Precipitation intensity and vegetation controls on geomorphology of the central Andes

Field observations and landscape evolution models indicate that landscape processes in active mountain belts are strongly dependent on vegetation and climate. In fluvial landscapes, erosional efficiency is commonly thought to depend on the intensity, frequency, and duration of precipitation events. We use Tropical Rainfall Measuring Mission (TRMM) observations to test the importance of precipitation intensity in determining geomorphology at the mountain belt scale. Precipitation metrics, including mean annual precipitation, and the mean intensity, duration, and frequency of precipitation events, are derived from the TRMM 3B42v7 product. The new precipitation datasets are then compared with different topographic metrics of the central Andes. Statistical analyses, including multiple linear regression, are used to quantify the importance of different precipitation metrics in controlling the regional topographic characteristics. In addition to climate properties, spatial variations in tectonic regime, bedrock lithology, and the amount and type of vegetation cover are accounted for in the statistical analyses. Our analysis indicates that in regions with high vegetation cover (>80%), mean precipitation intensity and mean interval correlate most strongly with mean hillslope (r = -0.51 and r = -0.66 respectively). In these regions, mean hillslope decreases from ~25° to ~ 10° with increasing mean event precipitation intensity (from 10 to 40 mm/day). In contrast, in sparsely vegetated (<40%) or shrub-dominated landscapes, precipitation intensity does not correlate with mean hillslope (r < 0.1). In regions with high vegetation cover, mean annual precipitation is weakly correlated with mean hillslope (r = 0.24). However, mean hillslope increases with increasing mean annual precipitation (r = 0.52) when all vegetation cover is considered. We interpret the results as evidence that vegetation is a key control on critical erosion thresholds at the landscape scale. Furthermore, the property of precipitation that governs surface processes is dependent on the amount and type of vegetation cover. Both grass and trees effectively increase erosion thresholds and act to stabilize the landscape. Only precipitation events that are capable of exceeding the erosion threshold set by the vegetation can do geomorphic work. High mean slopes (15 - 33°) develop where the mean precipitation intensity is low (<20 mm/day) and vegetation cover is high (>80%). In contrast, regions that are dominated by shrubs or that have sparse vegetation cover have a lower erosion threshold, and a greater proportion of precipitation events are effective erosion agents. Vegetation cover is also dependent on climate but is more closely correlated with mean annual precipitation and mean event duration than precipitation intensity. Precipitation controls on topography are therefore complex and mediated by vegetation cover but are evident at a mountain belt scale.