A FORTRAN-77 PROGRAM FOR PRELIMINARY EXTRACTION OF DRAINAGE NETWORKS BASED ON A DEM

Various authors have proposed methods for obtaining the automatic extraction of drainage networks from a Digital Elevation Model (DEM). That is an important aim to be achieved, in order to manage and process geomorphological information in soil conservation studies. Attempts have shown that procedures only based on a DEM are able to solve the problem but, in many situations, they require a post-processing step to introduce a global view of the drainage network and connect the disjoined stream segments to an appropriate whole. The results of these approaches depend on the DEM accuracy and the topographic features of the area at hand. This paper presents a computer program based on the local investigation of a DEM. A 3 x 3 mask is moved through the elevation matrix starting each time from the relative maximum point until a pit or a border pixel are met. A linking and an upstream matrix are stored; for each DEM's pixel, the first matrix represents the flow direction in one of the eight neighbors and the second one the number of upstreams. A threshold value on the upstreams permits a drainage network to be obtained ready for the post-processing step. Strahler's morphometric classification, lengths of selected segments, area of the relative basins and, finally, drainage density are the outputs for the user. The computer program was tested for an area in southern Italy. Theoretical arguments, and experimentation performed by several authors, suggest the use of Ordinary Kriging (OK) to ensure a better accuracy of the DEM.