Validation of the Cypress Offset and Mesic Wetland Offset for Establishment of Minimum Wetland and Lake Levels

This report revisits the Cypress Offset and Mesic Wetland Offset currently used by the Southwest Florida Water Management District to develop minimum levels for certain lakes and wetlands. A minimum level is the level of surface water at which further withdrawals would be significantly harmful to the water resources or ecology of the area. The offsets determine when a mesic wetland is likely to experience significant harm based on withdrawal-driven changes in water levels and are among several criteria the District evaluates during the development of minimum levels. The Cypress Offset finds that significant harm is likely to occur at a cypress dome when the long-term median water level (P50) falls more than 1.8 feet below the “normal pool” elevation, a field-derived high-water reference level. The Mesic Wetland Offset, which was derived from the Cypress Offset, finds that significant harm is likely to occur at a mesic wetland when the P50 is lowered by more than 0.8 feet relative to Historic conditions (i.e., no measurable impacts due to withdrawals). “Mesic” refers to waterbodies located in landscapes dominated by mesic soils, typically within a flatwoods shallow water table semi-confined hydrogeologic setting. The values of the Cypress Offset and Mesic Wetland Offset, and their ability to detect significant harm in mesic wetlands, were validated as robust and reasonable through literature review and analyses of updated and expanded datasets. This process included reexamining data used in the original development of the Cypress Offset, assessing work conducted as part of the Northern Tampa Bay Recovery assessment, considering findings from the Central Florida Water Initiative, and evaluating the hydrologic behavior of unimpacted cypress domes. Given that the Cypress Offset and Mesic Wetland Offset result in identical minimum levels at the average mesic waterbody with no structural alterations, the advantages of each were considered to develop updated recommendations for application of the offsets. The Mesic Wetland Offset offers the advantages of better accounting for structural alterations (often present at lakes) and removing the need to identify normal pool (often absent at lakes). The Cypress Offset offers the advantage that it does not incorporate a simplification made by the Mesic Wetland Offset regarding average Historic behavior. Therefore, in developing minimum levels, rather than assessing both offsets for the same waterbody, the Mesic Wetland Offset should be assessed for mesic lakes, while the Cypress Offset should be assessed for mesic cypress domes. For either offset, status assessment should be conducted using a weight-of-the-evidence approach that incorporates multiple lines of evidence and professional judgement. Finally, literature review and evaluation of water level data indicate differing hydrology and water level behavior between mesic and xeric waterbodies. Specifically, compared to mesic waterbodies, xeric waterbodies experience much larger water level fluctuations, and available information suggests that their median water levels can change substantially more before they exhibit stress. This underscores the need for development of a Xeric Wetland Offset for xeric lakes and wetlands.