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

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=38 incorporated into the development of the lake models developed by the UNRBA. For example, researchers found that denitrification in the lake arms reduces nitrogen before reaching the main part of the lake. They also measured rates of nitrogen fixation by blue-green algae where the algae can "import" nitrogen from the atmosphere to support their growth. The researchers found that rates of nitrogen fixation in Falls Lake are currently low. Reports on these and other studies funded by the NC Collaboratory are available online at htti3s://nutrients.web.unc.edu/resources/. The UNRBA models were developed with significant input from the Collaboratory researchers and modeling staff at DWR. UNRBA Lake Water Quality Models Developed to Support the Reexamination One of the process-based models is a complex hydrodynamic/water quality/sediment nutrient release model that uses the Environmental Fluid Dynamics Code (EFDC) model framework. One of the reasons EFDC was selected is because it was used by DWR to set the nutrient load reduction requirements specified in the Falls Lake Nutrient Management Strategy. The other UNRBA lake model is a simpler hydrologic/water quality model that uses the Watershed Analysis Risk Management Framework (WARMF). One reason this lake model was selected is because it is directly linked to the WARMF watershed model. This direct linkage generates results for lake water quality when changes in the watershed are simulated. The DWR Falls Lake WARMF model did not include the WARMF Lake model for Falls Lake in the development of the current Falls Lake Nutrient Management Strategy (other impoundments in the watershed were simulated with WARMF Lake). The UNRBA set up both the EDFC and WARMF Lake models for Falls Lake to use the information from the WARMF watershed model to simulate stream flows and associated sediment, nutrient, total organic carbon, and chlorophyll-a concentrations delivered to Falls Lake. This linked approach is a significant development in the assessment of changes in the watershed and how those are manifested in the lake. Previous modeling did not use a watershed model to provide input files to the lake model. Without this component of the modeling,there is no way to evaluate the feasibility of proposed management activities. The UNRBA reexamination also includes the development of a third lake model to evaluate how designated uses are affected by lake water quality.This statistical model is primarily data driven and incorporates Bayesian techniques that allow expert opinion to be considered when relationships between parameters and designated uses are difficult to quantify or costly to measure. The UNRBA convened a Technical Advisors Workgroup including users of the lake for feedback on how lake water quality affects their organizations use of Falls Lake for recreation and drinking water supply. This workgroup included representatives from Triangle Fly Fishers, Wake County Parks and Recreation, City of Raleigh Water Treatment Plant, and other local experts in the fields of water chemistry and lake processes. Comparison of UNRBA Lake Models to Water Quality Data and Algae Data Collected in Falls Lake The findings of the three lake models for the study period are consistent with the evaluations presented in the UNRBA 2019 comprehensive monitoring report: • Concentrations of nutrients in Falls Lake are usually relatively low for the study period (2014 to 2018). • Ammonia releases from lake sediments are higher in the deeper areas of the lake. • Chlorophyll-a can reach high concentrations even when nutrient concentrations remain low (Figure 10, 1984 to 2020)). Chlorophyll-a is better correlated to total nutrients than inorganic nutrients because algae consume inorganic nutrients and store nutrients as organic material in their cells. 19