Total Maximum Daily Load Studies Research Articles

Calibration and Validation of Watershed Models and Advances in Uncertainty Analysis in TMDL Studies

AbstractWatershed models are widely used in total maximum daily load (TMDL) studies to predict the impacts of pollutant discharges on the biochemical functioning and assimilative capacity of water ...

Impact of Parameter Uncertainty on Critical SWAT Output Simulations

Watershed models are increasingly being utilized to evaluate alternate management scenarios for improving water quality. The concern for using these tools in extensive programs such as the national Total Maximum Daily Load (TMDL) program is that the certainty of model results and efficacy of management scenarios are not often measured and therefore are not well known. In this study, we used the mean value first-order reliability method (MFORM), a computationally efficient uncertainty analysis method, to determine the contribution of parameter uncertainty to total model uncertainty in streamflow, sediment, and nutrient outputs in a small Maryland watershed. Examination of sensitive and uncertain parameters revealed that parameters not considered highly sensitive contributed to model output uncertainty to a large extent. Therefore, highly sensitive parameters should not be the only parameters considered in uncertainty or calibration analyses. Measures of output uncertainty showed that sediment had the largest amount of variance from its mean value (CV = 28%), while nitrate, phosphate, and streamflow had considerably less variance, with annual average CVs of 19%, 17%, and 15%, respectively. The largest amounts of model uncertainty occurred during wet periods. This study concluded that with improved knowledge of the true value and associated uncertainty of input parameters, and improved algorithms to capture the variability of rainfall and associated flow, watershed water quality models will be of much greater use to TMDL studies and studies alike.

A Tool for Automated Load Duration Curve Creation1

Abstract: A recent study by the Texas Bacteria Total Maximum Daily Load (TMDL) Task Force has recommended the use of load duration curves as a primary tool in calculating bacterial TMDLs. This method is attractive because it effectively integrates flow regimes into TMDL analyses, clearly communicates data through a method that is understandable to the general public, and has been successfully applied in TMDL studies in other states. To ease the creation of load duration curves, an automated load duration curve creation tool called LDCurve has been created within a Microsoft Excel framework. Web services and a webscraper are used to retrieve U.S. Geological Survey streamflow data and Texas Commission on Environmental Quality water quality data. Data are imported to the spreadsheet, combined to create flow and load duration curves, and plotted. Final steps result in a preliminary estimate of the overall load reductions needed to meet water quality standards in the modeled segment. LDCurve is currently only applicable in the state of Texas, but may be updated to model water quality throughout the nation using analogous web services from the EPA STORET database. By using automated data retrievals and computations, the LDCurve tool reduces the amount of time required to create curves and calculate load reductions to a matter of minutes. LDCurve and all supporting materials are available online for free download at: http://tools.crwr.utexas.edu/LDCurve/.

Resurrection of the Lake Michigan Eutrophication Model, MICH1

The Lake Michigan model, MICH1, was developed more than 30 years ago. This framework was evaluated using field data collected in 1976 and was later applied to predict total phosphorus and phytoplankton concentrations in Lake Michigan during the 1980s and early 1990s. With a renewed interest in the interaction of phytoplankton with toxics and the applicability to Total Maximum Daily Load studies, several new models have been developed and older models have been revived. As part of our interest in plankton dynamics in Lake Michigan, the MICH1 model was resurrected. The model was evaluated over the 1976–1995 period, with a surprisingly good model fit to lake-wide average total phosphorus (TP) field data. However, the model was less successful in mimicking the chlorophyll-a measurements, especially in the hypolimnion. Given the results, the model was applied to perform a few long-term TP model simulations. Using the model with average 1994–95 phosphorus loadings, a steady state was reached within approximately 20 years, and the lakewide phosphorus concentration was below the International Joint Commission water quality guideline of 7 μg/L. This exercise demonstrated that a relatively simple, four-segment model was able to mimic the TP lake-wide data well. However, this model was less suitable to predict future chlorophyll-a concentrations due to the limitation in the representation of the foodchain and the difficulty of the coarse segmentation of the model to capture the deep chlorophyll-a layer. Strengths and limitations of this model can guide future development of eutrophication models for Lake Michigan and the other Great Lakes.

GROUPWISE MODELING STUDY OF BACTERIALLY IMPAIRED WATERSHEDS IN TEXAS: CLUSTERING ANALYSIS

ABSTRACT: Under the Clean Water Act (CWA) program, the Texas Commission on Environmental Quality (TCEQ) listed 110 stream segments in the year 2000 with pathogenic bacteria impairment. A study was conducted to evaluate the probable sources of pollution and characterize the watersheds associated with these impaired water bodies. The primary aim of the study was to group the water bodies into clusters having similar watershed characteristics and to examine the possibility of studying them as a group by choosing models for total maximum daily load (TMDL) development based on their characteristics. This approach will help to identify possible sources and determine appropriate models and hence reduce the number of required TMDL studies. This in turn will help in reducing the effort required to restore the health of the impaired water bodies in Texas. The main characteristics considered for the classification of water bodies were land use distribution within the watershed, density of stream network, average distance of land of a particular use to the closest stream, household population, density of on-site sewage facilities (OSSFs), bacterial loading from different types of farm animals and wildlife, and average climatic conditions. The climatic data and observed instream fecal coliform bacteria concentrations were analyzed to evaluate seasonal variability of instream water quality. The grouping of water bodies was carried out using the multivariate statistical techniques of factor analysis/principal component analysis, cluster analysis, and discriminant analysis. The multivariate statistical analysis resulted in six clusters of water bodies. The main factors that differentiated the clusters were found to be bacterial contribution from farm animals and wildlife, density of OSSFs, density of households connected to public sewers, and land use distribution.

BSLC: A TOOL FOR BACTERIA SOURCE CHARACTERIZATION FOR WATERSHED MANAGEMENT

The Total Maximum Daily Load (TMDL) program is an integrated watershed assessment and management processdesigned to address surface water quality impairments. The presence of pathogen indicator bacteria [fecal coliform andEscherichia coli (E. coli)] is reported to be the most widespread cause of water quality impairment in the United States.Watershed-scale simulation models, when used in a what-if scenario assessment mode, can assist stakeholders andwatershed planners in making informed immediate and long-term land-use-based watershed management decisions designedto control bacterial contamination and improve water quality. However, to use such models, accurate characterization ofbacteria sources present in a watershed is necessary. The objectives of this article are to describe the process developed andused by the Center for TMDL and Watershed Studies at Virginia Tech to characterize bacterial source inputs needed for awatershed-scale simulation model and to introduce a software tool, the Bacteria Source Load Calculator (BSLC), that aidsin the source characterization and TMDL allocation scenario development process. The BSLC uses externally generatedinputs, such as land use distribution and livestock, wildlife, and human population estimates, to calculate monthly bacterialland loadings and hourly bacterial stream loadings. The program was developed using Visual Basic for Applications (VBA)in Microsoft Excel. The BSLC and its underlying methodology have been used in the development of 21 bacterial TMDLs inVirginia. The BSLC software greatly simplifies the creation of required data files needed by watershed-scale simulationmodels and provides consistency in data development and processing.