CSDMS 2014 annual meeting poster Jianwei Han
Measuring the imprint of orographic rainfall gradients on the morphology of steady-state numerical landscapes
Nicole Gasparini, Tulane University New Orleans Louisiana, United States. ngaspari@tulane.edu
Joel Johnson, The University of Texas at Austin Austin Texas, United States. joelj@jsg.utexas.edu
Abstract:
In this study we incorporate a 2-D orographic precipitation module into the CHILD numerical landscape evolution model to provide a quantitative tool for exploring the coevolution of rainfall patterns and fluvial topography, focusing on the imprint of spatial rainfall patterns on steady-state landscapes with uniform rock uplift. Our results suggest that network organization and planform morphology are strongly impacted by rainfall patterns. We find that rainfall gradients produce narrower watersheds, because channels show a tendency to flow along the rainfall gradient, rather than across it. The change in watershed shape is evidenced by smaller values of the exponent on distance in Hack’s law and a less peaked width function, which describes the distribution of points in a network at a given length from the outlet. Narrower watersheds also lead to an increase in the valley spacing ratio (mean mountain half width over mean distance between adjacent mainstem river outlets) and constrain trunk channels to follow a more direct path to the mountain foot. Rainfall gradients also influence the distribution of topography across a watershed. Channel profiles record rainfall patterns in both the channel concavity (downstream changes in slope) and the channel steepness index (ksn, or local slope normalized for drainage area). Small tributaries, in which the rainfall rate does not change as much relative to the mainstem channels, have a relatively clear relationship between ksn and mean rainfall across the tributary watershed. The hypsometric integral (HI), which increases with the amount of topography that is at relatively high elevations within a watershed, has a negative relationship with the profile concavity of the trunk channel, and high rainfall rates at the ridge top lead to mainstem channels that have relatively low concavity and watersheds with relatively higher HI in comparison with landscapes that have uniform precipitation. We contrast the impacts of rainfall patterns on landscape morphology with those resulting from a linear uplift gradient and uniform rainfall. We find that uplift patterns may have a similar impact on landscape morphology as rainfall gradients, making it challenging to decipher the relative roles of climate and tectonics on landscape evolution.
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