LANDSAP: a coupled surface and subsurface cellular automata model for landform simulation

Recent computer models of landform evolution have focused on surface processes of river networks and hillslopes (e.g., Ahnert, 1976; Kirkby, 1986; Willgoose et al., 1991a, b; Koltermann and Gorelick, 1992; Howard, 1994; Howard et al., 1994; Smith et al., 1997). Most of these models are based on complicated differential equations that are difficult to solve. Chase (1992) developed a cellular automata model (named Gilbert) that uses simple local rules of diffusion, erosion, and deposition to simulate the synoptic effects of fluvial processes. The Gilbert model applies simple rules iteratively to individual cells of a digital topographic grid after storm events (termed precipitons) randomly fall onto the grid. The rules are in a sense analogous to the natural processes. For example, precipitons move to lower elevations, simulating water running downhill; the amount of erosion is proportional to the local slope and to the erodibility of the rock, simulating speedier erosion of steeper slope and less erosion of hard rocks (Chase, 1992). The advantage of this approach is that it is simple and yet can still produce realistic first-order geomorphologic features and provide insights into landform evolution processes. The Gilbert model demonstrates that complex landscapes do not require complicated laws (Chase, 1992). Thus the cellular automata approach is ideal for simulating long-term landscape evolution that involves multiple interacting processes. This short note presents a model (LANDSAP) that extends the Gilbert model to combine both surface and subsurface processes. LANDSAP is designed to simulate the long-term landscape evolution from these processes and to test the effects of climatic changes. It is not designed to simulate every detail of the processes, but to help us understand the interactions among different processes and the overall effects of such interactions by comparing different simulation scenarios with field observations. A previous version of LANDSAP has been successfully used to model the groundwater sapping processes in Western Desert, Egypt and has provided insights into the paleoclimatic conditions of that area (Luo et al., 1997). The purposes of this Short Note are to describe the programming details in general terms and to make the updated program publicly available. LANDSAP could also be used as a teaching tool to demonstrate system interactions among surface, subsurface, and climatic processes.