On the prediction of channel heads in a complex alpine terrain using gridded elevation data

Threshold conditions for channel initiation are evaluated by using gridded elevation data derived from a lidar survey, a reliable algorithm for the determination of surface flow paths, and field observations of channel heads for a study area located in the eastern ItalianAlps. These threshold conditions are determined by considering the channel heads observed across a portion of the study area and computing the related values of (1) drainage area A, (2) area-slope function AS^2, with S being the local slope, and (3) Strahler order omega* of surface flow paths extracted from gridded elevation data. Attention is focused on the dependence of the obtained threshold values on the size of grid cells involved and on the ability of the identified threshold conditions to provide reliable predictions of channel heads across the entire study area. The results indicate that the threshold values of A, AS^2, and omega* are all significantly dependent on grid cell size, and the uncertainty in the determination of threshold values of omega* is significantly smaller than that affecting the determination of threshold values of A and AS^2. The comparison between predicted and observed channel heads indicates that the considered methods display variable reliability and sensitivity over different drainage basins and grid cell sizes, with a general tendency to predict more channelheads than can be observed in the field. Acceptable predictions are normally obtained where channel heads are formed essentially by surface runoff. More comprehensive methods seem, however, to be needed to predict channel heads affected by groundwater seeping upward.