What are LEM?

Landscape Evolution Models (LEMs) are computer codes. Basically, these codes route rainwater over a topography, eroding and transporting sediments, and thus progressively modifying the landscape. They allow the study of the evolution of topography over millions of years. They have been developed over the last three decades. However, they remain little taught and handling numerical models can be discouraging. This site is thus intended for non-modellers and future modellers, students, teachers, or simply curious people. The objective of this site is to provide educational material on Landscape Evolution Models, mainly for teachers and students at University.

Examples of simple questions that LEM can address

Landscape evolution models are based on an often very simplified description of hydrology, erosion and sediment deposition. They are difficult to calibrate, and therefore difficult to apply to concrete cases. Nevertheless, they are particularly useful tools for understanding the dynamics of the terrestrial and extraterrestrial relief. A simple question: if a mountain is uplifted twice as fast by tectonics, will it be twice as high? Only the study of natural cases provides a definitive answer, but quantifying even the tectonic uplift over the last few million years remains difficult in most cases. Besides, doesn’t the answer depend on how much rain falls on the mountain? Or the rocks that make it up? Moreover, how long does it take for the mountain to reach its final height? Is there a final height?… These and many other questions have been asked by geomorphological observers for a long time. Models of landscape evolution make it possible to provide answers. Oh of course, these models are uncertain, imperfect, simplistic etc… Yes, but they offer a theoretical framework for making predictions that can be tested in natural cases. Do these models produce counter-intuitive results? Good, it is the highest quality of this kind of model. These results sometimes allow researchers to understand things that have remained mysterious for centuries, or even better, to propose an alternative explanation to what was believed to be true.

There are two possible levels of use of this website:

– Without knowing anything about the physics behind the model, to qualitatively compare topographies according to the parameters tested, to describe the differences in relief shapes (more “angular”, more “rounded”, more “rivers”,  “higher”, “faster”, “changing” more, etc…) and to deduce rules for interpreting landscape shapes according to rainfall, tectonic uplift etc… To do this, simply compare the images and click on them to watch and compare the different movies. This first approach does not require any particular knowledge and can be used as a first step to define the notions of relief, erosion, erosion rate, drainage density, a prerequisite for geomorphology students. Posts for “beginners” at the bottom of this page may help.

– Make the link between a quantitative description of landscapes and theory, using the outputs of downloadable model files (ModelOutputFiles.zip) and use other softwares (e.g. GIS, Excel) to extract slope distribution, slope-drainage relationships, chi maps, mean denudation and elevation evolutions etc… and relate these measurements to the physical parameters of the model. This second approach is aimed at advanced geomorphology students at the University. Posts for “experts” at the bottom of this page may help.