Total Maximum Daily Load Analysis and Modeling: Assessment of the Practice

The EWRI Watershed Management Technical Committee (WMTC) chartered the total maximum daily load (TMDL) Analysis and Modeling Task Committee to identify why TMDL modeling is insufficient to develop reliable relationships between waste loads and receiving water impairments and whether professional consensus protocols are necessary to guide the TMDL modeling process. This chapter provides objectives of the original task committee. During its five-year tenure these objectives evolved as surveys of state agencies and member experience uncovered that cause-and-effect modeling is not always used for TMDL analysis. Recruiting the best available TMDL analysis experts led to the expansion of objective 2 to include receiving water quality models. Review of watershed modeling in isolation would unduly exclude some of the more important waste load allocation models and deemphasize using the waste load allocation program as a benchmark by which to evaluate TMDL model development.

A total maximum daily load (TMDL) is a regulatory term in the United States Clean Water Act that describes the maximum amount of a pollutant that a body of water can assimilate and still meet water quality standards. The TMDL procedure conservatively allocates the permissible loads of a specific pollutant between point sources and nonpoint sources. Section 303(d) of the United States Federal Water Pollution Control Act or Clean Water Act requires all states, territories, and authorized tribes to determine the TMDLs allowable for all impaired water bodies or waters threatened by impairment and to implement plans to remove or prevent all impairments. Legislative and regulatory events dominate the history of nonpoint source pollution management in the United States. This chapter describes some regulations before the 1987 Amendments to the Clean Water Act published by the USEPA.

Since 1925, engineers and scientists have developed water quality models to simulate many pollutants in a broad range of receiving waters, and since 1954, they have developed models of watershed hydrology. Following the 1987 Amendments of the United States Clean Water Act, water quality analysts combined these effective tools for total maximum daily load (TMDL) modeling. To minimize model implementation hurdles, analysts define how to apply TMDL modeling, which models to select, and then apply the appropriate models. The United States Clean Water Act and USEPA regulations for implementing the required TMDLs require that the process “take into account the critical conditions for stream flow, loading, and water quality parameters.” Depending on the uncertainty, the states and the USEPA set total maximum daily loads using simulations of the waste assimilative capacity, or use adaptive management that the USEPA calls a phased approach.

This chapter lists a few simple models and analytical procedures, including simplified mass balances; the Simple Method; load-duration curves; and the BATHTUB, SSTEMP, and RUSLE2 models. The simple, inexpensive tools introduced in the chapter may be used initially to estimate total maximum daily loads (TMDLs), but are usually employed to check and analyze TMDLs. The task committee is concerned that these methods are overused because they do not reliably relate loads to impairments to determine allocations rationally. The Simple Method is an approach to estimate loads rapidly based on available information such as catchment drainage area and impervious cover and storm water runoff concentrations. Some practitioners prize the insight that load-duration curves provide into the relationship between water quality impairment and hydrologic regime.

This chapter lists in alphabetical order the 14 watershed models that members of the task committee have used or developed or are aware of in total maximum daily load (TMDL) assessment and implementation. It simply overviews the model sources, and for some models it discusses the history of development, capabilities, and applicability as readily evident to the task committee without exhaustive investigation. Approximately 60% of all watershed models are based on the curve number relationship between rainfall and runoff. To better understand the limitations of this dominant class of watershed models, the chapter reviews the intended applications and the known limitations of the curve number method.

A common framework for total maximum daily load (TMDL) modeling links watershed and receiving water models. Receiving water includes both hydraulic and water quality modeling components. This chapter describes selected receiving water quality models with their source, capabilities, and applicability. The software package CCHE1D simulates one-dimensional unsteady flows, sediment transport, and water quality in dendritic channel networks. The CCHE1D hydrodynamic model includes special procedures to compute flow across hydraulic structures like culverts, low- and high-drop structures, bridge crossings, and measuring flumes. CCHE1D has been applied to various real-life hydrodynamics and sediment studies, including bed load transport in East Fork River, Wyoming; sedimentation in the Danjiangkou Reservoir on the Hanjiang River, China; and runoff and sediment yield in Goodwin Creek watershed, Mississippi. The CE-QUAL-W2 software directly links a hydrodynamic model and a water quality model for rivers, estuaries, lakes, and reservoirs.

In recent years, the difficulties in transferring information between models prompted development of integrated modeling frameworks known as decision support systems (DSS) and integrated water resources management (IWRM) tools. Notable integrations span economic and hydrologic models and surface and groundwater models. Two factors drive the need for model integration: capturing system complexity, such as that of large watersheds with large urban and rural areas and flow-regulated rivers, reservoirs, lakes, and estuaries, and modeling complex management goals, which could include capturing interactions of physical, ecological, economic, and social systems. The USEPA developed the BASINS framework to support the development of total maximum daily load (TMDL) for water bodies and to address the need for a watershed-based modeling system that could expedite the many water bodies that require TMDL development. BASINS 4.1 has four data categories: base cartographic, environmental background, environmental monitoring, and point source data.

Implementing total maximum daily load (TMDL) allocations for nonpoint source reductions depends on the regulatory authority of each state or territory. Such implementation may rely on voluntary adoption of agricultural or forestry best management practices by property owners, typically funded by federal resource management agencies. In each state, specific laws and local ordinances dictate implementation procedures. Monitoring in-stream water quality will determine the success of TMDL implementation. Allocation by class leads to at least two types of disjointed and inefficient TMDL assessment and implementation modeling. Several issues must be addressed during TMDL implementation, beginning with determining responsibilities. The implementation modeling process differs mainly from modeling for TMDL development in that receiving water models are not needed. Instead, models that can simulate the effects of management practices are required.

Integrating remote sensing data with geographic information systems has enabled more comprehensive analysis of water quality. Spatially distributed, parametric total maximum daily load (TMDL) models require topographic, soil, and land use information as input data. Geographic information systems and remote sensing are useful tools to process raw data for model input and to synthesize spatial data as part of the modeling process. States and the USEPA use TMDL calculations to limit point source and nonpoint source pollution by establishing water quality objectives for receiving waters and limiting watershed pollutant loading into these receiving waters in a manner consistent with meeting water quality objectives. Remote sensing and geographic information systems can also be used in formulating management plans that can result in improvements to the quality of impaired waters.

In professional practice, modeling to determine total maximum daily load (TMDL) or to implement TMDLs requires calibration, confirmation, and determination of the uncertainty in cause-and-effect simulations. Model calibration ensures accurate and useful TMDL allocations, and confirmation determines how well a model is calibrated using independent data. Reliable TMDLs are only determined by cause-and-effect simulations that relate violations of water quality criteria to contaminant sources that impair surface waters. In the Southwest United States, salt balance models are frequently used to great advantage in water supply and irrigation management. In more complex TMDL modeling, water balance and mass balance confirmations are the first step in model calibration to define transport uncertainty. The more extensive recalibration tends to reduce the uncertainty in forecasting the effects of the TMDL allocations.

Uncertainty analysis is a complementary activity to the calibration and confirmation of a model. This analysis is performed to investigate the impacts of sources of error in model simulations and ultimately to obtain a more informed evaluation of the relationship between loads and impaired waters. Uncertainty is classified as epistemic or ontological depending on the nature of the unknown information. In total maximum daily load (TMDL) studies, epistemic sources of uncertainty adversely impact the capacity of models to represent receiving water body responses to contaminant discharges accurately and the capacity to design effective strategies to meet water quality standards and protect designated uses. To address these problems in TMDL analysis, the United States Clean Water Act Section 303(d)(1)(c) mandates the use of a margin of safety to compensate for any source of uncertainty during the development of a TMDL.

The BASINS framework is a multipurpose total maximum daily load (TMDL) analysis system that helps USEPA regions, state and local agencies, and other organizations perform watershed and water quality studies. A geographic information system interface provides the integration framework for BASINS and various layers of data input and output to and from the model. The USEPA developed a case study document for Cottonwood Creek Watershed, Idaho County, Idaho, to demonstrate the use of BASINS in developing a TMDL. The USEPA’s step-by-step guide for the Cottonwood Creek Watershed case study describes how to reconstruct, in specific BASINS and NPSM projects, the work products described throughout the modeling report. This chapter provides links to sites from which TMDL reports can be downloaded and describes a tool for selecting reports based on impairment and modeling methods.

The task committee, comprising ASCE members from academia, federal-state-local government, and private consulting backgrounds, researched TMDL analysis and modeling including its inception, history, and current uses of models, which include modeling critical conditions based on available materials in the literature and experiences from their own work. Unlike earlier USEPA reviews, the committee grouped the models and procedures into four categories: simple models and analytical procedures, watershed models, receiving water models, and integrated modeling systems. Fourteen leading watershed and 13 leading receiving water models were critically reviewed and compiled and compared in matrices showing differences of certain critical features. Many TMDL assessments use models developed by other agencies for different purposes such as soil and water conservation or floodplain delineation. State agencies attempt phased TMDLs and use adaptive implementation to illustrate the penalties of not having an adequately planned and supported suite of TMDL models.