Advances in the mapping of flow networks from digital elevation data

Digital elevation models (DEMs) are a useful data source for the automatic delineation of flow paths, sub watersheds and flow networks for hydrologic modeling. Digital representation of the flow network is central to distributed hydrologic models because it encodes the model element linkages through which flow is routed to the outlet. The scale (drainage density) of the flow network, used controls the scale of hillslope and channel model elements. Although field mapping is acknowledged as the most accurate way to determine channel networks and drainage density, it is often impractical, especially for large watersheds, and DEM derived flow networks then provide a useful surrogate for channel or valley networks. There are a variety of approaches to delineating flow networks, using different algorithms such as single (drainage to a single neighboring cell) and multiple (partitioning of flow between multiple neighboring cells) flow direction methods for the computation of contributing area and local identification of upwards curvature. The scale of the delineated network is sometimes controlled by a support area threshold, which may impose an arbitrary and spatially constant drainage density. This paper examines methods for the delineation of flow networks using grid DEMs. We examine the question of objective estimation of drainage density and describe a method based on terrain curvature that can accommodate spatially variable drainage density. The methods presented have been incorporated as a component of the TMDL Toolkit software developed to support hydrologic and water quality modeling and available from http://www.engineering.usu.edu/dtarb/.