CREEPChanging Relief and Evolving Ecosystem Project
Nan Rosenbloom
Terrestrial Sciences Section
Climate and Global Dynamics
LandscapeImage   
landscape
National Center of Atmospheric Research
Boulder, CO  80305
nanr@ucar.edu
303.497.1617

The CREEP model is a theoretical, process-response model that links ecosystem dynamics with geomorphology at the landscape scale.  CREEP  focuses on the interaction of soil aggregation and organic matter with downslope particle size distribution (% sand, % silt, % clay) and the formation of catena sequences, defined by Milne [1935] as a lateral distribution of chemical, physical, and biological soil properties along a hillslope.  The CREEP model simulates edaphic controls on ecosystem dynamics by simulating the downslope movement of soil and nutrients while monitoring the effect on soil carbon accumulation.

 Background

The CREEP model simulates the sediment transport processes that fractionate soil particles by texture into a soil catena.  A soil catena is the cumulative result of natural downslope interactions between pedogenic and geomorphic processes, including drainage conditions, differential transport of eroed material, and leaching, translocation, and redeposition of mobile chemical constituents. Within a classically defined catena, finer particles, particularly clays, are winnowed from the upper slopes and redeposited at the foot and toe slopes by hillslope transport processes (e.g., Riecken and Poetsch, 1960).  Thus a catena implies both a lateral soil sequence and the geomorphological and pedological processes instrumental to its formation.

Sediment Transport

Sediment transport is simulated numerically as texturally dependent diffusive fluxes.  Mass transport is parameterized by a linear diffusion model analog, transporting mass downslope in response to gradients in the local surface topography.  However, CREEP differs from previous hillslope models by partitioning mass transport into texturally dependent diffusive fluxes where each textural component (sand/silt/clay) diffuses independently, responding to gradients in the local surface topography with a texturally dependent rate.  To accomodate this textural dependence we take advantafge of the finite difference format found in the alternating direction implicit (ADI) numerical scheme but write separate differential equations for each texture component.

Isotopic Tracers

We introduced a stable isotope (10Be) to trace the movement of soil layers.  Using a conservative isotopic tracer allows us to observe the motion of surface layers, and contrast this movement with the signature of soil carbon, which has a more complex soil residence history.   10Be is introduced into the system through dust deposition.  Once deposited, it is assigned to the clay fraction, where it assumed to advect downslope by adhering to clay particles.  

Code development:

Algorithm development:

Dust deposition
10Be deposition
Topographic rejuvenation
Soil Carbon Dynamics/ Soil Decomposition (Jason Neff Algorithm)

Soil carbon dynamics are now simulated explicitly by the fully coupled Neff soil carbon model.  The Neff soil decomposition algorithm implements a system of differential equations, associated utilities and an ODE solver to simulate the changes in carbon content and isotopic composition in various forms (pools) in soil.  The soil structure evolved from two primary sources - Tom Hilinski's soil base class used in the implementation of erosion in the Century 5 ecosystem model, and Jason Neff's differential equations which quantify changes in content and composition of soil pools.

References
 

Rosenbloom, N.A., S. C. Doney, and D. S. Schimel (2001) Geomorphic evolution of soil texture and organic matter in eroding landscapes. Global Biogeochemical Cycles. vol. 15, no. 2, p. 365-381.

Rosenbloom, N.A., S. C. Doney and D. S. Schimel (1998) Hillslope mass transport, catenary sequences, and soil organic matter: Numerical simulations and model-data comparisons of the CREEP model for Great Plains grassland environments. Presented at the 1998 Fall Meeting of the American Geophysical Union. San Francisco, CA. December 1998. EOS Abstract. 79(45):F264.

Rosenbloom, N.A., R. McKeown and D. S. Schimel (1996) Predicting the interaction between long-term hillslope mass transport and vegetation dynamics. Presented at the 1996 Fall Meeting of the American Geophysical Union. San Francisco, CA. December 1996. EOS Abstract. 77(46):F253.

Rosenbloom, N.A. (1996) A model to predict the interactions between long-term hillslope mass transport and vegetation dynamics. Presented at the 1996 Fall Meeting of the Geological Society of America. Denver, CO. October 1996.

Rosenbloom, N.A., and R. S. Anderson (1994) Hillslope and channel evolution in a marine terraced landscape, Santa Cruz, California.  Journal of Geophysical Research.  99. p. 14,013-14,029.