Natural experiments in landscape evolution

Abstract

A natural experiment in landscape evolution is a case study of landform development in which only one element varies significantly, and for which the driving forces, initial conditions, and/or boundary conditions are well constrained. Natural experiments provide a means of testing landscape evolution theory on the large space and time scales to which that theory applies. Natural experiments can involve either steady or transient conditions. Cases with steady conditions allow one to test predictions about the relationships among topography, erosion rates, and various attributes related to climate and material properties. Transient cases are valuable for distinguishing between models whose predictions might be similar, and therefore indistinguishable, under steady conditions. Essential ingredients of a natural experiment include minimal variation in all but one factor, good constraints on timing and/or rates, well-characterized processes, and high quality topographic data. Other useful ingredients include information about intermediate topographic states (such as a former valley profile revealed by strath terraces), and knowledge of the time history of erosion rates. In order to deepen our understanding of the physics and chemistry of longterm landscape evolution, there is a pressing need to identify natural experiments and develop the necessary databases to take advantage of them.

Modelling landscape evolution

“…This has prompted the use of paired ancient and modern stream profi les as test cases (Stock and Montgomery, 1999;Tomkin et al, 2003;van der Beek and Bishop, 2003), as well as studies of transient incision Whittaker et al, 2007a) and spatially variable erosion and rock uplift (Snyder et al, , 2003aKirby and Whipple, 2001;Lavé and Avouac, 2001;Duval et al, 2004;Whittaker et al, 2007aWhittaker et al, ,b, 2008Attal et al, 2008). Some examples of studies that use natural experiments to test landscape models are reviewed by Tucker (2009). More such studies are needed to provide a basis for evaluating and refi ning the current generation of water erosion models.…”

“…In addition, both armouring and clast weathering play a vital role in the erosional development of engineered landforms, motivating the development of 1D models of these processes and their interaction with erosion and transport Willgoose and Sharmeen, 2006). In addition to the problem of grain sizes, the broader issue of lithology also stands out as an important unknown: while variations in lithology clearly impact landform evolution and erosion rates, there remains a need to quantify the relationship (Cohen et al, 2008;2009).…”

“…Natural experiments can range widely in both geologic setting and in terms of space and time scales. Tucker (2009) provides an overview and examples of the use of natural experiments in testing landscape evolution models. Transient natural experiments, in which the landscape is responding to a change in forcing (e.g., tectonic/baselevel, climate, vegetation, etc.)…”

The SPACE 1.0 model: a Landlab component for 2-D calculation of sediment transport, bedrock erosion, and landscape evolution

“…Uncertainties in initial and boundary conditions, as well as spatial and temporal heterogeneity in rock type and sediment size, introduce substantial complexity into model verification exercises. Nevertheless, studies exploiting natural experiments, or landscapes where initial conditions, boundary conditions, and/or parameter values are particularly well constrained (Tucker, 2009), have seen some success in validating models (e.g., Stock and Montgomery, 1999;Tomkin et al, 2003;van der Beek and Bishop, 2003;Valla et al, 2010;Hobley et al, 2011). The SPACE model makes predictions for steady-state channel slope and sediment thickness that could be validated by field or experimental studies.…”

Weaving the Common Threads of Simulation and Formation studies in Archaeology

“…It was also observed that erosion in some places had winnowed away topsoils, exposing a greater abundance of smaller artefacts, while concentrated flows in other parts of the study area decreased the number of small flakes. This serendipitous co-occurrence of an archaeological survey and a large-scale fluvial event is an example of a 'natural experiment'; an opportunity to evaluate the effects of a natural process that is otherwise difficult or impossible to control (Tucker, 2009). The effects observed at Nundooka were not uniform across Fowlers Gap landscape: for example, stream action in another study area knocked out previously recorded surfaces dating back to the mid-Holocene.…”