Hydrology, Water Resources and Ecology in Headwaters (Proceedings of the HeadWater'98 Conference held at Meran/Merano, Italy, April 1998). IAHS Publ. no. 248, 1998. 545 The definition of stream channel head location using digital elevation data G. B. BISCHETTI, C. GANDOLFI & M. J. WHELAN Istituto di Idraulica Agraria, Université degli Studi di Milano, via Celoria 2, 1-20133, Milano, Italy Abstract We examine the practicalities of estimating stream channel head location from digital elevation data using single and multiple flow direction algorithms within the context of field observations for two small catchments in northern Italy. The main conclusions were: (a) field observed contributing areas to channel heads vary significantly and suggest that a constant contri- buting area is not a good assumption; (b) field evidence confirmed an inverse relationship between drainage area and local slope immediately upslope of the channel head for one catchment but not the other; (c) when best fit parameters for field observed channel heads were used to define slope dependent thresholds in the DEM algorithms the resulting drainage densities were too high; (d) the single direction algorithms generally produced more extensive channel networks with shorter hillslope flow pathways. INTRODUCTION The identification of the permanent channel network is important from both a theoretical and practical perspective in geomorphology and hydrology. It defines the relative extent of hillslope and channel processes in a catchment which, in turn, have important influences on catchment hydrological response. Most algorithms for flow path definition from grid based Digital Elevation Models (DEMs) involve comparisons between the elevations of neighbouring cells. In single direction algorithms (SDAs) outflow from each cell is allowed in only one direction whilst multiple direction algorithms (MDAs) allow outflow to more than one neighbour. Relatively little attention has been paid to the identification of grid cells containing channels. The most widely applied method assumes an arbitrary catchment-wide constant contributing area required to support a permanent channel, although more recently spatially variable (slope dependent) thresholds have been proposed. Few attempts have been made to use parameters derived from field observations of channel head location in automatic network generation. In this paper we examine some of the practicalities of channel head estimation using the SDA of O'Callaghan & Mark (1984), by far the most widely used algorithm for DEM flow- path generation, and the MDA described by Quinn et al. (1991, 1995), within the context of field observations of channel head location. The latter, which is theoretically better able to cope with areas of divergent flow such as spurs and ridges, was modified (a) so as to allow a variable slope angle-dependence in weighting the proportion of flow given to neighbouring downslope cells which increases with increasing contributing area (see Quinn et al, 1995) and (b) so as to force flow (area accumulation) in a single direction in cells containing channels.