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[http://pubs.usgs.gov/tm/04/a10/](http://pubs.usgs.gov/tm/04/a10/)
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Note: The "official EGRET User Guide" currently (2014-11-12) shows a workflow that has been superseded by
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the workflow shown in this vignette. However the science and math is the User Guide is correct. The User
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Note: The "official EGRET User Guide" currently (2014-11-12) shows a workflow that has been superseded by the workflow now used in `EGRET` and `dataRetrieval` software.. However the science and math is the User Guide is correct. The User
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Guide is in the process of being updated and will be available at the URL shown above in the near future.
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An recent presentation on an overview of `dataRetrieval` and `EGRET` can be found here:
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Background Information
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WRTDS is a method of analysis for long-term surface water quality data to evaluate trends and average concentrations and fluxes. It is intended for use with data sets of more than about 200 observations of water quality over a time span of about 20 years or more, but can be used with somewhat shorter data sets. There also needs to be a daily time series of streamflow data covering the entire period of the water quality data collection. The method can be used with smaller data sets, but some adjustments by the user are needed to make it work with less than 100 water quality observations. The best way to learn about the WRTDS approach and to see examples of its application to multiple large data sets is to read two journal articles. Both are available, for free, from the journals in which they were published.
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WRTDS is a method of analysis for long-term surface water quality data to evaluate trends and average concentrations and fluxes. It is intended for use with data sets of more than about 200 observations of water quality over a time span of about 20 years or more, but can be used with somewhat shorter data sets. There also needs to be a daily time series of streamflow data covering the entire period of the water quality data collection. The method can be used with smaller data sets, but some adjustments by the user are needed to make it work with less than 100 water quality observations. The best way to learn about the WRTDS approach is to read the [User Guide](http://pubs.usgs.gov/tm/04/a10/) and two journal articles which are available, for free, from the journals in which they were published.
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The first relates to nitrate and total phosphorus data for 9 rivers draining to Chesapeake Bay:
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