Purpose

Brief Review

Strengths and Limitations of the Selected Modeling Approach

Alternative Scenarios Identified for Regional Modeling-SFWMM

Table 1. Initial Scenarios/alternatives identified for modeling

Possible Hydrologic Effects of Creating Large Lakes

Key Assumptions Used For Regional Modeling

Table 2. Key Assumptions used in the Regional Model (SFWMM)

Evapotranspiration of Melaleuca versus Open Water

Regional Modeling Results

Summary

Table 3. Summary of regional modeling result for initial alternatives

Lake Belt Project Sensitivity Analysis of Melaleuca ET using South Florida Water Management Model (SFWMM)

Table 4: Rainfall and ET (65-90 average annual values in inches/yr), for a typical Melaleuca and Open Water cell (cell area 2560 acres).

Figure 4: Comparison of seepage lost to LEC across levee L-30 in WCA-3B for Base (solid line) and ET Sensitivity runs (dashed lines).

Literature Citied

Appendix 1

Purpose 

This report provides a summary of the regional modeling studies conducted by the staff of the South Florida Water Management District for evaluating the first four scenarios/alternatives associated with the Northwest Dade County Freshwater Lake Plan. This regional modeling is designed to address the hydrologic impacts/benefits of the first four scenarios that include varying degrees of rock mining in the Lake Belt region. The results of this preliminary analysis should assist the committee and other working groups in designing the future scenarios/alternatives.

Brief Review 

 
The overall approach for the Analysis of Lake Belt alternatives was presented at the April, 1996 Committee meeting, and it is shown in the figure 1 below.

Using the input from the Lake Belt Committee and various other working groups, the initial conceptual design of scenarios and alternatives was completed. During this phase, the design tools (screening models) such as Visual MODFLOW, to develop the so-called "strip models" in an attempt to understand relative effects of hydrologic processes within the Lake Belt region. While these screening level models, if applied properly, are useful for initial design purposes, they are inadequate for comprehensive evaluation of Lake Belt alternatives.

Since June, 1995, the District staff of the Planning Department has focused much of their attention on developing comprehensive models necessary for analyzing the complex scenarios and alternatives identified by the committee and the working groups. These models are:

a. South Florida Water Management Model Version 2.8 (SFWMM)

d. Protect the Environment

Plans are being made to preliminarily evaluate the water quality concerns of different alternatives. It is expected that the hydrologic modeling results generated from this work will provide the basis for future work on water quality concerns. The hydrologic modeling results can also provide useful information for evaluating alternatives with respect to biological aspects of local and regional wetlands, estuaries (Biscayne Bay and Florida Bay), and the littoral areas around the proposed lakes.

The regional simulation model (SFWMM) was selected to estimate regional impacts/benefits that would result from the implementation of various structural and non-structural alternatives within the Lake Belt region. The scale of the regional model (2 mile x 2 mile) must be considered in evaluating its modeling results. Its strength, as with most models, is primarily in demonstrating the incremental differences between successive alternatives. The regional model will be particularly useful for evaluating regional-scale hydrologic effects of structural changes within the study area such as backpumping and seepage control. To a more limited extent, the model should also be useful for demonstrating the trends and relative changes resulting from the nonstructural alternatives such as various scenarios of rock mining within the Lake Belt region. There is no other appropriate tool available to investigate regional effects of Lake Belt alternatives.

For the initial modeling studies reported here, the selected approach is to first look at the results of the regional model to investigate the trend and relative differences in the first group of "non-structural" scenarios with varying extent of rock mining. The analysis of the different scenarios will be repeated by using the MODFLOW model. In the final analysis, the MODFLOW model will be used to determine the specific quantitative effects of various rock mining scenarios and other alternatives that will be modeled in the future. It is clear that both models are necessary to address the questions being asked regarding the impacts/benefits of various alternatives.
 

Alternative Scenarios Identified for Regional Modeling-SFWMM 

The six (6) scenarios included in this analysis are essentially, "non-structural" alternatives that simply address increasing extent of mining in the Lake Belt region (Table 1). The results presented in this document address only these scenarios. Alternatives 5, 6 and 7 will be addressed in a later study.

In particular, two of the scenarios (a and b) have been created for the alternatives 3 and 4 each. These reflect the variations of alternatives 3 and 4 with respect to "1990 base condition" and the "2010 base condition" used for the Lower East Coast Regional Water Supply Plan (LEC Plan) development efforts. In general, they are two "snapshots" of system-wide landuse and associated structures and their operations, which are used to evaluate the performance of changes within the Lake Belt region. For this reason, care should be taken when comparing modeling results of different scenarios. In presenting modeling results, the alternatives 1, 2, 3a, and 4a will be lumped into a single group as they represent the same "snapshot." The alternatives 3b and 4b corresponds to the 2010 "snapshot."

The modeling and analysis of the first group of scenarios can help address the following key question:

Possible Hydrologic Effects of Creating Large Lakes 

There are many hydrologic changes that can occur due to creation of extensive lakes within the Lake Belt region. They include, but are not limited to, the following:

The key modeling assumptions used for the regional simulation model (SFWMM) are shown in Table 2.

For the regional modeling in this study, assumptions were made to reflect a particular condition or snapshot being simulated. For convenience, the same version of the model developed to simulate the "1990 base condition" of the Lower East Coast Water Supply Plan (LEC Plan) was used for alternatives 1, 2, 3a, and 4a. This version of the model simulates the 1965-1990 hydrology and the operational rules of the more recent period which are assumed to be static during the entire period of simulation. The "1990 base" version of SFWMM actually uses 1988 landcover data. However, the 1994 landuse map (EAS Associates, 1994) available for Lake Belt region was used to reflect more realistic "present conditions" within the Lake Belt study area.

For alternatives 3b and 4b, the "2010 base condition" of the LECRWSP was used. This snapshot assumes the implementation of such regional projects as Kissimmee River Restoration, C-111 GRR, Everglades Construction Project including STAs, and Modified Water Deliveries to the Everglades National Park (ENP). It also uses the projected landuse for the year 2010 and the associated public water supply and irrigation demands.

Table 2. Key Assumptions used in the Regional Model (SFWMM) 
 

It is important to understand that the model cannot exactly mimic the fine-scale landuse "polygons" of various shapes and details depicted in the landuse map. Typically, the model grid (2 mile x 2 mile) is overlaid on a detailed landuse map and a landuse category is assigned to the each cell based on the predominant landuse category. The acreage corresponding to model cells assigned for various scenarios are shown in Table 2.

Evapotranspiration of Melaleuca versus Open Water 

The 1994 landcover map within the Lake Belt area shows that the study area consists of large strands of Melaleuca. Unfortunately, there is limited information available regarding the evapotranspiration (ET) rates of Melaleuca. For this study, the recent report on ET rates of Melaleuca (Chin, 1996) was used to select appropriate parameters for modeling melaleuca ET in the regional model. The method includes two factors for determining melaleuca losses in the lake belt region. First, Chin (1996) calculated the actual evapotranspiration rates for melaleuca utilizing the Penman-Monteith method. In addition to the Penman-Monteith ET rates calculated for melaleuca, Chin (1996) suggests that a second component be added which accounts for evaporative losses due to interception of rainfall within the melaleuca canopy.

It is assumed that these interception losses are removed from the system during heavy rainfall events and not available as recharge to the aquifer or for utilization by the vegetation. In order to determine the interception losses, Chin (1996) utilized Woodall's (1984) empirical equation for estimating interception losses. The result is that Chin (1996) determined that the maximum ET rate for melaleuca is approximately 51 in/year plus an interception loss of approximately 12 in/year resulting in a total loss of 63 in/year.

At first glance, it would seem that the ET rates for melaleuca and open water are essentially the same and, therefore, the change from melaleuca forest to open water systems should have minimal impact in the overall water balance for the region. However, this is rarely the case. The reason is the time variant nature of ET and rainfall in south Florida and how that impacts these two very different systems.

The lake systems are large open water bodies directly connected to the Biscayne aquifer. Because of this, rainfall which falls on the lake system directly recharges the Biscayne aquifer. However, rain does not occur constantly and there are extended periods of time when the lakes are evaporating with no rainfall to offset the losses. Because the lakes are directly connected to the Biscayne aquifer, the lakes will evaporate at 63 in/year from the Biscayne aquifer regardless of the amount of rainfall which falls.

On the other hand, the total losses from the melaleuca forests are a combination of interception losses during rainfall events and ET from the melaleuca plants (Chin, 1996). In this case, when it does not rain for an extended period of time, the maximum ET rate for melaleuca is about 51 in/year which is less than the 63 in/year for the lake systems. The actual ET rate from the Biscayne aquifer for melaleuca would tend to be less than 51 in/year during the dry periods due to reduced water availability in the unsaturated zone for use by the plant. The depth to the water table also can affect the ET rate. Melaleuca ET rates are assumed to start declining when the water table drops 18 inches or more below ground surface.

Because of the uncertainty associated with the ET rates of Melaleuca versus Open Water, a sensitivity analysis will be conducted to investigate the implications of the assumptions made in this analysis.

Regional Modeling Results 

From the results of the regional modeling runs, the specific measures of success were quantified and summarized in Table 3. It should be noted that only a limited number of measures that are relevant for evaluating the performance of the initial set of scenarios are reported here. Again, it is noted that Scenarios 1,2,3a, and 4a correspond to the "1990 base condition" and are comparable. Scenarios 3b and 4b shown in the last two columns correspond to the "2010 base condition." The measures of success relevant to urban water supply issues are separated from those important for environmental enhancement and protection.

Without conducting rigorous statistical tests, the following preliminary observations were derived from the review of Table 3 and the other results from the regional modeling.

Water Supply

The most significant effect of the increasing the extent the extent of lakes as modeled modeled in Scenarios 1 through 4a appears to be in the form of declining hydroperiods in the Pennsuco wetlands and in eastern portions of WCA-3B. The modeling results show that the Miners Proposed Initial Plan would increase the seepage to LEC near WCA-3B when compared with the Scenarios of Permitted Lakes or lesser. The regional system deliveries to Service Area 3 appear to decrease slightly when plan are compared from 1 through 4a. The water delivery to North West Well Field appears to decrease significantly in the case of Miners Proposed Initial Plan with extensive lakes in the Lake Belt area. No effort has been made to determine the ecological implications of hydrologic changes demonstrated by the modeling results.
 

Table 3. Summary of regional modeling result for initial alternatives 
 

1. A perfect match with NSM hydroperiod would show all 69,000 acres in WCA-3B in the 30 days longer/shorter category

In the SFWMM, Melaleuca ET starts declining when the water table drops to 18 inches (shallow root zone assumed for Melaleuca) or more below ground. When the water table drops to 3 feet (extinction depth assumed for Melaleuca) or more below ground, model simulated Melaleuca ET drops to zero. Thus, among other things, the actual Melaleuca ET simulated by the model depends on these parameters. An upper bound of Melaleuca ET can be obtained by increasing the extinction depth to a large value (so that Melaleuca ET is close to its maximum potential rate of 51 inch/yr) and see if the important performance measures are affected significantly.

The four scenario runs (#1, #2, #3a and #4a as discussed in the report presented to the Lake Belt Committee on July 19, 1996) were re-run after increasing the Melaleuca shallow root zone depth and extinction depth to large values (from 18 inches and 3 feet to 100 and 200 feet, respectively). In addition, the irrigated acreage was set to zero for those Melaleuca cells in the Lake Belt region of the model that had any type of irrigation. This was done because the SFWMM performs unsaturated zone accounting for all irrigated cells in the model and thereby the saturated ET coming out of the non-irrigated portion of the cell is limited by the composite crop PET for the cell (for further discussion, please see memorandum "Revised methodology for integrating pre-processed irrigation demands and Unsaturated zone evapotranspiration into the SFWMM" dated November 23, 1993 from Cal Neidrauer to Ken Ammon). Setting the irrigation to zero in the Sensitivity runs further pushes Melaleuca ET towards its upper bound value.

Table 4, shows rainfall and ET for typical Melaleuca (with no irrigation) and open water cells in the study area. Row1 is the annual average rainfall amount input to the cell while row2 and row4, respectively, are the rainfall intercepted and returned to the atmosphere by the Melaleuca canopy. Row3 represents the balance rainfall that falls on the ground while row5 is the total ET from the aquifer.

As can be seen from the table, for the Lakebelt Present Condition Base run, the ET from the aquifer is 41 in/yr and the net RF-ET = +0.9 in/yr. In contrast, for the Lakebelt Present Condition sensitivity run, ET from the aquifer increases as expected to about 50 in/yr which is closer to the potential ET of Melaleuca used in the model (51 in/yr). In addition, the net RF-ET also changes to a deficit value of -8.5 in/yr which is closer to RF-ET for open water (-6.2 in/yr from column (4) in the table). Thus, the combined effect of increasing the extinction depth and turning the irrigated acreages to zero was to bring up the Melaleuca ET close to that of open water (as determined by Chin(1996)).

Table 4: Rainfall and ET (65-90 average annual values in inches/yr), for a typical Melaleuca and Open Water cell (cell area 2560 acres).
 

 
Figure 4: Comparison of seepage lost to LEC across levee L-30 in WCA-3B for Base (solid line) and ET Sensitivity runs (dashed lines).

The primary interest from these sensitivity runs is to see what effect this increased ET over the region has on the model results and conclusions drawn from the previous set of base runs. Fig. 4 displays the comparison of seepage lost from WCA-3B to the east across L-30 levee for the two sets of runs. It can be seen that increasing ET from Melaleuca has an insignificant effect on the increasing trend of seepage lost from WCA-3B to the east as more mining is done. The slightly higher seepage in 1989 (dry condition) is to compensate for the higher ET loss occurring over the region??. A decreasing difference between the 1989 total seepage can be noticed as mining acreage increases. This is due to the fact that the more lakes scenario has lesser Melaleuca acreage and so the base and sensitivity runs are similar.

The ponding depths and the duration of flooding in Pensucco wetlands (fig. 2) also show similar results as in the previous set of base runs. The area is significantly affected in Scenario 4a as compared with Scenario 1 (as was also seen shown from the earlier set of base runs). A few other key regional performance measures were also examined (flows to Biscayne Bay, hydroperiods and Water levels in WCA-3B, etc) to see the effect of increased Melaleuca ET. The results that were noted from the earlier set of runs (that used 3 feet extinction depth for Melaleuca) are insensitive to changes in the extinction depth.

In summary, the uncertainty associated with ET rates for Melaleuca does not have any significant impacts on model results and conclusion.

Literature Cited

Chin, D.A, 1996. Research Plan for the Study of Melaleuca Evapotranspiration. Technical Report No. CEN-96-1. University of Miami, FL, 133 p.

Woodall, S.L., 1984. Rainfall interception losses from melaleuca forest in Florida. Research Note SE-323, U.S. Department of Agriculture, Southeastern Forest Experiment Station, Forest Resources Laboratory, Lehigh Acres, FL.

APPENDIX I 

Expanded Measures of Success

Incorporates changes made at Technical Workshop Sessions on June 7 & 14, 1996. Refined measures of success are shown in italics. Revisions to measures of success are underlined.

Objective 1: Enhance Water Supply for Dade County and the Everglades

Measures of Success

!Extent to which water supply for Dade County is enhanced under the following pumping demands:

155 MGD*
198 MGD**
245 MGD**
380 MGD**

- Water Levels for Salt Water Intrusion
- Number of Water Shortage Cutbacks
- Regional Surface Water Deliveries
- Spatial Maps of Drawdowns
- One foot drawdown at Dade Broward Levee in a drought with a ten year return period
- Maintenance of Snapper Creek Canal as hydrologic barrier

! Extent to which hydroperiod and flows are enhanced for the Everglades including Florida Bay

- Flow across Tamiami Trail (compared to NSM)
- Hydroperiod Maps (compared to NSM)
- Hydroperiod Maps (compared to MF&L)

-Wet and Dry season surface water flows into Northern, Central and Southern Biscayne Bay.
- Wet and dry period flow hydrographs flows into Northern, Central & Southern Biscayne Bay.

! Amount of water available during drought conditions for Dade County and the Everglades system including Biscayne Bay, Florida Bay, and South Florida estuaries.

- No detrimental changes in Primary and Secondary Standards
- No incidence of contamination with Criptosporidium or Giardia
- No changes in water classification of wellfield
- No degradation of surface water quality within the lake belt (except for water quality treatment areas) resulting from
a. Recreation
b. Mining
c. Backpumping

! Quality of the water being made available for Dade County and the Everglades system

! Water Supplied to the Everglades must meet applicable Everglades water quality standards
 

Objective 2: Maximize Efficient Rockmining

Measures of Success

! Total volume of rock available for mining through 2050

! Proximity of lands available for rockmining to processing and transportation facilities

- Quantity of minable lands within a reasonable distance of appropriate crusher locations r rock processing facilities
- Efficient mining pattern or layout

- Within the confines of the Plan, the ability of miners to quantify reserves with certainty

- The extent to which the dredge and fill permitting process is made more certain

! Design and phasing such that blasting impacts are minimized

! Design and phasing such that impacts of quarry operations such as dust and noise are minimized

! Design and phasing such that attractive nuisance aspects of quarry operations are minimized.

! Flexibility of quarry operators to operate within the Lake Belt Plan.
 

Objective 3: Promote the Social and Economic Welfare of the Community

Measures of Success

! Extent of recreational opportunities
- Provides for Managed public access to recreational resources
- Provides supply for existing and proposed recreational demand
- Provides for diversity of compatible recreational opportunity
- Provides for cost effective management of recreational areas.

! Economic vitality of rock mining industry (narrative)

! Economic vitality of other industries (narrative)

! Compatibility of land uses (narrative)

! Addresses rights of all private and public land owners, large and small (narrative)

! Economic value of clean, quality environment (narrative)

! Compatibility with transportation plans (narrative)

! Extent to which the Lake Plan provides for a sustainable South Florida (narrative)

! Costs of infrastructure construction, operation, and maintenance (narrative)

! Protection of public health (narrative)

! Avoidance of risk to potable water quality and preservation of groundwater designation of the
Northwest Wellfield (narrative)

! Provision for the acquisition or compatible lawful use of parcels not intended for rock mining (narrative)
 

Objective 4: Protect the Environment

Measures of Success

! Extent to which the Everglades including WCAs and Florida Bay are preserved, enhanced, and restored

- Seepage lost from everglades
- Flow across Tamiami Trail (compared to NSM)
- Hydroperiod Maps (compared to NSM)
- Hydroperiod Maps (compared to MF&L)
- Hydroperiod and ponding maps

-Wet and Dry season surface water flows into Northern, Central and Southern Biscayne Bay.
- Wet and dry period flow hydrographs flows into Northern, Central & Southern Biscayne Bay.

! The extent to which the habitat created, preserved, enhanced, or restored offsets the biological functions of the existing habitat lost.

! Amount of exotic vegetation removed and controlled

! Extent to which habitat functions resulting from the Plan are protected from impacts from future development.

! Extent to which water quality is enhanced
 

Revised: December 12, 1996