Characterizing Flow and Phosphorus Loads for Lake Okeechobee Watershed
using WAM
Barry M. Jacobson and Adelbert (Del) B. Bottcher
Soil and Water Engineering Technology, Inc., Gainesville, FL
Jeffrey G. Hiscock
Palm Beach Gardens, FL
Watershed Assessment Model (WAM) was set up and calibrated / validated for
simulating the existing conditions within the 20 basins in the Lake Okeechobee
Watershed (see Figure 1). This was done as part of the larger Lake Okeechobee
Comprehensive Everglades Restoration Plan (CERP) project to first develop the
hydrologic/water quality characterization of tributaries, and then to use WAM to
assess various management scenarios for phosphorous reduction. WAM is a
useful tool that provides both ease of use and the robustness and technical
defensibility of a complex solution. Because WAM was designed to assess
current conditions and to analyze the impacts of land use and management
alternatives, it was selected as the primary analytical tool for this project.
Modeling efforts included the simulation of flows and phosphorus loadings for
every stream reach within the watershed. The basins within the watershed contain
a variety of land uses including forests, wetlands, dairies, beef pastures, row
crops, citrus, residential, and commercial. As part of the overall project,
conservation easements, agricultural BMPs, and reservoir-assisted storm water
treatment areas (RASTAs) were included in the final WAM model.
South Florida Water Management District (SFWMD) maintains an environmental
database named “DBHydro” which contains hydrological, meteorological,
hydrogeological, and water quality data. WAM was calibrated for flow and
phosphorus concentration for the 20 basins using available data from DBHydro
and average annual measured loads provided by the District (see Figure 2).
Measured average annual water flows and phosphorus loads for the period 19951999 were provided by SFWMD for 4 basins (see Table 1). Since direct
measurements of phosphorus loads were unavailable, loads were calculated from
daily measured flows and phosphorus concentrations. Daily flow data were only
available for a few sites, whereas phosphorus concentrations were available for
over 100 sites. Most concentration measurements were obtained from bi-weekly
grab samples. Samples taken during periods of no flow were eliminated from the
load concentrations. Daily concentrations were calculated by interpolating
between the grab samples. This technique gives reasonable estimates of
phosphorus loads; however, the errors associated with this technique are expected
to range between 25 and 50% of actual loads.
Since WAM is a process-based model, traditional model calibration was not
utilized. However, all possible field parameters which can be physically
measured to ensure accurate model rendering of the basin were considered. For
example, locations of culverts and other flow control structures were determined
and accurately reflected in the simulated physical representation. Also, site visits
and stage data were used to improve stream profile definitions. Stage, flow, and
phosphorus data were gathered and quality checked for use in model verification.
Daily stages, flows, and phosphorus loads were simulated for each stream reach in
each basin for the period 1991 through 2000. Based on visual comparisons of
simulated and measured phosphorus concentration levels, responses to rainfall
events, and the accuracy of the measured data, the simulated values matched the
measured values quite well.
The comparisons of measured and simulated values indicate that WAM can
reliably simulate flows and phosphorus loads and concentrations discharged from
the basins. Since many subbasins and basins have little or no measured data,
these simulations are the best estimates of current flows and loads available.
WAM’s demonstrated ability to simulate the impacts of land use and management
makes it a powerful tool in assisting planners and decision makers to develop
strategies to maintain and improve water quality.
Table 1. Measured and Simulated Phosphorus Loads and Concentrations
Basin
S-191
S-154
C-41
Fisheating
Creek
Source
Measured
Phosphorus Phosphorus
Discharge Concentration
Load
(acre-feet)
(ppb)
(tonnes)
113,467
653
91.2
Simulated
119,237
617
90.2
Measured
31,885
828
32.5
Simulated
30,867
540
20.8
Measured
52,630
433
28.0
Simulated
76,718
286
27.1
Measured
249,378
176
54.1
Simulated
265,654
197
64.5
Figure 1. Project Basin Locator Map
4
Simulated -- Reach 62
Measured -- Station TCNS213
Jan-94
Jan-97
3.5
Total Phosphorus (ppm)
3
2.5
2
1.5
1
0.5
0
Jan-91
Jan-92
Jan-93
Jan-95
Jan-96
Jan-98
Jan-99
Jan-00
Figure 2. Total Phosphorus Concentration for Basin S-191 Station TCNS213
Jan-01
Barry M. Jacobson, PhD, PE
Vice-President
Soil & Water Engineering Technology, Inc.
3448 NW 12th Ave.
Gainesville, FL 32605
352-378-7372
352-378-7472 (fax)
jacobson@swet.com