We first proposed a digital terrain model for mapping the pattern of potential shallow slope instability (Dietrich et al., 1992, Dietrich et al., 1993, Montgomery and Dietrich, 1994) by building upon the hydrologic model, TOPOG, developed by O'Loughlin (1986) and his colleagues at CSIRO in Australia. TOPOG uses a "contour-based" digital terrain model in which cells are created by projecting across the landscape approximate flow lines normal to contour lines (each cell being bounded by two contour lines and two flow lines). While such an approach captures beautifully the effects of surface topography on shallow runoff and overland flow, the model turned out to be very difficult to use over large areas. Consequently, since 1994 we have shifted our efforts to a more conventional grid-based model and we rely on tools in ARC/INFO for data display. The basic code is a combination of C++ programs and ARC/INFO amls, and most of it was created by Rob Reiss with modifications by Dino Bellugi and Harvey Greenberg.

Since converting the model to a more conventional grid-based model, we have given copies of the programs to various groups including the Bureau of Land Management, the U.S. Forest Service, and Weyerhaeuser Company. The model has recently been demonstrated as an effective tool in landslide mapping in British Columbia (Pack and Tarboton, 1997) and through inquiries to us, various colleagues in other foreign countries have begun to explore the utility of the model.

To date, although we have published significant extensions of the model (Dietrich et al., 1995) and described applications in a watershed context (Montgomery et al, 1998), we haven't offered a detailed discussion of the model in a manner that could serve to instruct others in its use. Furthermore, we have found through conversations with many others that some basic assumptions that led to this very simple model need to be made clearer and that testing and application of the model needs further discussion, particularly now that the model is being considered for use as a regulatory tool.