Coupled Evolution of Topography and Orographic Precipitation in Varied Climates

Landscapes respond to climate change. However, even in the absence of external drivers of climate change, topography and regional climate evolve together over timescales of thousands to millions of years. Topography itself is a strong control on precipitation patterns and produces persistent precipitation gradients of 150-500 percent over spatial scales of 5-30 km. As precipitation fundamentally affects the ability of rivers and glaciers to erode, these same precipitation gradients directly influence topographic development. A coupled model of landscape evolution and orographic precipitation is used to explore the co-evolution of climate and topography under a range of climatic conditions. The CASCADE landscape evolution model simulates fluvial erosion with a threshold slope condition over a uniformly uplifting surface. An orographic precipitation model simulates flow over topography and the production and advection of precipitation particles. The delay time (timescale for advection of precipitation from its formation until it reaches the land surface) is a strong control on the steady-state landscape form produced by the coupled model. Delay time controls the peak elevation, hypsometric integral, channel concavity and ridge-valley relief in modeled landscapes. These results indicate that in addition to the clear impact of precipitation amounts on topography, spatial patterns of precipitation - which are controlled by delay time - also strongly influence topography. The delay time can be interpreted to represent the temperature of the region. Short delay times are consistent with fast-falling rain and a warm climate. Long delay times are consistent with climates in which precipitation falls more slowly, as snow, for a large portion of its descent. Thus, large differences in peak elevation, hypsometry, channel concavity and ridge-valley relief are predicted in different climatic settings. Moreover, a transition from a snow-dominated to a rain-dominated climate is predicted to increase peak elevation and channel concavity. This behavior is simply due to a shift in the spatial pattern of precipitation and lacks consideration of a transition from fluvial to glacial erosion. Further research is needed to refine understanding of the impact of spatial patterns of precipitation on landscape evolution, but initial results demonstrate that spatial variability in precipitation and its relationship to topography has a strong impact on both mountain geomorphology and moutain climates.