Significance of microtopography as a control on surface-water flow in the Everglades

Microtopographic variation of 30-cm or more often is present in wetlands, influencing water budgets by affecting the cross sectional area available for surface flow. Surface-water exchange with sediment is also affected if water levels decline enough to expose microtopographic high points. Microtopographic variation also plays a role, along with vegetation, in increasing the resistance to surface flow through wetlands. Relatively few field studies and modeling efforts have attempted to quantify explicitly how microtopography influences surface flow in wetlands. Our study evaluated the role of microtopography on surface-water flow modeled along a 10-km transect in Water Conservation Area 2A in the north-central Everglades. While previous models of surface flow through wetlands advanced by Hammer and Kadlec (1986) and Kadlec (1990) consider the role of microtopography along with vegetation stem density, the applications to field sites do not take full advantage of detailed microtopographic measurements to partition the microtopographic effects. Our model equations followed previous derivations, but were extended in several aspects to account for independent effects of microtopography on (1) characteristic depth controlling cross-sectional area of surface flow, (2) storage exchange with porewater of unsaturated sediment, and (3) stage-dependent flow resistance. Whereas the addition of the characteristic depth formulation and storage-exchange term accounted for only modest improvements in fitting to the data, the addition of stage-dependent flow resistance specified by the measured microtopographic distribution improved model results (RMSE) by 40% over the baseline model simulation that did not consider microtopography independently. Our conclusion is that the effect of microtopography on flow resistance can be very significant to understanding the controls on surface-water flow at wetlands, especially when the surface water declines to depths that begin to expose microtopographic highs.