Landscape evolution models

Geomorphology is the study of Earth’s landforms and the processes that shape them. From its beginnings, geomorphology has embraced a historical approach to understanding landforms; the concept of evolving landforms is firmly ingrained in geomorphic thought. Modern process geomorphologists use the term landscape evolution to describe the interactions between form and process that are played out as measurable changes in landscapes over geologic as well as human time scales. More traditionally defined landscape evolution describes exclusively time-dependent changes from rugged youthful topography, through the rounded hillslopes of maturity, to death as a flat plain. Bridging the considerable gulf in these different views of landscape evolution is the task of a different paper altogether. Rather, this chapter provides some historical perspectives on landscape evolution, identifies the key qualitative studies that have moved the science of largescale geomorphology forward, explores some of the new numeric models that simulate real landscapes and real processes, and provides a glimpse of future landscape evolution studies. In 1965, the year of the last INQUA meeting held in the United States, thoughts on landscape evolution were still dominated by the classic, philosophy-based arguments of William Morris Davis, Lester King, and John Hack. Geomorphology, on the other hand, had become more quantitative and the number of process studies was growing rapidly (Leopold et al., 1964). Systems theory was on still on the horizon, but a landmark paper by Schumm & Licthy (1965) had laid the framework for scaling the coming generation of process and physical modeling studies into the landscape context. In 1965, landscape evolution models were becoming stale; process studies were viewed as more scientifically rigorous and more directly applicable to human dimension problems. At the same time, plate tectonics was emerging and thought at the orogen scale enjoyed growth and acceptance in the structure-tectonics community. Decades later that community would begin questioning basic characteristics of active orogens such as: what limits mean elevation or mean relief of a landscape or what controls the rate that an orogen erodes? Orogen-scale geomorphology became relevant again to geologic and tectonic questions about the uplift and erosion of mountains. Particularly in the past decade, interests shared by the structure-tectonics community and the geomorphic community have inspired new thinking at the orogen scale and a new generation of landscape evolution models. Landscape evolution models come in two basic flavors, qualitative and quantitative, that can be applied across a wide range of spatial and temporal scales. This chapter will primarily consider models that address large-scale landforms and processes over the graded and cyclic scales of Schumm & Licthy (1965) (spatial dimensions equivalent to an orogen or physiographic province, temporal dimensions equivalent to 104 to 106 years). This scale of observation is useful because it encompasses investigations common to physical geography, process geomorphology, paleoclimatology, and geodynamics. Qualitative models are well known to most students of geomorphology and they form the basis for the more quantitative approaches. Louis Agassiz (1840) is best figured as the grandfather of all landscape models. Agassiz’s approach to understanding the impact of glaciation on landscapes forced him to think in terms of form and process as well as irreversible changes in the overall configuration of landforms as a function of time. Ironically, Agassiz’s integrated approach diverged with the next generation of geomorphologists and landscape evolution models. By the late 19th century William Morris Davis had published his two seminal papers on the geographic cycle (Davis, 1889, 1899a) and Grove Karl Gilbert laid the foundation for process-oriented approaches with his influential chapter on land sculpture in his monograph on the Henry Mountains (Gilbert, 1877). The middle part of the 20th century saw the blossoming of physical analogue models and more aggressive pursuit of process-oriented studies, particularly with respect to hillslope hydrology (e.g. Horton, 1945) and fluvial geomorphology (e.g. Leopold et al., 1964). Growth of interdisciplinary studies in the latter part of the 20th century allowed the qualitative and quantitative approaches to begin to find common ground, spurring a proliferation of numeric approaches (e.g. Willgoose et al., 1991) and ultimately, the coupled geodynamic-surface process model (e.g. Beaumont et al., 1992; Koons, 1989). This chapter begins by defining terms and suggesting a taxonomy for types of landscape evolution models. It then discusses the classic qualitative paradigms of landscape evolution as a basis for explaining where the science is today. It follows with an exploration of physical models where the physical bases for geomorphic processes were first explored. Finally, it summarizes four different types of numeric landscape evolution models.