We explored the possible causes of change in Mississippi River nutrient load trends through an impact evaluation that utilizes counterfactual scenarios to compare observed changes in river loads to changes in river load that might have occurred in the absence of potential causal factors. Prior to the counterfactual analysis, we developed a multiple linear regression model to predict TN and TP load changes over time. We modeled annual FN river loads as a function of current nutrient balances, lagged nutrient balances, and a latent variable representing the aggregate effect of other potential causal factors. We examined two different counterfactual scenarios, using hypothetical inputs to the calibrated TN and TP regression models. For Counterfactual A, the hypothetical inputs were current and lagged nutrient balances held constant at 1975 levels through 2017, and the Year terms were the same as the original inputs. The objective of holding the nutrient balance inputs constant was to investigate how river nutrient loads might have changed between 1975 and 2017 in the absence of any variability in nutrient balances after 1975. For Counterfactual B, the hypothetical inputs were the latent Year term held constant at 1975 levels through 2017, and the current and lagged nutrient balance inputs were the same as in the original inputs. The objective of holding the Year input constant at 1975 was to investigate how river nutrient loads might have changed between 1975 and 2017 in the absence of any variability in latent processes, potentially including BMP implementation, watershed buffering capacity, and other factors. The impact analysis compared the mean annual counterfactual analysis results to the mean original regression results for the time period 2013 to 2017. The original regression results refer to the predicted river loads estimated from the calibrated regression model using the original data.

We explored the possible causes of change in Mississippi River nutrient load trends through an impact evaluation that utilizes counterfactual scenarios to compare observed changes in river loads to changes in river load that might have occurred in the absence of potential causal factors. Prior to the counterfactual analysis, we developed a multiple linear regression model to predict TN and TP load changes over time. We modeled annual FN river loads as a function of current nutrient balances, lagged nutrient balances, and a latent variable representing the aggregate effect of other potential causal factors. We examined two different counterfactual scenarios, using hypothetical inputs to the calibrated TN and TP regression models. For Counterfactual A, the hypothetical inputs were current and lagged nutrient balances held constant at 1975 levels through 2017, and the Year terms were the same as the original inputs. The objective of holding the nutrient balance inputs constant was to investigate how river nutrient loads might have changed between 1975 and 2017 in the absence of any variability in nutrient balances after 1975. For Counterfactual B, the hypothetical inputs were the latent Year term held constant at 1975 levels through 2017, and the current and lagged nutrient balance inputs were the same as in the original inputs. The objective of holding the Year input constant at 1975 was to investigate how river nutrient loads might have changed between 1975 and 2017 in the absence of any variability in latent processes, potentially including BMP implementation, watershed buffering capacity, and other factors. The impact analysis compared the mean annual counterfactual analysis results to the mean original regression results for the time period 2013 to 2017. The original regression results refer to the predicted river loads estimated from the calibrated regression model using the original data.

This dataset consists of input and output datasets for estimating nitrogen and phosphorus balances for the Mississippi River Basin from 1950 to 2017. The N balance was calculated as the difference between inputs (fertilizer, manure, wastewater treatment facility effluent, N20 fixation, and atmospheric deposition) and outputs (crop uptake and removal in harvest and gaseous emissions to the atmosphere). The P balance was calculated as the difference between inputs (fertilizer, manure, wastewater treatment facility effluent, and weathering) minus outputs (P harvested and removed in crops, hay, and pasture).

This dataset consists of input and output datasets for estimating nitrogen and phosphorus balances for the Mississippi River Basin from 1950 to 2017. The N balance was calculated as the difference between inputs (fertilizer, manure, wastewater treatment facility effluent, N20 fixation, and atmospheric deposition) and outputs (crop uptake and removal in harvest and gaseous emissions to the atmosphere). The P balance was calculated as the difference between inputs (fertilizer, manure, wastewater treatment facility effluent, and weathering) minus outputs (P harvested and removed in crops, hay, and pasture).

This dataset consists of input and output datasets for estimating nitrogen and phosphorus balances for the Mississippi River Basin from 1950 to 2017. The N balance was calculated as the difference between inputs (fertilizer, manure, wastewater treatment facility effluent, N20 fixation, and atmospheric deposition) and outputs (crop uptake and removal in harvest and gaseous emissions to the atmosphere). The P balance was calculated as the difference between inputs (fertilizer, manure, wastewater treatment facility effluent, and weathering) minus outputs (P harvested and removed in crops, hay, and pasture).

This dataset consists of input and output datasets for estimating trends in river loads using the Weighted Regressions on Time Season and Discharge (WRTDS) model. The input datasets includes a discharge datafile and concentration datafiles for 6 water quality constituents: Total Phosphorus, Total Nitrogen, Nitrate plus Nitrite, Ammonium, Orthophosphate and Suspended Sediment. The period of record for the water quality data is from 1975 to 2017. The concentration and discharge data are for one site, the Mississippi River Outflow site, which is based on concentration data from USGS site 07373420, Mississippi River near St. Francisville, LA, and the discharge data are the sum of US Army Corps of Engineers Sites 01100 (Tarbert Landing) and 02600 (Old River Outflow) . Output dataset has the annual WRTDS estimates for time period 1975 to 2017 for all six constituents.

This dataset consists of input and output datasets for estimating trends in river loads using the Weighted Regressions on Time Season and Discharge (WRTDS) model. The input datasets includes a discharge datafile and concentration datafiles for 6 water quality constituents: Total Phosphorus, Total Nitrogen, Nitrate plus Nitrite, Ammonium, Orthophosphate and Suspended Sediment. The period of record for the water quality data is from 1975 to 2017. The concentration and discharge data are for one site, the Mississippi River Outflow site, which is based on concentration data from USGS site 07373420, Mississippi River near St. Francisville, LA, and the discharge data are the sum of US Army Corps of Engineers Sites 01100 (Tarbert Landing) and 02600 (Old River Outflow) . Output dataset has the annual WRTDS estimates for time period 1975 to 2017 for all six constituents.

This dataset consists of input and output datasets for estimating trends in river loads using the Weighted Regressions on Time Season and Discharge (WRTDS) model. The input datasets includes a discharge datafile and concentration datafiles for 6 water quality constituents: Total Phosphorus, Total Nitrogen, Nitrate plus Nitrite, Ammonium, Orthophosphate and Suspended Sediment. The period of record for the water quality data is from 1975 to 2017. The concentration and discharge data are for one site, the Mississippi River Outflow site, which is based on concentration data from USGS site 07373420, Mississippi River near St. Francisville, LA, and the discharge data are the sum of US Army Corps of Engineers Sites 01100 (Tarbert Landing) and 02600 (Old River Outflow) . Output dataset has the annual WRTDS estimates for time period 1975 to 2017 for all six constituents.

This dataset consists of input and output datasets for estimating trends in river loads using the Weighted Regressions on Time Season and Discharge (WRTDS) model. The input datasets includes a discharge datafile and concentration datafiles for 6 water quality constituents: Total Phosphorus, Total Nitrogen, Nitrate plus Nitrite, Ammonium, Orthophosphate and Suspended Sediment. The period of record for the water quality data is from 1975 to 2017. The concentration and discharge data are for one site, the Mississippi River Outflow site, which is based on concentration data from USGS site 07373420, Mississippi River near St. Francisville, LA, and the discharge data are the sum of US Army Corps of Engineers Sites 01100 (Tarbert Landing) and 02600 (Old River Outflow) . Output dataset has the annual WRTDS estimates for time period 1975 to 2017 for all six constituents.

This dataset consists of input and output datasets for estimating trends in river loads using the Weighted Regressions on Time Season and Discharge (WRTDS) model. The input datasets includes a discharge datafile and concentration datafiles for 6 water quality constituents: Total Phosphorus, Total Nitrogen, Nitrate plus Nitrite, Ammonium, Orthophosphate and Suspended Sediment. The period of record for the water quality data is from 1975 to 2017. The concentration and discharge data are for one site, the Mississippi River Outflow site, which is based on concentration data from USGS site 07373420, Mississippi River near St. Francisville, LA, and the discharge data are the sum of US Army Corps of Engineers Sites 01100 (Tarbert Landing) and 02600 (Old River Outflow) . Output dataset has the annual WRTDS estimates for time period 1975 to 2017 for all six constituents.