360-scale DEM data for Alaska. It has a resolution of one arc-second (approximately 30 meters) for the conterminous United States, Hawaii, and Puerto Rico and a resolution of two arc-seconds for Alaska. NED is designed to provide National elevation data in a seamless raster format with a consistent datum, elevation unit, and projection. Data corrections were made in the NED assembly process to minimize artifacts, perform edge matching, and fill sliver areas of missing data. These processing steps ensure that NED has no void areas and artificial discontinuities have been minimized. As higher-resolution or higher-quality data become available, the NED is periodically updated to incorporate best-available coverage. Also, as more data become available at a finer resolution than NED, the feasibility of developing a finer resolution NED will be investigated.
The National Elevation Dataset - Hydrologic Derivatives (NED-H) is intended to develop a hydrologically correct version of the NED and systematic derivation of standard hydrologic derivatives. The recent completion of the NED and the NHD combined with advances in GIS application of terrain modeling have made possible the development of these topographically derived hydrologic data layers at a scale of 1:24,000. Some of the benefits of a nation-wide development of hydrologic derivatives are:
The following applications intend to utilize this national hydrologic framework and are representative of the significant applications potential it supports.
SPARROW (SPAtially Referenced Regressions On Watershed Attributes) is a method developed by the USGS Water Resources Division for regional interpretation of water-quality monitoring data. It relates in-stream water-quality measurements to spatially referenced characteristics of watersheds, including contaminant sources and factors influencing terrestrial and stream transport. The model empirically estimates the origin and fate of contaminants in streams, and quantifies uncertainties in these estimates based on model coefficient error and unexplained variability in the observed data.
The SPARROW team is collaborating with the NED and NHD teams in a multi-year project to estimate flow volume and velocity for NHD reaches in support of SPARROW and other water quality modeling applications.
Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) is a system developed by the EPA to meet the needs of pollution control agencies that are increasingly emphasizing watershed and water quality-based assessment and integrated analysis of point and nonpoint sources. BASINS combines a GIS, national watershed data, and state-of-the-art environmental assessment and modeling tools into one convenient package. It supports a variety of applications at multiple scales including the development of total maximum daily loads (TMDLs) which require a watershed-based approach that integrates both point and nonpoint sources.
The National Weather Service's Flash Flood Monitoring and Prediction (FFMP) Program will be used to assist in flash flood warning decisions at 121 National Weather Service (NWS) offices throughout the country. The FFMP is based on the Areal Mean Basin Estimated Rainfall (AMBER) Program which accumulates areal average values of precipitation on a basin level. The accumulations are compared to Flash Flood Guidance values or to user-defined threshold values to determine flash flooding potential.
The USGS in cooperation with state cooperators in Massachusetts has developed streamflow statistics and basin characteristics for more than 700 locations in the State where the USGS has collected streamflow data. The GIS-based process used employs an automated procedure that measures characteristics of the land surface area (basin) that drains to the stream and inserts those characteristics into equations that estimate the streamflow statistics.
The Center for Research in Water Resources of the University of Texas at Austin, and Esri have established a Consortium for developing and implementing new GIS capabilities in Water Resources. The initial focus of the consortium is on the design of a new GeoDatabase Model for Rivers and Watersheds for ArcInfo Version 8.