Collaborative Research: Modeling and monitoring of landscape evolution along a climate gradient: Kohala Peninsula, Hawaii

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Climate, and more specifically rainfall, is amongst the most important forces that shape a landscape, yet quantifying the impacts of rainfall on landscape development remains a challenge. It is easy to see the differences between the topography in arid versus humid landscapes, but it is has proven difficult to quantify how physical processes such as soil development, surface and subsurface water flow, and hillslope and fluvial erosion, differ between these two environments. The Kohala Peninsula, on the northern tip of the Big Island of Hawaii, offers an excellent field environment to address this question. Across the approximately 20 km wide peninsula, rainfall rates very from upwards of 4,000 mm/year to less than 200 mm/year. Deeply incised gulches dominate the topography of the wet side of the peninsula, while on the dry side, shallow gulches disappear and reappear across the landscape. The wet side of the peninsula has developed deep soils and the process of weathering is so intense that a fingernail can scrape a groove in many rocks. In contrast, on the dry side, surface soils are often shallow and rocky and exposed rock surfaces are much less weathered. This project will quantify the differences in hydrologic and erosion processes on the contrasting sides on the peninsula using both field monitoring and numerical modeling. Two watersheds, one on the dry side and one on the wet side, will be equipped with devices to measure rainfall rates, water flow depths on both the hillslopes and in the gulches, soil moisture content, and erosion rates. These devices will operate continually throughout the duration of the project. Field surveys will provide detailed observations of the geometry of the gulches and the variables that control incision rates, such as sediment grain size, rock hardness, and degree of fracturing. These data will be incorporated into a numerical landscape evolution model. The model will be used to systematically explore feedbacks between key climatic controls and surface process responses over the time scales at which landscapes evolve. Field data will be used to both constrain physical processes in the model and inspire the specific modeling scenarios.This project will address a fundamental scientific question of how rainfall patterns actually influence the processes of weathering and erosion. Rainfall gradients are present across almost every mountain range on Earth and the findings from this study will have broad significance beyond the Hawaiian Islands. Knowledge of the detailed interactions among climate, hydrology, and erosion can be applied to landscapes across the planet. Furthermore, by quantifying links between landscape development and rainfall, the effects of potential climate change on landscape evolution can be predicted more effectively. This project will develop a number of numerical models that will be freely available for use throughout the scientific community, so that the findings from this study can be easily applied in other settings and to a wide range of scientific questions.