DELINEATION OF DRAINAGE BASINS IN A LARGE WATERSHED FROM DIGITAL ELEVATION MODEL DATA

D.A. Freehafer (U.S. Geological Survey, 425 Jordan Road, Troy, NY 12180)

Drainage divides were delineated from 1:250,000-scale digital-elevation-model (DEM) data as part of the U. S. Geological Survey (USGS) National Water-Quality Assessment of the Hudson River basin. The drainage divides define the topographic high points around a basin and thereby delineate boundaries of areas where precipitation can contribute to streamflow. The delineation of drainage basins is crucial for development of ancillary data necessary for surface-water-quality analysis, such as land use, geology, and soils. The method used to automate the processing of DEM data is based on Fortran software developed in 1988 by the USGS. Programs written in ARC (1) macro language were used to convert raster data to a vector format and to provide a user interface with the ARC/INFO geographic information system.

More than 5.35 million elevation points, sampled at a 3-arc-second grid spacing from ten DEM files, were required to cover the 13,265 square-mile Hudson River basin. The DEM files were first processed to create a depressionless DEM data file to simplify overland-flow calculations. Additional data sets, including a flow-direction file and a flow-accumulation file, were also created. A drainage-network coverage was then produced for each data set and inspected for areas that were incorrectly delineated because of a sparsity of topographic detail and (or) errors in DEM data. A point coverage was created from sites for which surface-water-quality data was stored in the USGS National Water Information System data base. The sites were checked for correct location and relocated on the nearest DEM-generated drainage-network, if necessary. These site locations were used as the basin-outlet locations for generating drainage divides from the DEM data. The drainage basins were converted to ARC/INFO polygon coverages, edge-matched, and appended into a single coverage.

(1) Use of brand and trade names in this report is for identification purposes only and does not imply endorsement by the U.S. Geological Survey.

Presented at the 7th Annual Northeast ARC/INFO Users Group Conference, Stamford CT.
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