Climate and vegetation effects on sediment transport and catchment properties along an arid to humid climatic gradient

Abstract

Recent attempts to elucidate a climatic effect on erosion rates at the catchment scale have generally found little or no correlation between precipitation and erosion rates, yet climate has been shown to exert a significant control on landscape properties such as drainage density, slope and relief. That erosion rates to do not directly reflect climatic conditions may not come as a surprise, since erosion rates will tend to keep pace with uplift rates in a tectonically active landscape. The interplay between erosion rates and climate may therefore be better understood with reference to the erosional efficiency of the landscape. Erosional efficiency governs how steep the landscape must become to balance uplift rates, and has also recently been postulated to affect the width to length (or spacing) ratio of first order basins, and the distribution of hillslopes within a landscape, via the relative inputs of diffusive and advective transport. This study constrains the efficiency of sediment transport along a climatic transect spanning a precipitation range of over two orders of magnitude in the Chilean Coastal Cordillera (26˚-41˚S), combining long-term erosion rates derived from concentrations of cosmogenic Be-10 in quartz in fluvial sediments with topographic metrics. The effects of changes in the relative input of diffusive and advective processes is investigated by studying the basin spacing ratios and distribution of hillslopes for a variety of natural landscapes and landscapes generated using the CHILD model. Sediment transport efficiency was found to peak at the transition between arid and semiarid climates, where herbaceous vegetation has almost entirely replaced bare ground, and to level off as climate becomes more humid, providing a background sediment transport efficiency value which will be applicable in both semi-arid and humid landscapes. Basin spacing ratios in natural landscapes show little variation along the transect, suggesting that changes in climate have little effect on this apparently universal catchment property, although maximum basin length attained appears to be linked to sediment transport efficiency. Slopes are consistently lower in the southern region where vegetation and sediment transport efficiency are uniform; here, lower slopes are maintained despite relatively high erosion rates thanks to higher sediment transport efficiency than in the north. Results from the CHILD landscapes show an increase in width to length ratio with decreasing sediment transport efficiency; this relationship is at odds with both the data from the study area and with data from previous studies. Results therefore indicate that, in natural landscapes, climate and vegetation cover exert a first order control on sediment transport efficiency. While climate and vegetation play little or no part in controlling the ratio of catchment dimensions, they may exert some control on the maximum dimensions of catchments and may help to modify the distribution of mean basin slope via their effects on hillslope processes.