The coupled development of terrain and vegetation : the case of semiarid grasslands

Abstract

The distribution of vegetation in semiarid landscapes organizes as a function of moisture availability, which is often mediated by the form of the land surface. Simultaneously the processes that shape the land surface are influenced by vegetation, mainly because vegetation reduces fluvial erosion. This thesis presents the study of the bidirectional interaction between vegetation and the land form in semiarid grasslands. Remotely sensed data and digital elevation maps are used to infer the relationship between vegetation cover and topographic attributes at two field sites in the semiarid southwestern United States. A positive relationship between drainage area (a proxy for water accumulation) and vegetation, and a negative relationship between solar radiation incidence and vegetation, are identified. The Channel-Hillslope Integrated Landscape Development (CHILD) model is modified to include a detailed representation of soil moisture dynamics, vegetation dynamics, seasonality of the solar forcing and rainfall, spatial variability of the incidence angle of solar radiation, and intrastorm variability of rainfall intensity.

(cont.) The soil moisture and vegetation dynamics components of the model are calibrated and compared to observations from a small catchment in southeastern Arizona. The representation of the evolution in time of evapotranspiration, soil moisture and above ground vegetation, and of the spatial distribution of vegetation with the model, are satisfactory. CHILD was used to simulate the evolution of the landscape under bare surface, uniform and stationary vegetation, and dynamic vegetation conditions to evaluate the effects of a dynamic vegetation cover in ten thousand years. In these simulations the emergence of bedrock outcrops that lead to significant runoff, erosion, and the accumulation of sediments and moisture downstream of the bedrock-regolith interface, had the most significant impact in the landscape. Erosion was greatest in the bedrock outcrops, and bands of sediment and moisture accumulation in their downstream ends lead to the development of bands of vegetation that augmented the accumulation of sediment and runoff infiltration.