Agenda 11-02-23; 4-a - Resolution Endorsing Consensus Principles II for Revised Falls Lake Rules

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=37 approximately half of those delivered during a very wet rainfall year like 2018 (approximately 60 inches of rain at RDU). The predictive models were developed and calibrated to observed water quality data from the monitoring period (2015-2018). The calibrated models were used to evaluate the impacts of different management options and model assumptions on delivered nutrient loading, lake water quality, and designated uses. The UNRBA convened a Scenario Screening Workgroup comprised of UNRBA members and representatives from DWR, agriculture, the US Forest Service, NC DOT, and environmental advocacy groups to prioritize these evaluations. Sensitivity analyses evaluate how much the parameters of concern (like chlorophyll-a) change when a model input or model parameter is changed (e.g., how much do simulated chlorophyll-a concentrations change if the modeled growth rate of algae is increased or decreased by 20 percent?). Scenarios were used to simulate different conditions than the model was originally calibrated for (e.g., how would simulated chlorophyll-a concentrations change if the USACE operated the lake differently?) Several model sensitivity analyses and model scenarios (e.g., 20 percent more or less rainfall) were evaluated using the watershed model to understand the impacts on delivered nutrients. The scenario with the greatest impact to delivered loading was the change in rainfall amount (Appendix H of the UNRBA Watershed Modeling Report). The majority of delivered load comes from non-point sources, and most of these areas are natural, unmanaged lands. This finding further demonstrates the limitations on additional, large-scale nutrient reductions as natural areas like forests are considered a balanced condition of nutrient inputs and outputs. Implications for a Revised Nutrient Management Strategy The watershed modeling results support the conclusion that achieving measurable load reductions to Falls Lake will require a systems approach directed at realistic and incremental change. The relatively small amount of managed land, constraints on the ability to install retrofits, relative effectiveness of load reduction projects, and the long response time of the watershed and lake will require a long-term effort to make progress. The revised rules should provide an adaptive management provision to reevaluate conditions in the future and make changes to the approach as needed (discussed later in this document). Any effective management approach must be flexible and include specific provisions for modifying the strategy as projects and improvements are tracked and as understanding about the system is supplemented. These recommendations for adaptive management are similar to the Chesapeake Bay STAC: "refining restoration goals over time should be considered as knowledge evolves about what future conditions are possible, what local communities and the partnership at-large see as priorities, and what is required to attain those possible futures." The current Falls Lake Rules require a 40 percent reduction of total nitrogen and a 77 percent reduction of total phosphorus load delivered to Falls Lake relative to the baseline year of 2006. While significant progress has been made,there is no feasible means to achieve these reductions, and as a result, a reexamination of the Rules is needed. Lake Water Quality Modeling Three lake water quality models for Falls Lake were developed. Two of these simulate the physical, chemical, and biological processes (e.g., settling of sediment and algae, growth and decay of algae, nutrient uptake rates) in the lake and have been calibrated to match water quality observations collected in Falls Lake. Researchers funded by the NC Collaboratory have been studying nutrient-related processes in Falls Lake since 2019. Some of these researchers also provided third-party and subject matter expert review of the lake and watershed models. The results of these research studies have been 18