Modeling Changes in Flow Induced by Anisotropy of Vegetation

Modeling Changes in Flow Induced by Anisotropy of Vegetation Patches and
Topographic Features
Stuart Stothoff
Southeast Environmental Research Center, Miami, FL
Sherry Mitchell-Bruker
Everglades National Park, Homestead, FL
Surface-water flow in the Greater Everglades has historically been modeled using
an isotropic Manning's n, implying that resistance to flow is the same in all
directions. This approximation may be reasonable when grid cells are much
smaller than vegetation patches or topographic features such as tree islands or
sawgrass ridges. However, in large-scale models such as the Everglades Natural
System Model and the South Florida Water Management Model, grid cells may
contain several sawgrass ridges or tree islands.
A groundwater and surface water flow model that includes several options for
modeling vegetative resistance to flow is used to explore the importance of
anisotropy of large scale resistance features. The impact of these features is
examined by considering a fine-scale grid with explicit delineation of the
topographic and vegetation features with corresponding distribution of resistance
parameters and topography. By imposing gradients parallel and perpendicular to
the features, the fine-scale model provides a mechanism to assess the effect of
large-scale anisotropy in resistance and topography on the flow system.
The impact of the surface anisotropy on system behavior is found to be dependent
on both flow depth and permeability of the underlying aquifer. When the
underlying aquifer has low permeability, both surface-water and groundwater
flow directions are significantly modified. On the other hand, when the
underlying aquifer has high permeability and the surface water is shallow, the
overall system is dominated by the groundwater component so that the system is
insensitive to surface anisotropy. As the surface water deepens, the surface water
system dominates the overall system flow and the surface water flow can be
significantly changed by the surface anisotropy.
This result may have important implications for large scale flow models of the
Florida Everglades. Failure to simulate large scale surface anisotropies could lead
to erroneous predictions in flow directions and could also explain differences in
the observed tree island distributions and modeled flow directions in the current
version of the Natural System Model. Methods for including directional surface
anisotropy in large scale models are tested.
Stuart Stothoff, 880 Lockland Avenue, Winston-Salem, NC 27103
Phone 336-723-9332, Fax 336-724-1470