Development and Application of Watershed Regressions for Pesticides (WARP) for Estimating Atrazine Concentration Distributions in Streams

By Steven J. Larson, Charles G. Crawford and Robert J. Gilliom



Water–Resources Investigations Report 03-4047

Sacramento, California 2004
Prepared in cooperation with the
U.S. Environmental Protection Agency

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     Regression models were developed for predicting atrazine concentration distributions in rivers and streams, using the Watershed Regressions for Pesticides (WARP) methodology. Separate regression equations were derived for each of nine percentiles of the annual distribution of atrazine concentrations and for the annual time-weighted mean atrazine concentration. In addition, seasonal models were developed for two specific periods of the year--the high season, when the highest atrazine concentrations are expected in streams, and the low season, when concentrations are expected to be low or undetectable. Various nationally available watershed parameters were used as explanatory variables, including atrazine use intensity, soil characteristics, hydrologic parameters, climate and weather variables, land use, and agricultural management practices. Concentration data from 112 river and stream stations sampled as part of the U.S. Geological Survey's National Water-Quality Assessment and National Stream Quality Accounting Network Programs were used for computing the concentration percentiles and mean concentrations used as the response variables in regression models. Tobit regression methods, using maximum likelihood estimation, were used for developing the models because some of the concentration values used for the response variables were censored (reported as less than a detection threshold). Data from 26 stations not used for model development were used for model validation.

     The annual models accounted for 62 to 77 percent of the variability in concentrations among the 112 model development stations. Atrazine use intensity (the amount of atrazine used in the watershed divided by watershed area) was the most important explanatory variable in all models, but additional watershed parameters significantly increased the amount of variability explained by the models. Predicted concentrations from all 10 models were within a factor of 10 of the observed concentrations at most model development and model validation stations. Results for the two sets of seasonal models were similar. Concentration distributions derived from the seasonal-model predictions provided additional information compared to distributions derived from the annual models.





Purpose and Scope



Atrazine Data Used for Model Development

Sample Collection and Analysis Methods

Sampling Station Selection

Atrazine Data Used for Model Validation

Watershed Characteristics Used as Explanatory Variables

Estimation of Atrazine Use

Other Watershed Characteristics

Statistical Analysis

Calculation of Annual Percentile and Mean Concentrations

Transformation of Response and Explanatory Variables

Selection of Explanatory Variables

Analysis of Model Fit

Estimation of Confidence and Prediction Intervals

Atrazine Models

Analysis of Significant Explanatory Variables

Relation Between Predicted Concentrations and Atrazine Use Intensity

Model Performance

Model Development Stations

Model Validation Stations

Uncertainty in Model Predictions

Prediction of Annual Concentration Distributions

Model Development Stations

Model Validation Stations

Year-to-Year Variability

Seasonal Atrazine Models

Model Limitations

Summary and Conclusions

References Cited

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Water Resources of California

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