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API Publ 4736-2006 pdf free download

API Publ 4736-2006 pdf free download.Identification of Key Assumptions and Models for the Development of Total Maximum Daily Loads.
This report focuses on the types of models used for TMDLs, the key assumptions underlying the models, how models are selected for specific surface waters and impairments, the data required to apply the models to a specific surface water and impairment, and how the predictive capability of the models is assessed. EPA has published a report entitled Compendium of Tools for Watershed Assessment and TMDL Development. 2 EPA’s Compendium provides detailed descriptions of most of the models that are included in this review. The Water Environment Research Foundation (WERF) has also published a survey and assessment of water quality models that is available as a CD-ROM. 3 This API review is not intended to be a substitute for EPA’s Compendium, other EPA guidance, the WERF assessment, and published technical references on water quality modeling. The reader should refer to the EPA compendium and WERF report for more detailed descriptions of watershed and water quality models. A list of published references on modeling is included in this report. Scope This review covers different types of water quality models applied to TMDLs performed to date. It also includes models recommended for, but not necessarily applied to, TMDLs. Because there are literally dozens of water quality models, both public and proprietary, this review focuses on well-documented models that are in the public domain and most commonly recommended for TMDL use. A range of model complexity is represented in this review. Models that are applicable to a wide range of surface water constituents were evaluated for this study. This report is designed to inform users about the application of models to specific types of TMDL problems, with emphasis on assuring that predictions of water quality are as reliable and accurate as practical, given the time and resources available to conduct the TMDL. This review does not recommend any specific model for any particular application.
The objective of this study was to review federal, state, and regional TMDL methodologies and guidance to identify key assumptions, variables, and input data required to develop waste load allocations (WLA) for point sources and load allocations (LA) for non-point sources that, when implemented, will restore water quality in an impaired water body so that it meets applicable water quality criteria and designated uses. The review focused on the models available to federal, state, regional, and local regulatory authorities to perform TMDL studies. The term “models” is used in its broadest sense, ranging from simple desktop calculations to complex mathematical models that must be run on powerful computers. The review is broken into 3 categories: (1) an overview of TMDL modeling including how models are selected and how the boundaries of the water body to be modeled are specified; (2) watershed models used to predict point and non-point source pollutant loadings from sub-watersheds and watershed; and (3) receiving water quality models that are used to predict the transport and fate of specific water quality constituents in a surface water body. The conclusions drawn from this review are presented in this summary section. TMDL Fundamentals Fundamental steps in the TMDL process are: identifying water quality constituents causing the impairment, determination of the geographic boundaries, selection of the modeling approach for developing quantitative WLAs and LAs, selecting the appropriate critical hydrologic conditions, and calibrating and validating the selected model(s). The principal considerations in these steps are summarized in Table 2-1.

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