As concerns over how to restore the Everglades intensify, the need to improve capabilities of surface water flow models becomes increasingly important. One of the physical factors not often considered in surface flow modeling is the microtopography of the wetland surface. At the local scale (1-m horizontal), the elevation of the wetland surface undulates between hummocks associated with macrophytes and the depressions between them. At a larger spatial scale (100 m), topography varies between the tops of ridges and the bottom of nearby sloughs. We refer to all of these topographic variations as "microtopography."

Microtopography affects the cross-sectional area of a wetland that is available for surface-water flow. As water levels decline seasonally, the tops of ridges and hummocks become exposed, making flow paths more sinuous, and therefore, increasing the resistance to surface flow. Microtopography also plays a role in the water exchange between wetland surface water and porewater of sediments.

To characterize microtopography, elevations of the top of the peat surface and the top of the layer of flocculent organic material, or "floc," were estimated in Water Conservation Area 2A (WCA-2A) at sites F1 and U3 (fig. 1). These two sites are located along a research transect where water generally flows toward the southwest.

Figure 1. (left) Location of data-collection sites in WCA-2A, central Everglades, south Florida. [larger image]	Figure 2. (middle) Top view schematics of type I (A) and type II (B) measuring tools. [larger image]	Figure 3. (right) Location of microtopography measurements at site F1 (A) and site U3 (B), WCA-2A, central Everglades, south Florida. [larger image]

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Table 1. Microtopographic variability of peat surface elevation (type II tool).
Horizontal Scale of Measurement	2 Standard Deviations of the Peat Surface Elevation
	F1	U3
1-meter	0.20 ft	0.14 ft
100-meter	0.76 ft	0.48 ft

Two types of measurement tools were used to determine the micro-topography at different vertical and horizontal scales. Schematic diagrams of the measurement tools are shown in figure 2. The type I tool generally did not penetrate the floc, and estimated the elevation of the top of the floc layer. Due to its more open footprint, the type II tool penetrated the floc and was used to measure the elevation of the peat surface, which is considerably firmer than the floc. Field measurements of microtopography were made in different vegetative conditions near sites F1 and U3 (fig. 3), and the measurements were distributed so that the variability of the wetland surface could be characterized for 100-meter and 1-meter spatial scale. Measurements show that the elevation of the peat surface varies 3-4 times more at the 100-m scale compared with the 1-m scale (table 1, and fig. 4).

Microtopography data for sites F1 and U3 are summarized in figure 5 by plotting an inverse distribution function, i.e. elevation of the peat or floc surface versus the probability of occurrence when sampling at the 100-m scale. The x-axes in figure 5 can be interpreted as the fraction of wetland cross-section that has an elevation equal to or less than a given elevation. Using this function allows estimation of the average wetland cross-section available for surface flow at a given surface-water level.

Our ongoing work focuses on using the microtopography distributions in a surface-water flow model. In addition to using microtopography data to compute the cross-sectional area of surface flow over a period of fluctuating water levels, these data are being used to quantify the water exchange fluxes that occur between the surface water and the porewater of sediments that become exposed at low water levels. The micro-topography measurements are also being used to identify critical surface-water levels below which the microtopography becomes a dominant factor in flow resistance.

Figure 4. (above) Variability of peat surface elevation (type II tool) at 1-meter and 100-meter measurement scale. Site F1 (A) and site U3 (B), WCA-2A, central Everglades, south Florida. [larger image]

Figure 5. (above) Inverse cumulative distribution function of elevation measurements at site F1 (A) and site U3 (B), WCA-2A, central Everglades, south Florida. [larger image]

Contact: Judson W. Harvey, U.S. Geological Survey, 430 National Center, Reston, VA 20192, Phone: 703-648-5876, Fax: 703-648-5484, jwharvey@usgs.gov, Hydrology and Hydrologic Modeling

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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