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

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=31 Two additional sources of nutrient loading affect the lake directly. Atmospheric deposition contributes nitrogen, phosphorus, and organic carbon directly to the surface of Falls Lake as well as to the contributing drainage area. Atmospheric deposition data incorporated into the monitoring and modeling shows total nitrogen loads from atmospheric deposition have declined by approximately 20 percent since 2006 (DWR 2021). The bottom sediments within Falls Lake accumulate, release, and cycle nutrients. The UNRBA 2019 monitoring report estimated that lake sediments release approximately 200,000 pounds of total nitrogen each year(10 percent to 25 percent of the watershed load depending on rainfall and the year The UNRBA 2019 monitoring report estimated that referenced) and 14,000 pounds of total lake sediments release approximately 200,000 phosphorus (5 percent to 15 percent of pounds of total nitrogen each year(10 percent to the watershed load depending on 25 percent of the watershed load depending on rainfall and the year referenced). rainfall and the year referenced) and 14,000 These preliminary estimates of lake pounds of total phosphorus (5 percent to 15 sediment releases have been refined percent of the watershed load depending on through the UNRBA lake modeling rainfall and the year referenced) efforts and Collaboratory studies. To summarize,the hydrology, morphology, retention time, depth, and characteristics of the different areas of the lake contribute significantly to nutrient loading and chlorophyll-a levels in the lake. These aspects of the lake are beyond the control of the regulated entities in the watershed. While the UNRBA aims to manage nutrient loading in the watershed, it recognizes it is not the only factor to consider in developing a revised nutrient management strategy. Many important data results and relationships contribute to an updated and improved understanding of this lake and watershed. The best way to examine those in detail is to access the UNRBA 2019 Monitoring Report. UNRBA Watershed Modeling The UNRBA has worked extensively with local governments, researchers, state and federal agencies, and utilities to obtain the most comprehensive and complete data available to develop a revised watershed model for Falls Lake. The UNRBA selected the Watershed Analysis Risk Management Framework (WARMF) model partly because it is the same model that DWR used to support development of the Falls Lake Nutrient Management Strategy. Data summaries and discussions related to the Falls Lake and watershed WARMF model development are documented in Modeling and Regulatory Support Workgroup (MRSW) meetin materials and the UNRBA watershed modeling report(BC and Systech Water Resources 2023, available at https://unrba.org/reexamination). Development of a watershed model is an essential component of a fully informed management approach. The UNRBA has invested significant resources into an effective watershed model development process. The following summary of the watershed modeling effort is provided as context for the UNRBA recommendations for a revised nutrient management strategy. The modeling was reviewed extensively by subject matter experts,third-party model reviewers funded by the NC Collaboratory, and DWR modeling staff. The UNRBA has a high level of confidence in the results of this modeling. A more complete description of the model and the development process is available in the UNRBA Watershed Modeling Report(BC and Systech Water Resources 2023, available at https://unrba.orVreexamination). 12