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A conceptual model starts with a desired condition, and aims to show approximately 80% of what causes variation in its status. Based on the desired condition, specific objectives can be defined to break the desired condition up into parts. Objectives must be measureable, time-limited and specific. The desired condition is shown as a blue box on the right-hand side of the diagram, and the objectives are light blue ovals inside the desired condition box. Triangles in the objective ovals represent metrics, or measurements that can be made to determine the status of the objectives and the desired condition. Metrics can be in any box in the conceptual model diagram. A white diamond indicates a point in the system that is a research priority.
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In the Lake Tahoe Clarity Conceptual Model, the desired condition comes from the Pathway planning process. Specifically, the aim of the Lake Tahoe Clarity desired condition is to “restore, and then maintain, the waters of Lake Tahoe for the purposes of human enjoyment and preservation of its ecological status as one of the few large, deepwater, ultraoligotrophic lakes in the world with unique transparency, color and clarity.” To cover the various aspects of the Lake Clarity desired condition, it is split into three objectives – Mid-Lake Clarity, Trophic Status and Nearshore Aesthetic. The primary indicator for Mid-Lake Clarity is Secchi depth, and the proposed metrics for Nearshore Aesthetic are periphyton cover, nutrient concentration and nearshore turbidity. Trophic status is based on (1) nearshore nutrient concentration and periphyton, and (2) mid-lake Secchi depth and phosphorus and chlorophyll α concentrations. Knowledge of the drivers of Mid-Lake Clarity is strong; however there is less confidence about the drivers of both Nearshore Aesthetic and Trophic Status.
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