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Do vegetated patches on hillslopes act like immobile zones with heavy-tailed residence times?
Ciaran Harman1,2,3, Kathleen Lohse4, Peter Troch2, Siva Sivapalan1
1 Department of Civil and Environmental Engineering, University of Illinois
Department of Hydrology and Water Resources, University of Arizona
Department of Geography and Environmental Engineering, Johns Hopkins University
Department of Biological Sciences, Idaho State University
Surface soils in semi-arid landscapes can vary over very small distances, with a great deal of variation
associated with 'resource islands' created and maintained by woody vegetation. These patches tend to
have high levels of organic matter, mostly generated in situ, and are an important carbon and nutrient
resource in stark landscapes. Studies in the past have shown that these islands persist and retain carbon
for very long periods of time. Given the low bulk density, low cohesiveness and relative lack of surface
armoring, it is amazing that these soils can persist in landscapes dominated by rapid overland flow and
sheet-flow erosion.
Observations of hillslopes underlain by both granite and schist parent materials in the Santa Catalina
mountains (part of the JRB-SCM Critical Zone Observatory) suggest that the carbon rich surface soils
are protected from erosion by a combination of higher infiltration capacities, divergent flow around
micro-topography, and probably (although no data on this was collected) protection by the canopy from
rainsplash and trampling by cattle and scientists.
Scaling up from these small-scale behaviors is important for understanding the water, sediment, carbon
and nitrogen dynamics, and their delivery to streams in semi-arid areas. In this presentation I will
discuss how we are using a combination of field data and modeling to explore how these small-scale
processes control these fluxes at hillslope and small watershed scale.. One hypothesis relevant to the
aims of STRESS that we are pursuing is that the dynamics of carbon in these patchy hillslopes is
something like mobile-immobile transport, but with particles being generated and retained within the
immobile zone (the patches beneath vegetation). Furthermore, due to the increased fertility of these
resource islands and the role that vegetation plays in protecting the soils, the rates of generation and
retention may increase with local concentration.