Identification of surface flow paths, slopes, and channel networks from gridded elevation data

Stream channels enhance the hydrological connectivity between land surface elements. The representation and prediction of stream channels in drainage basins has therefore been a perennial concern in geomorphology, with implications for understanding of drainage basin origin, scale and morphology, basin hydrology, and effects of natural and man-induced process changes. The channel (or stream) network is defined by channels with well-defined banks and sources. In theory, the channel network includes all the minor rills which are definite watercourses, even including all the ephemeral channels in the furthermost headwaters. In practice, the direct survey of all channels is normally a prohibitive task, and the detail with which the channel network is represented is dependent on the scale of the map used to trace the channels. In actual fact, the headward limits of the blue lines do not reflect any statistical characteristic of streamflow occurrence, but they are drawn to fit a rather personalized aesthetic. On the other hand, the explicit description of the mechanisms determining the channel heads is a nontrivial task since it requires complex fluvial and/or landsliding processes to be considered singly or in combination. However, the increasing availability of highly accurate digital elevation data derived from LiDAR surveys, reliable terrain analysis methods, and observations collected in the field or remotely, offers new potential for developing and/or evaluating prediction models for channel initiation. In addition to detailed models, simpler generalizations from field facts can be sought by incorporating the broad features of climate, topography, and geology. In the present study, methods for the determination of surface flow paths are briefly reviewed. Three methods for the prediction of channel networks are then evaluated by using gridded elevation data derived from high-precision LiDAR surveys, a reliable algorithm for the determination of surface flow paths, and accurate field observations of channel heads for sites located in the eastern Italian Alps. These three methods are based on different threshold conditions for channel initiation, defined in terms of: (1) drainage area, (2) monomial function of drainage area and local slope, and (3) Strahler order of surface flow paths extracted from gridded elevation data. Attention is especially focused on the dependence of threshold conditions on the size of grid cells involved. The results indicate that: (i) all threshold conditions for channel initiation are grid cell size depended, (ii) the methods based on drainage area and Strahler order provide robust predictions of channel heads, and (iii) the threshold in the Strahler order of surface flow paths extracted from gridded elevation data follows quite well a scaling relation of grid cell size. The analysis carried out shows that the considered methods may provide a sound rationale for the prediction of channel heads formed essentially by surface erosion, but it reveals that more comprehensive methods are needed to predict channel heads influenced by groundwater seeping upwards.