Sediment Transport & Vegetation Change: A Study Using Medium-Scale Landscape Units as Indicators of the Influence of a Vegetation Transition on Sediment Production.

Set against a background of vegetation change from grassland to shrubland, this project used the small agricultural stock ponds of the Jornada Experimental Range, in the semi-arid Chihuahuan Desert of New Mexico, to quantify sediment production from the expanding shrubland vegetation communities. In quantifying sediment production, conclusions can be drawn about the importance of land cover in rangeland management, but more significantly for this study, a valuable dataset is generated at a generally under-researched scale. Small pond studies are necessary to expand the existing knowledge on up-scaling of erosion datasets. Sediment yield data are primarily collected from erosion plots, or at a much larger scale using erosion models. These models rely on data from plots for calibration and validation. However, data collected at the plot scale do not accurately represent sediment production at larger scales, often resulting in the propagation of errors. New methods of considering sediment routing through a catchment are necessary if understanding at an intermediate (catchment) scale is to be gained. Three approaches were used to generate comparable datasets: repeated surveys, sediment dating, and reconstructing runoff coefficients from aerial photographs. The results from these projects show internally consistent results, as well as agreement with similar studies in the wider erosion-study literature. This demonstrates the potential of this technique to produce viable datasets. The principal findings of this research are that runoff coefficients calculated at the catchment scale do not show the expected reduction from those gained from plots. This is primarily thought to be a methodological problem. However, the principal aim of the research was met with two complimentary datasets showing variations in sediment fluxes from shrubland vegetation. The dataset was insufficient to conclude this was statistically different from the historic grasslands, but this does appear to be the case. The idea of travel distance of particles as a control on sediment production was only partially substantiated by this work: fining of sediment is evident only within the catchments of the ponds. No statistical difference was found between the particle-size distribution of pond and catchment samples.