Dissertation

Development and testing of a coupled vegetation/sediment-transport model for dryland environments

Mayaud, Jerome

2017

Overview

Drylands are characterised by patchy vegetation, erodible surfaces and erosive aeolian processes. The extreme nature of dryland environments means that semi-arid vegetation cover is often dynamic through time and space, due to complex relationships among plants, soil and transport processes. Understanding how ecogeomorphic processes interact and shape landscape evolution is critical for managing potential environmental and anthropogenic impacts in drylands, since these vulnerable regions are often used for pastoralism, agriculture and habitation. Despite a wealth of vegetation distribution models that simulate distinctive patterning that is often observed in drylands, relatively little is known about the effects of vegetation patch distribution, geometry and scale on the entrainment and transport of aeolian sediment. Empirical and modelling studies have shown that vegetation elements provide drag on the overlying airflow, thus affecting wind velocity profiles and altering erosive dynamics on desert surfaces. However, these dynamics are significantly complicated by turbulence, porosity and pliability effects in canopies. This thesis therefore presents new high-resolution field data for parameterising wind flow around individual plants and vegetation patches. Wind transport models have played a key role in simplifying aeolian processes in partly vegetated landscapes, but they remain challenging in some respects. Most models do not recognise the heterogeneous nature of desert surfaces, and those that do are often computationally expensive to run. Cellular automaton (CA) modelling has been successfully used to simulate dryland ecogeomorphic processes over large spatial and temporal scales. However, no existing CA model explicitly links vegetation growth, wind flow dynamics and sediment flux over a surface. This thesis presents a new CA approach that couples a sophisticated vegetation distribution model with a sediment transport model. The Vegetation and Sediment TrAnsport model (ViSTA) is verified and validated against existing dunefield theory and field datasets. ViSTA is then forced with potential 21st century climate and land use change scenarios, to characterise possible transition scenarios between environmental states in the Kalahari Desert. ViSTA is shown to be a robust geomorphological tool for predicting landscape responses to a variety of human and environmental stresses.

• Publisher: ProQuest Dissertations & Theses
• Subject: Sediment transport / Vegetation