Empirical Reservoir Eutrophication Modelling with Oklahoma Calibration // Empirical reservoir water quality modelling using Walker's 'BATHTUB' Model 1 (second-order available- phosphorus sedimentation) from Walker (1985) https://hdl.handle.net/11681/13884 and Walker (1996) https://hdl.handle.net/11681/4353 as the default retention model. The Vollenweider (1976) hydraulic- residence form and the equivalent formulation of Larsen and Mercier (1976) are available as alternatives. Predicts in-lake total phosphorus, total nitrogen, chlorophyll-a, and Secchi depth from tributary nutrient and hydraulic loading inputs, and computes Carlson (1977) doi:10.4319/lo.1977.22.2.0361 Trophic State Indices. Optional Oklahoma-specific chlorophyll and Secchi regression coefficients are provided, calibrated from publicly available state lake monitoring data. Supports single-segment and multi- segment reservoir configurations and load-reduction scenario analysis. Designed to complement watershed loading models such as the Soil and Water Assessment Tool ('SWAT'; https://swat.tamu.edu) and the U.S. EPA Hydrologic and Water Quality System ('HAWQS'; https://hawqs.tamu.edu) in a two-model nutrient management workflow.