Skip to content

Analyzing the effects of spatial gradients and temporal variability in rainfall on long-term erosion in tropical environments using landscape evolution modeling on Réunion Island, Indian Ocean

Daniel O\'Hara, Loraine Gourbet, Jean Braun

Abstract

Understanding whether long-term rainfall averages or the occurrence of low-frequency, high-magnitude events are most efficient in driving landscape erosion remains unresolved. Studies from Hawaii and the Himalayas suggest average rainfall drive erosion; while theoretical studies and field-based analysis in the San Gabriel Mountains suggest that erosion more closely follows rainfall and discharge variability. Réunion Island, a tropical volcanic island in the Indian Ocean composed of both an active and dormant volcano, represents an ideal natural laboratory to analyze the effects of rainfall on erosion. The island has uniform lithology and geologically-dated surfaces from its construction over the past 2.2 Ma, and experiences significant spatial and temporal rainfall gradients. Rainfall varies over several orders of magnitude between the leeward (~0.01 m/yr) and windward (~10 m/yr) flanks of the island. However, yearly cyclones and large storms create extreme rainfall events over the entire island. We analyze rainfall gradient effects using the stochastic-threshold stream power law landscape evolution model, which accounts for precipitation variability within the long-term erosive record. After reconstructing the non-eroded surface of Réunion for use as a simplified initial model topography, we analyze rainfall effects on drainage basin development and morphometric evolution through three test cases of increasing complexity: 1) using Réunion's background rainfall spatial gradient from small precipitation events; 2) incorporating precipitation variability associated with large storms, and 3) adding Réunion's general construction history to test the impact of volcanic construction and partial-resurfacing. In all models, we compare model basin morphometrics and erosion rates to Réunion Island observations.