Assessing the TMDL Approach to Water Quality Management (2001) / Chapter Skim
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4 Modeling to Support the TMDL Process
4 Modeling to Support the TMDL Process
Pages 68-88
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From page 68... ...
Because they represent our scientific understanding of how stressors relate to appropriate designated uses, models play a central role in the TMDL program. Models are the means of making predictions not only about the TMDL required to achieve water quality standards, but also about the effectiveness of different actions to limit pollutant sources and modify other stressors to reach attainment of a designated use.
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From page 69... ...
Water quality models for TMDL development are typically classified as either watershed (pollutant load) models or as waterbody (pollutant response)
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From page 70... ...
As discussed below, accounting for uncertainty and representing watershed processes are two of the possible criteria that need to be considered when selecting an analytical model for TMDL development. TMDIJs, which are typically evaluated through predictive modeling, lead to decisions concerning controls on pollutant sources or other stressors.
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From page 71... ...
Given the limitations of existing models, it should not be viewed as a required checklist for attributes that all present-day TMDL models must have. EPA has supported water quality model development for many years and, along with the U.S.
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From page 72... ...
72 Assessing the TMDL Approach to Water Quality Management
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From page 73... ...
It is important to recognize that placing priority on ultimate process description often will come at the expense of the other model selection criteria. For one thing, an emphasis on process description tends to favor complex mechanistic models over simpler mechanistic or empirical models and may result in analyses that are more costly than is necessary for effective decision-making.
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From page 74... ...
notes that since parameters involved in the TMDL determination are probabilistic and the MOS is a measure of uncertainty, the MOS should be determined through a formal uncertainty and error propagation analysis. There is also a compelling practical reason for explicit and thorough quantification of uncertainty in the TMDL via the MOS reduction of the MOS can potentially lead to a significant reduction in TMDL implementation cost.
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From page 75... ...
Most error analyses conducted on mechanistic water quality models have also focused on eutrophication, so relatively little is known of prediction error for toxic pollutants, microorganisms, or other important stressors. In one of the few relatively thorough error propagation studies, Di Toro and van Straten (1979)
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From page 76... ...
Indeed, recent TMDL modeling and assessment guidance from EPA often mentions the importance of formal uncertainty analysis in determining the MOS (EPA, 19991. However, EPA has consistently failed to either recommend predictive models that are amenable to thorough uncertainty analysis or provide adequate technical guidance for reliable estimation of prediction error.
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From page 77... ...
There have been some developments in modeling biological responses as a function of chemical water quality. One approach-attempts to describe the aquatic ecosystem as a mechanistic model that includes the full sequence of processes linking biological conditions to pollutant sources; this typically results in a relatively complex model and depends heavily on scientific knowledge of the processes.
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From page 78... ...
Success with this empirical approach has been primarily limited to models of relatively simple biological metrics such as chlorophyll a (Peters, 1991; Reckhow et al., 1992~. For reasons that are not entirely clear, empirical models of higher-level biological variables, such as indices of biotic integrity, have not been widely used.
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From page 79... ...
The latter, of course, increases the requirement for observational data. Despite these limitations, in the near term, empirical models may more easily fill the need for biological response models than would mechanistic models.
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From page 80... ...
ADDITIONAL MODEL SELECTION ISSUES Data Required The use of complex mechanistic models in the TMDL program is warranted if it helps promote the understanding of complex systems, as Tong as uncertainties in the results are reported and incorporated into decision-making. However, there may be a tendency to use complex mechanistic models to conduct water quality assessments in situations with little useful water quality data and/or involving major remediation expenditures or legal actions.
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From page 81... ...
Modeling to Support the TMDL Process 81
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From page 82... ...
82 Assessing the TMDL Approach to Water Quality Management
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From page 83... ...
. TMDL development will also likely require data on point' nonpoint sources and pollutant loads, atmospheric deposition, the effectiveness of current best management practices, and legacy/upstream pollutant sources.
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From page 84... ...
84 Assessing the TMDL Approach to Water Quality Management
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From page 85... ...
I EPA should not advocate detailed mechanistic models for TMDL development in data-poor situations.
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From page 86... ...
The focus on detailed mechanistic models has resulted in complex, costly, time-consuming modeling exercises for single TMDLs, potentially taking away resources from hundreds of other required TMDLs. Given the variety of existing watershed and water quality models available, and the range of relevant model selection criteria, EPA should expand its focus beyond mechanistic process models to include simpler models.
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From page 87... ...
1996. Development of a TMDL to reduce nonpoint source sediment pollution to Deep Creek, Montana.
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From page 88... ...
Beck and G van Straten (Editors)
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