What are MFLs?
“MFLs” is short for minimum flows and levels. Minimum flows are set for rivers, streams, estuaries and springs, while minimum water levels are established for lakes, wetlands and aquifers. In short, an MFL sets a limit on how much water can be withdrawn from various water resources to prevent significant harm occurring to those resources or the ecology of the area.
Why does the Southwest Florida Water Management District set MFLs?
Florida law (Chapter 373.042, Florida Statutes) requires the state’s water management districts or the Florida Department of Environmental Protection to establish MFLs. MFLs are used to protect water resources from significant harm that could be caused by water withdrawals. Within the Southwest Florida Water Management District (SWFWMD or District), water levels in some lakes can fluctuate as groundwater levels fluctuate. Because groundwater is a significant source of water supply, lake MFLs help the District balance public water supply permitting with environmental and ecologic resource needs of the area.
How is a Minimum Lake Level set?
First, District staff consider how a lake is affected by changes in hydrologic conditions. For each priority lake where minimum levels are to be set, the District collects and evaluates a large amount of information. We typically collect and compile lake water-level data every month and have done so for many years at hundreds of lakes. For some lakes we have a long-term record of historic, measured water-level data that provides a picture of what water levels were like without any withdrawal effects. However, because withdrawals directly from a lake, or more commonly nearby groundwater withdrawals, have affected the lake’s levels, we often must use a water budget model and other models to help generate what water levels were like without any withdrawal effects.
It’s important that we develop a long-term historic water-level record, measured or modeled, to help set Minimum Lake Levels because the historic record minimizes the potential bias caused by short-term rainfall conditions and provides the best picture of lake water levels unaffected by withdrawal impacts. This long-term historic “best picture” of lake levels serves as a basis for identifying how much water levels can change due to withdrawals before significant harm may occur.
Developing a water budget model is similar to balancing a checkbook, in that we determine how much water is added to the lake (via rainfall, inflows from other lakes and runoff), as well as how much water leaves the lake (via evaporation, taken up and used by plants, through losses to groundwater and through outflows). In a lake water budget model, we also account for existing structural alterations, such as drainage ditches, dams and culverts used to control water levels. Then, in conjunction with a groundwater model that helps us predict groundwater level changes near the lake that may affect water levels, we use the water budget model to develop a long-term historic water level record for the lake.
District staff also collect biological data and elevations of low floor slabs, low roads, docks and anything else of interest at the lake where Minimum Lake Levels are to be set. We use all of this information to identify standards and other criteria for setting Minimum Lake Levels that best protect wetlands on the lake, the plant and animal species that live on or use the lake, water quality, ecological processes associated with nutrient filtration, movement of detritus and sediment loads, and the use of the lake for navigation, aesthetic, scenic and recreational purposes.
How does the District use Minimum Lake Levels?
MFLs are used by the District to plan for current and future regional water needs, which may include the need to offset groundwater use through projects that encourage conservation and provide alternative water supplies. MFLs also are an important tool for the District’s water use and environmental permitting programs to ensure that withdrawals do not adversely impact an established MFL and cause significant harm to the water resources or ecology of the area.
What is done if a Minimum Lake Level is not met?
A recovery strategy is implemented if an MFL is not currently met. A prevention strategy is implemented if an MFL is projected to not be met in the next 20 years. Prevention and recovery strategies allow for providing sufficient water supplies for all existing and projected water uses through development of additional water supplies, implementation of conservation and efficiency measures and regulatory measures. The District funds the initiatives associated with prevention and recovery strategies with help from local governments and regional water supply authorities. The intent of these strategies is to achieve recovery to the established MFL as soon as possible or prevent the existing flow or water level from falling below the established MFL.
What are lake Guidance Levels?
Guidance levels for lakes are typically developed at the same time as minimum levels. The high guidance level is the water level that the lake can be expected to be at or above 10% of the time over the long-term, while the low guidance level is the water level that the lake can be expected to be at or above 90% of the time over the long-term, both in the absence of water withdrawals. Guidance levels are developed as advisory information.
What if a lake’s water levels are too high and there is flooding?
While flood elevations are considered in the process of setting minimum levels, establishment of Minimum Lake Levels does not result in projects that address flood control issues. Minimum levels are set to support permitting and water supply planning decisions that prevent a lake’s water levels from becoming too low. High water levels, or flooding, are addressed with watershed management projects overseen by either the District and/or other agencies such as county governments and the U.S. Army Corps of Engineers.
Does anyone besides District staff get to review proposed Minimum Lake Levels before they are set?
Yes, an important component of the process for setting Minimum Lake Levels involves getting input and feedback from stakeholders, including the public, local governments, environmental groups and water supply utilities. All comments are considered and, as appropriate, used to refine Minimum Lake Levels prior to District staff recommending that the District Governing Board approve them for adoption into District rules. The District’s Governing Board, consisting of 13 Governor-appointed volunteers, considers the staff’s recommendation and the formal public comments prior to deciding if the levels should be approved for adoption. If the Governing Board votes to adopt a Minimum Lake Level, rulemaking proceeds and the levels are adopted into District rules (Chapter 40D-8, Florida Administrative Code).
How do I get more information about Minimum Lake Levels?
Additional information on Minimum Lake Levels and MFLs in general is available on the District’s website at WaterMatters.org/MFLs. We maintain a specific webpage at WaterMatters.org/projects/mfl/documents-and-reports, where you can read reports about the development of our peer-reviewed Minimum Lake Level methods, as well as reports with the details for the Minimum Lake Levels we have and are currently developing.