Significance of Microtopography as a Control on Surface-Water Flow in
Wetlands
Jungyill Choi1, Judson W. Harvey, and Jessica T. Newlin
U.S. Geological Survey, Reston, VA
1
– now at S.S. Papadopulos and Associates, Inc., Bethesda, MD
Microtopography rarely has been considered in wetland surface-water flow
models, even though the ground surface often undulates significantly. We define
microtopography as topographic variations in the wetland occurring at a small
spatial scale (1-m or less) between hummocks and depressions, as well as an
intermediate spatial scale (tens or hundreds of meters) between the tops of ridges
and the bottom of nearby sloughs. To our knowledge, no previous model of
surface-water flow in the Everglades has considered how microtopography (1)
decreases the cross-sectional area available for flow at low water levels, (2)
increases surface-water exchange with sediment porewater, and (3) increases flow
resistance due to flow over and around microtopographic features.
The goal of the present project was to expand on the concepts and modeling of
Hammer and Kadlec (1986) and Kadlec (1990) by developing a governing
equation that more explicitly isolates the effects of microtopography on surfacewater flow in wetlands,
h
h  
h 
fw  Ss 
 1  f w   S y 
  f w  K f  d β 1    (P  ET  GWi ) ,
(1)
t
t x 
x 
where fw is the fraction of free surface water normal to flow (a function of water
level and microtopographic distribution), Ss is the surface-water storage
coefficient, h is the surface-water elevation, Sy is the specific yield of the wetland
sediments (i.e. subsurface-water storage coefficient), Kf is the flow conductance, d
is the surface-water depth,  is the exponent on depth, P is precipitation, ET is
evapotranspiration, and GWi is ground-water inflow.
A schematic of Everglades topography (fig. 1) illustrates how the cross-sectional
area available for surface-water flow is dependent on the microtopographic
distribution as well as on surface-water stage. According to Harvey and others
(this volume), the ground-surface elevation in Water Conservation Area 2A
(WCA-2A) (fig. 2) varies as much as 0.4 meters vertically over a horizontal
distance of 100 meters, which is one third of the typical vertical fluctuation in
surface-water depth (1.2 m) in that part of the Everglades. Variability in
topography and surface-water depth at WCA-2A makes this an ideal location for
testing a model of the effects of microtopography on surface-water flow.
Figure 1: Schematic of wetland topography
showing change in cross-sectional area for surfacewater flow for “critical” surface-water stages.
Figure 2. Water Conservation Area 2A,
central Everglades, south Florida.
Three different models were applied and compared by selectively combining three
effects of microtopography on surface-water flow. Model 1 was the base model
simulation, which did not incorporate any of the effects of microtopography.
Model 2 included the effects of microtopography on cross-sectional area of
surface flow, and surface and porewater exchange. Model 3 included the depthdependent influence of microtopography on flow resistance in addition to those
considered by model 2. All three models used daily water levels measured by
South Florida Water Management District at sites F1, F4 and U3 in WCA-2A and
field measurements of fw, d, Ss, Sy, P, ET, and GWi. The two reaches that were
modeled (F1-F4 and F4-U3) differed mainly in their vegetative characteristics
(fig. 2). The inverse modeling program, UCODE (http://www.usgs.gov/software/
ucode.html), was used to objectively estimate the optimal values for Kf, and for
each model (table 1).
Surface-water flow simulations from model 2 showed a 15 percent improvement
of the Root Mean Squared Error (RMSE) over the model 1 results, demonstrating
that consideration of the effect of microtopography on flow cross-sectional area
and storage-exchange improves the accuracy of the surface-water flow model (fig.
3). We observed additional improvements in the model 3 simulation (40 percent
decrease in RMSE from that of model 1) through incorporating stage-dependence
in the flow parameters, Kf and . The stage-dependent parameters were
determined from separate inverse modeling runs of the low-stage period (first 45
days, fig. 3) and the high-stage period (remaining 65 days, fig. 3). Flow
parameters in model 3 are varied according to the critical stages defined by field
measurements of microtopography at sites F1 and U3 in WCA-2A (fig. 1).
Table 1. Optimized flow parameters (Kf, ) for models 1, 2, and 3 as determined from inverse
modeling. Notes: Model 3 parameters vary with critical stage (see fig. 1), Parameters vary linearly
for depth ranges (c) to (d) and (a) to (b).
Model 1
Model 2
Model 3
Stages greater than (d)
Stages (c) to (b)
Stages less than (a)
Reach 1 (F1 to F4)
Kf

(m/d/m-1)
1.8 x 107
0.60
3.4 x 107
0.41
2.5 x 107
8.2 x 106
0.98
0.39
0.46
0.00
Reach 2 (F4 to U3)
Kf

(m/d/m-1)
4.7 x 107
0.64
8.9 x 107
0.16
5.6 x 107
5.6 x 107
0.98
0.56
0.56
0.00
Figure 3: Comparison of simulation results from models 1, 2, and 3 at site F4 for the entire
modeling period (A) and the low-stage modeling period (B).
Results of this study indicate that microtopography is a significant control on
surface-water flow in the Everglades, especially when the surface-water elevation
declines to depths that begin to expose microtopographic highs. Our current
modeling effort focuses on objectively determining the critical stages that affect
stage-dependence in flow parameters through inverse modeling.
References:
Hammer, D.E., and Kadlec, R.H. 1986. A model for wetland surface water
dynamics: Water Resources Research, vol. 22, no. 13, pp. 1951-1958.
Kadlec, Robert H. 1990. Overland flow in wetlands – vegetation resistance:
Journal of Hydraulic Engineering, vol. 116, no. 5, pp. 691-706.
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