This data release contains simulated orthophosphate concentrations (milligrams per liter) as a function of year, season, and flow for 53 monitoring stations in the Chesapeake Bay watershed. These data were generated for a study published in Science of the Total Environment (https://doi.org/10.1016/j.scitotenv.2018.10.062). They were extracted from the Weighted Regressions in Time, Discharge and Season (WRTDS; Hirsch et al. 2010) models developed for each monitoring station using two functions in the EGRET R package: the flowDuration function and the plotConcTimeSmooth function. The flowDuration function in the EGRET package (Hirsch and DeCicco, 2015) was used to quantify the 5th and 95th percentile flows to represent low and high flows, respectively, for each season at each watershed. These discharge values are then used as input for the plotConcTimeSmooth function, along with a date to represent the midpoint of each calendar year season (January-March = winter, etc.). The midpoints for the seasons were 15-February, 15-May, 15-August, and 15-November for the winter, spring, summer, and fall seasons, respectively. The plotConcTimeSmooth function provides estimates of concentration for those dates at those flows for all years included in the analysis. Simulated concentrations for the years 2006 and 2014 were extracted from this output, and are the only years included in this data release. Sources: Hirsch, R.M., Moyer, D.L., and Archfield, S.A., 2010, Weighted regressions on time, discharge, and season (WRTDS), with an application to Chesapeake Bay river inputs: Journal of the American Water Resources Resources Association, v. 46, no. 5, p. 857-880 Hirsch, R.M. and De Cicco, Laura, 2015, User guide to Exploration and Graphics for RivEr Trends (EGRET) and dataRetrieval: R packages for hydrologic data (version 2.0, February 2015): U.S. Geological Survey Techniques and Methods book 4, chap. A10, 93 p., http://dx.doi.org/10.3133/tm4A10.(accessed May 24, 2018)

Streamflow and phosphorus concentrations were monitored in the Assabet River in central Massachusetts in order to evaluate concentrations and loads in the river before, during, and after changes in the amount of total phosphorus that was discharged to the river from three wastewater-treatment plants. At four locations the U.S. Geological Survey collected weekly flow-proportional, composite samples of water from the Assabet River for analysis of concentrations of total phosphorus and orthophosphate. Streamflow and concentration data were used to estimate total phosphorus and orthophosphate loads in the river and compare them with total phosphorus load outputs from three wastewater-treatment plants. Data were collected from October 2008 through April 2014. The data are analyzed in a report that describes changes in phosphorus concentrations and loads during the study period. This data release consists of tables of (1) instream daily streamflow, concentrations of total phosphorus and orthophosphate from weekly composite samples, and daily load estimates for the four monitoring stations on the Assabet River; (2) daily outflow, daily measured and estimated total phosphorus and concentrations, and daily total phosphorus load estimates from the three wastewater-treatment plants; and (3) concentrations of total phosphorus and orthophosphate, and associated daily mean streamflows, for quality-control samples collected for the project.

Streamflow and phosphorus concentrations were monitored in the Assabet River in central Massachusetts in order to evaluate concentrations and loads in the river before, during, and after changes in the amount of total phosphorus that was discharged to the river from three wastewater-treatment plants. At four locations the U.S. Geological Survey collected weekly flow-proportional, composite samples of water from the Assabet River for analysis of concentrations of total phosphorus and orthophosphate. Streamflow and concentration data were used to estimate total phosphorus and orthophosphate loads in the river and compare them with total phosphorus load outputs from three wastewater-treatment plants. Data were collected from October 2008 through April 2014. The data are analyzed in a report that describes changes in phosphorus concentrations and loads during the study period. This data release consists of tables of (1) instream daily streamflow, concentrations of total phosphorus and orthophosphate from weekly composite samples, and daily load estimates for the four monitoring stations on the Assabet River; (2) daily outflow, daily measured and estimated total phosphorus and concentrations, and daily total phosphorus load estimates from the three wastewater-treatment plants; and (3) concentrations of total phosphorus and orthophosphate, and associated daily mean streamflows, for quality-control samples collected for the project.

These datasets allow users to acquire necessary input data to replication Zhang et al. (2023) and also review the model output results. This dataset is associated with the following publication: Zhang, Q., J. Bostic, and R. Sabo. Effects of point and nonpoint source controls on total phosphorus load trends across the Chesapeake Bay watershed, USA. Environmental Research Letters. IOP Publishing LIMITED, Bristol, UK, 19: 014012, (2023).

In 1991, the U.S. Geological Survey (USGS) began a study of more than 50 major river basins across the Nation as part of the National Water-Quality Assessment (NAWQA) project. One of the major goals of the NAWQA project was to determine how river water quality has changed over time. To support that goal, long-term consistent and comparable monitoring has been conducted by the USGS on streams and rivers throughout the Nation. Outside of the NAWQA project, the USGS and other Federal, State, and local agencies also have collected long-term water-quality data to support their own assessments of changing water quality. In 2017, data from these multiple sources were combined to support one of the most comprehensive assessments to date of water-quality trends in the United States (Oelsner and others., 2017; De Cicco and others, 2017). This data release updates these water quality trends, which ended in 2012, with 5 more years of data and now end in 2017. The three zipped folders below contain the input data used to calibrate the WRTDS trend models for three separate runs on the Yeti supercomputer. The initial run contained the majority of the screened sites and parameters and later runs included additional sites and reruns for sites with corrected data. The output from later runs always superseded earlier runs for any site-parameter combination that was run more than once. The data used in each run is contained in a zipped folder, each of which contains a "data" folder with 2 files and 1 folder with the same beginning part of the filename and same formatting. "WRTDS_2017data...csv" contains discrete water quality data. "WRTDS_2017info...csv" contains site information and model specifications. The folder "flowScaled" contains individual "Q_....csv" files of daily mean streamflow for each trend site. This metadata describes the format of those 3 objects within the zipped folders.

In 1991, the U.S. Geological Survey (USGS) began a study of more than 50 major river basins across the Nation as part of the National Water-Quality Assessment (NAWQA) project. One of the major goals of the NAWQA project was to determine how river water quality has changed over time. To support that goal, long-term consistent and comparable monitoring has been conducted by the USGS on streams and rivers throughout the Nation. Outside of the NAWQA project, the USGS and other Federal, State, and local agencies also have collected long-term water-quality data to support their own assessments of changing water quality. In 2017, data from these multiple sources were combined to support one of the most comprehensive assessments to date of water-quality trends in the United States (Oelsner and others., 2017; De Cicco and others, 2017). This data release updates these water quality trends, which ended in 2012, with 5 more years of data and now end in 2017. The three zipped folders below contain the input data used to calibrate the WRTDS trend models for three separate runs on the Yeti supercomputer. The initial run contained the majority of the screened sites and parameters and later runs included additional sites and reruns for sites with corrected data. The output from later runs always superseded earlier runs for any site-parameter combination that was run more than once. The data used in each run is contained in a zipped folder, each of which contains a "data" folder with 2 files and 1 folder with the same beginning part of the filename and same formatting. "WRTDS_2017data...csv" contains discrete water quality data. "WRTDS_2017info...csv" contains site information and model specifications. The folder "flowScaled" contains individual "Q_....csv" files of daily mean streamflow for each trend site. This metadata describes the format of those 3 objects within the zipped folders.

This data release contains model inputs, outputs, and source code (written in R) for the calculation of daily total phosphorus (TP) concentrations at a sampling site operated by the Klamath Tribes (WR6000), which is located 5 miles downstream of USGS streamgage 11502500, Williamson River below Sprague River near Chiloquin, Oregon. Daily streamflow data from 11502500 were used to calibrate the model along with data provided for the Klamath Tribes for site WR6000. Daily TP concentrations were calculated from 1991-2019 using the USGS Weighted Regression on Time, Discharge, and Season (WRTDS) model. Calibration data are bi-weekly (every two weeks) TP concentrations collected at site WR6000 and analyzed by the Klamath Tribes at the Sprague River Water Quality Laboratory (SRWQL). All concentrations are reported in milligrams per liter (mg/L).

In 1991, the U.S. Geological Survey (USGS) began a study of more than 50 major river basins across the Nation as part of the National Water-Quality Assessment (NAWQA) project. One of the major goals of the NAWQA project was to determine how river water quality has changed over time. To support that goal, long-term consistent and comparable monitoring has been conducted by the USGS on streams and rivers throughout the Nation. Outside of the NAWQA project, the USGS and other Federal, State, and local agencies also have collected long-term water-quality data to support their own assessments of changing water quality. In 2017, data from these multiple sources were combined to support one of the most comprehensive assessments to date of water-quality trends in the United States (Oelsner and others, 2017; De Cicco and others, 2017). This data release updates these water quality trends, which ended in 2012, with 5 more years of data and now end in 2017. These datasets contain the output from the WRTDS trend models that characterize changes in water quality in rivers and streams across the Nation. The "compiledResults_final" folder contains 3 output tables: "allAnnualResults.csv", "bootOut.csv", and "parisOut.csv". "allAnnualResults.csv" contains several types of estimates of annual mean concentration and fluxes for the entire calibration period for each combination of site and parameter. These estimates include "true condition" estimates determined using the original implementation of WRTDS ("_orig" suffix). "true condition" estimates determined using WRTDS_K (i.e. using a Kalman filter: "_K" suffix), flow normalized (FN) trend estimates ("FN prefix"), lower and upper 90% confidence intervals of FN trend estimates ("FN" prefix with "Low" or "High" suffix), and FN trend estimates assuming a stationary flow regime ("SFN" suffix). Water quality changes were calculated for up to six trend periods (1972-2017, 1982-2017, 1992-2017, 2002-2017, and 2007-2017) per site and parameter. "bootOut.csv" gives the results of the bootstrap trend test, including uncertainty estimates, by site parameter and trend period. "pairsOut.csv" gives the trend component estimates (concentration-discharge trend component (CQCT), also referred to as the "management" trend component (MTC)) and discharge trend component (QTC) and related information, by site, parameter, trend period, and estimate type (i.e. concentration or flux). Finally, the "eList" for each WRTDS model (site-parameter combination) is available in the zipped folder according to whether the model was accepted or rejected; "outLists_accepted.zip" and "outLists_rejected.zip", respectively. The output tables in "compiledResults_final.zip" contain results only for models that had acceptable fits, as determined by an estimated probability of acceptance or by manual evaluation of residual and diagnostic plots. eLists are compiled according to whether the model was accepted or rejected.

In 1991, the U.S. Geological Survey (USGS) began a study of more than 50 major river basins across the Nation as part of the National Water-Quality Assessment (NAWQA) project. One of the major goals of the NAWQA project was to determine how river water quality has changed over time. To support that goal, long-term consistent and comparable monitoring has been conducted by the USGS on streams and rivers throughout the Nation. Outside of the NAWQA project, the USGS and other Federal, State, and local agencies also have collected long-term water-quality data to support their own assessments of changing water quality. In 2017, data from these multiple sources were combined to support one of the most comprehensive assessments to date of water-quality trends in the United States (Oelsner and others, 2017; De Cicco and others, 2017). This data release updates these water quality trends, which ended in 2012, with 5 more years of data and now end in 2017. These datasets contain the output from the WRTDS trend models that characterize changes in water quality in rivers and streams across the Nation. The "compiledResults_final" folder contains 3 output tables: "allAnnualResults.csv", "bootOut.csv", and "parisOut.csv". "allAnnualResults.csv" contains several types of estimates of annual mean concentration and fluxes for the entire calibration period for each combination of site and parameter. These estimates include "true condition" estimates determined using the original implementation of WRTDS ("_orig" suffix). "true condition" estimates determined using WRTDS_K (i.e. using a Kalman filter: "_K" suffix), flow normalized (FN) trend estimates ("FN prefix"), lower and upper 90% confidence intervals of FN trend estimates ("FN" prefix with "Low" or "High" suffix), and FN trend estimates assuming a stationary flow regime ("SFN" suffix). Water quality changes were calculated for up to six trend periods (1972-2017, 1982-2017, 1992-2017, 2002-2017, and 2007-2017) per site and parameter. "bootOut.csv" gives the results of the bootstrap trend test, including uncertainty estimates, by site parameter and trend period. "pairsOut.csv" gives the trend component estimates (concentration-discharge trend component (CQCT), also referred to as the "management" trend component (MTC)) and discharge trend component (QTC) and related information, by site, parameter, trend period, and estimate type (i.e. concentration or flux). Finally, the "eList" for each WRTDS model (site-parameter combination) is available in the zipped folder according to whether the model was accepted or rejected; "outLists_accepted.zip" and "outLists_rejected.zip", respectively. The output tables in "compiledResults_final.zip" contain results only for models that had acceptable fits, as determined by an estimated probability of acceptance or by manual evaluation of residual and diagnostic plots. eLists are compiled according to whether the model was accepted or rejected.

In 1991, the U.S. Geological Survey (USGS) began a study of more than 50 major river basins across the Nation as part of the National Water-Quality Assessment (NAWQA) project. One of the major goals of the NAWQA project was to determine how river water quality has changed over time. To support that goal, long-term consistent and comparable monitoring has been conducted by the USGS on streams and rivers throughout the Nation. Outside of the NAWQA project, the USGS and other Federal, State, and local agencies also have collected long-term water-quality data to support their own assessments of changing water quality. In 2017, data from these multiple sources were combined to support one of the most comprehensive assessments to date of water-quality trends in the United States (Oelsner and others, 2017; De Cicco and others, 2017). This data release updates these water quality trends, which ended in 2012, with 5 more years of data and now end in 2017. These datasets contain the output from the WRTDS trend models that characterize changes in water quality in rivers and streams across the Nation. The "compiledResults_final" folder contains 3 output tables: "allAnnualResults.csv", "bootOut.csv", and "parisOut.csv". "allAnnualResults.csv" contains several types of estimates of annual mean concentration and fluxes for the entire calibration period for each combination of site and parameter. These estimates include "true condition" estimates determined using the original implementation of WRTDS ("_orig" suffix). "true condition" estimates determined using WRTDS_K (i.e. using a Kalman filter: "_K" suffix), flow normalized (FN) trend estimates ("FN prefix"), lower and upper 90% confidence intervals of FN trend estimates ("FN" prefix with "Low" or "High" suffix), and FN trend estimates assuming a stationary flow regime ("SFN" suffix). Water quality changes were calculated for up to six trend periods (1972-2017, 1982-2017, 1992-2017, 2002-2017, and 2007-2017) per site and parameter. "bootOut.csv" gives the results of the bootstrap trend test, including uncertainty estimates, by site parameter and trend period. "pairsOut.csv" gives the trend component estimates (concentration-discharge trend component (CQCT), also referred to as the "management" trend component (MTC)) and discharge trend component (QTC) and related information, by site, parameter, trend period, and estimate type (i.e. concentration or flux). Finally, the "eList" for each WRTDS model (site-parameter combination) is available in the zipped folder according to whether the model was accepted or rejected; "outLists_accepted.zip" and "outLists_rejected.zip", respectively. The output tables in "compiledResults_final.zip" contain results only for models that had acceptable fits, as determined by an estimated probability of acceptance or by manual evaluation of residual and diagnostic plots. eLists are compiled according to whether the model was accepted or rejected.