Climate Change and Landscape Evolution

Objectives

The primary motivation for studying the hydrologic cycle of the Susquehanna River Basin (SRB) in the context of the EOS program is to understand its behavior under current climatic conditions and predict how it may respond to a changed climatic regime. This understanding in turn will provide insight into the impact of climate change on the physical landscape, vegetation, and the human activities which depend on them.

SRBEX is explicitly addressing only the evolution of the physical landscape, with the specific objective of

Understanding how catchment morphometrics and sediment yields in drainage basins will evolve in response to changing boundary conditions, especially changing climate.

Although the impacts on vegetation and human activities are outside the scope of the present study, they fall within the course offerings and research interests of a number of Penn State faculty members. In particular, a group of faculty are currently seeking support from other sources for an integrated assessment of hydrologic, human, and decision-making systems, with a major focus on the SRB.

Current Activities

In order to develop quantitative predictions of characteristic geomorphic responses to different climate change scenarios, numerical experiments are being conducted with GOLEM (Geomorphic/Orogenic Landscape Evolution Model), a physically-based model of coupled hillslope and channel geomorphic evolution. GOLEM is currently being augmented by:

Adding algorithms and equations to predict channel head locations as a function of exceedence of a critical shear stress by overland flow, and
Improving the model's treatment of shallow landsliding by incorporating equations for slope instability resulting from excess pore pressure.

The planform geometry, drainage network, and bedrock geology of the WE38 catchment area within the Mahantango Creek watershed are being used as boundary conditions for the model experiments. Digital elevation and related land surface properties data serve both to evaluate models and to provide insight into the relationship between catchment form and geomorphic processes.

Digital elevation and lithologic data have also been used to investigate catchment morphometrics for the Mahantango Creek Watershed as a whole. This analysis reveals that the shape of modern stream profiles in the watershed is consistent with the hypothesis that long-term channel erosion rates are proportional to bed shear stress (or some power thereof). Fluvial profile concavity varies little among the different major valley-forming lithologies, suggesting that the use of a single channel erosion law for all lithologies is appropriate.

Plots of drainage area versus local channel or hillslope gradient derived from digital elevation data reveal two characteristic inflection points. The more pronounced of these has been recognized elsewhere, and has been interpreted as reflecting the transition between hillslope and valley. However, in the SRB the primary slope/area inflection point corresponds poorly with the hillslope-valley transition. Modeling is currently being used to test the hypothesis that these inflection points seen in the slope/area data are characteristic signatures of the catchment's recent (late glacial to post-glacial) geomorphic history.

This research is connected with an ongoing investigation at Penn State into the long-term evolution of topography and coupling between climatically-driven denudation, isostasy, and active tectonism (Tucker and Slingerland, 1994, 1995).

Achievements

Two sets of software tools for studying landscape evolution in the SRB have been implemented:

The initial version of GOLEM, which includes modules to simulate both large-scale landscape evolution (in which hillslope processes occur on a sub-grid scale) and landscape evolution on the scale of a small watershed, in which channel initiation, bedrock weathering, soil creep, and landsliding are explicitly computed.
GIS tools for hydrologic modeling and analysis of catchment morphometric properties.

Last change: 11 May 1995, R. A. White / raw@essc.psu.edu