Computers & Geosciences Vol. 18, No. 6, pp. 747-761, 1992 0098-3004/92$5.00+ 0.00 Printed in Great Britain PergamonPress Ltd NUMERICAL DEFINITION OF DRAINAGE NETWORK AND SUBCATCHMENT AREAS FROM DIGITAL ELEVATION MODELS L. W. MARTZ l and J. GARBRECHT 2 ~Arts Building, Department of Geography, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0W0 and 2National Agricultural Water Quality Laboratory, P.O. Box 1430, Durant, OK 74702-1430, U.S.A. (Received 14 January 1992; accepted 17February 1992) Abstract--A set of ten algorithms to automate the determination of drainage network and subcatchment areas from Digital Elevation Models (DEMs) is presented. The algorithms perform such tasks as: DEM aggregation; depression identification and treatment; relief incrementation of fiat areas; flow vector determination; watershed boundary delineation; drainage network and subcatchment area definition and systematic indexing; tabulation of channel and subcatchment area properties; and evaluation of drainage network composition. A computer program (written in FORTRAN 77) that integrates these algorithms for a full DEM evaluation also is discussed. The primary purpose of the algorithms and computer program is to parameterize rapidly drainage network and subcatchment properties from widely available DEMs for subsequent use in hydrologic surface runoff models, watershed discretizations, or statistical and topological evaluation of drainage networks. Selected results of a DEM evaluation are presented for illustration purposes. Key Words: Digital Elevation Model, Drainage basin, Drainage network, Watershed discretization, Hydrology, Geomorphology. INTRODUCTION Numerical algorithms and a computer program to delineate automatically a drainage network from a raster-type Digital Elevation Model (DEM) and to measure selected morphometric properties of that network are presented and discussed. The algorithms and corresponding computer program are designed primarily for applications to raster DEMs similar to those distributed by U.S. Geological Survey for 7.5'× 7.5' topographic quadrangles. These DEMs have grid dimensions of approximately 400 × 500 cells with horizontal and vertical resolutions of 30 and 1 m, respectively. Although application of the computer program is not restricted to DEMs of this resolution, its development did focus on the major difficulties encountered in automated network analysis at this resolution. The development of the program also was directed toward the analysis of larger watersheds that have a network of well-devel- oped channels. Such watersheds usually drain an area ranging from a few to hundreds of square kilometers. At this scale the manual determination of the drainage network and subwatershed parameters is a tedious, time consuming and error prone task. The automated evaluation greatly simplifies and expedites the task of data preparation and organization. The primary object of the computer program is to provide a tool for rapid parameterization of drainage network and subcatchment properties from available DEMs for subsequent use in hydrologic surface runoff models. This is achieved by the direct measure- ment of network, subcatchment, and basin properties from a DEM; the unique identification of each and every channel segment and corresponding direct drainage area; the application of a consistent drainage network node indexing system, and the supply of detailed network and drainage boundary maps in raster format. The drainage network data from the proposed algorithms and computer pro- grams provide the necessary information to perform an automated determination of the channel-flow routing sequence for cascade-type routing in complex networks (Garbrecht, 1988). The resulting network organization also can be used to structure the channel and subwatershed database. The channel number, Strahler order (Strahler, 1957), length and longitudi- nal slope, and starting and ending coordinates for individual channel segments are tabulated and ready for routing applications, mapping, and statistical and topologic evaluations. The usefulness of the drainage network data from the DEM evaluation is illustrated by a recent application of channel-flow routing in networks (Garbrecht and Brunner, 1991a, 1991b, 1991c). The raster map of the channel network and subwatersheds also provides an interface to other spatial data available for the drainage basin. It can be registered to other data layers such as land use, soil type, soil cover, permeability, climate, etc. and used as a template to extract data for individual 747