This data release contains data in support of "Diverging trends in Nitrate and Phosphorus Loads and Yields Across Illinois Watersheds, 1997–2022" (Kamrath and others, 2025). It contains input and output data used to estimate total nitrate + nitrite, dissolved phosphorous and total phosphorous loads for sites in Illinois from water years 1974 to 2022. The input data includes "input-data_WQ.csv," which contains water quality data for nitrate + nitrite, dissolved phosphorous and total phosphorous. The water quality data comes from (1) the Water Quality Portal (WQP), (2) the Illinois Environmental Protection Agency (IEPA), and (3) the Environmental Protection Agency (EPA) STORET warehouse. The WQP includes the most complete record of U.S. Geological Survey and IEPA water quality samples. The most recent IEPA water quality samples, which are not yet in the WQP, were provided directly from IEPA. Legacy IEPA data that is housed via the EPA STORET warehouse was used to fill in any gaps in the data record. See the Entity and Attribute section of the metadata for details. The output data provides results calculated using the water-quality method Weighted Regressions on Time, Discharge, and Season (WRTDS) from the R package 'EGRET' (Exploration and Graphics for RivEr Trends, available from https://doi-usgs.github.io/EGRET/). The file "model-runs.csv" includes a list of all the models run using WRTDS with additional information for each run; for each run, WRTDS stationary and generalized flow normalization (SFN and GFN, respectively) were used along with the WRTDS Kalman (WRTDS-K) post processing. The results of each WRTDS run are saved in 'eList,' the native output format from 'EGRET,' in "eLists.zip." There are two eLists per run, one with SFN results and the other with GFN and WRTDS-K results. For more information on 'EGRET' and the WRTDS method see https://doi-usgs.github.io/EGRET/index.html. Additional output data includes estimated annual and monthly loads and concentrations of nitrate + nitrite, dissolved phosphorous and total phosphorous for all sites. The files are broken up by constituent, nitrate + nitrite (N) or phosphorous (P), and time frame, annual or monthly.

This data release supports "Diverging trends in nitrate and phosphorus loads and yields across Illinois watersheds, 1997–2022" (Kamrath and others, 2024) and the State of Illinois's Nutrient Loss Reduction Strategy (NLRS; IEPA and others, 2015). This data release contains estimates of incremental nitrate (NO3) and total phosphorus (TP) loads and yields for 49 watersheds in Illinois, defined using 8-digit hydrologic unit codes (HUC8; Jones and others, 2022). An "incremental" load or yield represents the load or yield attributable to each HUC8 watershed area determined by removing nested upstream drainage areas where present. For each HUC8 this data release also includes the proportion of the incremental load attributable to point and nonpoint sources, the proportion of the total phosphorus load attributable to dissolved and particulate phosphorus, and water yields. Data are provided as average annual values for three periods between 1997 and 2022: water years 1997–2011 (the baseline period used in the initial NLRS; IEPA and others, 2015), 2012–2017 (the period of previous update reported in the NLRS 2019 biennial report; IEPA and others, 2019), and 2018–2022 (the recent update). Data in "avg_loads_yields.csv" contains average annual loads (million pounds per year) and yields (pounds per acre per year) for each nutrient of interest, HUC8, and period. Data in "avg_water_yield.csv" contains average annual water yields (inches per year) for each HUC8 and period. Data in "avg_Pspecies.csv" contains the percentage of the TP loads attributable to dissolved and particulate phosphorus for each HUC8 and period. Supporting datasets include "site_list.csv," which contains the descriptions of each Illinois HUC8 and "site_calc_m23.csv," which contains the associated USGS and IEPA water quality sites used for HUC8-based computations IEPA, IDOA, and University of Illinois Extension, 2015, Illinois Nutrient Loss Reduction Strategy. Illinois Environmental Protection Agency and Illinois Department of Agriculture; Springfield, Illinois. University of Illinois Extension; Urbana, Illinois. IEPA, IDOA, and University of Illinois Extension, 2019, Illinois Nutrient Loss Reduction Strategy Biennial Report 2019: IISG19-RCE-RLA-071, accessed at https://epa.illinois.gov/content/dam/soi/en/web/epa/topics/water-quality/watershed-management/excess-nutrients/documents/nlrs-biennial-report-2019-final.pdf. Jones, K.A., Niknami, L.S., Buto, S.G., and Decker, D., 2022, Federal standards and procedures for the national Watershed Boundary Dataset (WBD) (5 ed.): U.S. Geological Survey Techniques and Methods 11-A3, 54 p., https://pubs.usgs.gov/tm/11/a3/. Kamarath and others, 2024 placeholder

This data release supports "Diverging trends in nitrate and phosphorus loads and yields across Illinois watersheds, 1997–2022" (Kamrath and others, 2024) and the State of Illinois's Nutrient Loss Reduction Strategy (NLRS; IEPA and others, 2015). This data release contains estimates of incremental nitrate (NO3) and total phosphorus (TP) loads and yields for 49 watersheds in Illinois, defined using 8-digit hydrologic unit codes (HUC8; Jones and others, 2022). An "incremental" load or yield represents the load or yield attributable to each HUC8 watershed area determined by removing nested upstream drainage areas where present. For each HUC8 this data release also includes the proportion of the incremental load attributable to point and nonpoint sources, the proportion of the total phosphorus load attributable to dissolved and particulate phosphorus, and water yields. Data are provided as average annual values for three periods between 1997 and 2022: water years 1997–2011 (the baseline period used in the initial NLRS; IEPA and others, 2015), 2012–2017 (the period of previous update reported in the NLRS 2019 biennial report; IEPA and others, 2019), and 2018–2022 (the recent update). Data in "avg_loads_yields.csv" contains average annual loads (million pounds per year) and yields (pounds per acre per year) for each nutrient of interest, HUC8, and period. Data in "avg_water_yield.csv" contains average annual water yields (inches per year) for each HUC8 and period. Data in "avg_Pspecies.csv" contains the percentage of the TP loads attributable to dissolved and particulate phosphorus for each HUC8 and period. Supporting datasets include "site_list.csv," which contains the descriptions of each Illinois HUC8 and "site_calc_m23.csv," which contains the associated USGS and IEPA water quality sites used for HUC8-based computations IEPA, IDOA, and University of Illinois Extension, 2015, Illinois Nutrient Loss Reduction Strategy. Illinois Environmental Protection Agency and Illinois Department of Agriculture; Springfield, Illinois. University of Illinois Extension; Urbana, Illinois. IEPA, IDOA, and University of Illinois Extension, 2019, Illinois Nutrient Loss Reduction Strategy Biennial Report 2019: IISG19-RCE-RLA-071, accessed at https://epa.illinois.gov/content/dam/soi/en/web/epa/topics/water-quality/watershed-management/excess-nutrients/documents/nlrs-biennial-report-2019-final.pdf. Jones, K.A., Niknami, L.S., Buto, S.G., and Decker, D., 2022, Federal standards and procedures for the national Watershed Boundary Dataset (WBD) (5 ed.): U.S. Geological Survey Techniques and Methods 11-A3, 54 p., https://pubs.usgs.gov/tm/11/a3/. Kamarath and others, 2024 placeholder

This data release contains seasonal source-specific estimates of total nitrogen (TN) and total phosphorus (TP) loading to streams across the Illinois River basin (25,620 reaches) for 21 years from 2000 through 2020 using a dynamic SPARROW (Spatially Referenced Regressions on Watershed attributes) model. Input data including calibration loads, output predictions, model control files, and model source code are provided to fully reproduce results. The modeled period was from December 1999 through November 2020 in seasonal timesteps, or 84 periods, and the following seasonal definitions were applied: winter includes December, January, and February; spring includes March, April, and May; summer includes June, July, and August; and fall includes September, October, and November. See Schmadel and others (2024) for further details.

This data release contains seasonal source-specific estimates of total nitrogen (TN) and total phosphorus (TP) loading to streams across the Illinois River basin (25,620 reaches) for 21 years from 2000 through 2020 using a dynamic SPARROW (Spatially Referenced Regressions on Watershed attributes) model. Input data including calibration loads, output predictions, model control files, and model source code are provided to fully reproduce results. The modeled period was from December 1999 through November 2020 in seasonal timesteps, or 84 periods, and the following seasonal definitions were applied: winter includes December, January, and February; spring includes March, April, and May; summer includes June, July, and August; and fall includes September, October, and November. See Schmadel and others (2024) for further details.

Estimated seasonal total nitrogen and total phosphorus loads during 1999 through 2020 in selected streams of the conterminous United States and water-quality and stream flow data used to generate those estimates are presented in this dataset. Loads were generated as part of the Integrated Water-Availability Assessment (IWAA) Program of the U.S. Geological Survey (www.usgs.gov/mission-areas/water-resources/science/integrated-water-availability-assessments#overview) using Fluxmaster (Schwarz and others, 2006, https://doi.org/10.3133/tm6B3) and Weighted Regression on Time, Discharge, and Season (WRTDS) (Hirsch and De Cicco, 2015, https://doi.org/10.1016/j.envsoft.2015.07.017). Loads were estimated initially for the Illinois River Basin (IRB) and later for the wider conterminous United States (including the IRB). Along with the water-quality data, streamflow data, and estimated nitrogen and phosphorus loads, this release also includes computer code that can be run to recreate the load estimates or modified for similar applications.

This USGS data release contains long-term mean annual total nitrogen and total phosphorus load estimates, and the model coefficients used to obtain the load estimates, for streams in the Midwest Region of the United States. The loads were estimated using the Fluxmaster program (Schwarz and others, 2006, https://pubs.usgs.gov/tm/2006/tm6b3) with a 5-parameter model and detrending to 2012 following the methods described in Saad and others, 2011 (https://doi.org/10.1111/j.1752-1688.2011.00575.x). A subset of these load estimates are described in Robertson and Saad, 2021 (https://doi.org/10.1111/1752-1688.12905) and were used to evaluate differences in load estimates in the Mississippi/Atchafalaya River Basin based on this method and the methods described in Saad and others, 2019 (https://dx.doi.org/10.3133/sir20195069). The flow and water-quality input files necessary to reproduce the load estimates included in this data release can be found in Saad and others, 2019 (https://doi.org/10.5066/F7DN436B).

This USGS data release contains long-term mean annual total nitrogen and total phosphorus load estimates, and the model coefficients used to obtain the load estimates, for streams in the Midwest Region of the United States. The loads were estimated using the Fluxmaster program (Schwarz and others, 2006, https://pubs.usgs.gov/tm/2006/tm6b3) with a 5-parameter model and detrending to 2012 following the methods described in Saad and others, 2011 (https://doi.org/10.1111/j.1752-1688.2011.00575.x). A subset of these load estimates are described in Robertson and Saad, 2021 (https://doi.org/10.1111/1752-1688.12905) and were used to evaluate differences in load estimates in the Mississippi/Atchafalaya River Basin based on this method and the methods described in Saad and others, 2019 (https://dx.doi.org/10.3133/sir20195069). The flow and water-quality input files necessary to reproduce the load estimates included in this data release can be found in Saad and others, 2019 (https://doi.org/10.5066/F7DN436B).

The Illinois River receives nutrients from point and non-point runoff from upstream urban and agricultural areas. These excess nutrients could be contributing to harmful algal blooms in the Illinois River and its tributaries. Successfully mitigating the blooms requires an understanding of the underlying nutrient processes occurring in the river. Phosphorus is often the nutrient that limits phytoplankton growth and is a substantial concern in aquatic ecosystems because it can be stored long-term in the streambed sediment. Streambed sediment can act as a source or sink for phosphorus, and this dynamic is influenced by site conditions, including historic phosphorus loading and current stream water phosphorus concentrations. This study quantified streambed sediment phosphorus dynamics at 10 stream sites in the Illinois River Basin during the summer and fall of 2024.

In this data release, we provide the data used to compute total annual phosphorus loads from tributaries and direct drainages to the Great Lakes during 2012-18 using the model load ratio approach and the unit area load approach and the resulting annual loads for 2012–18. This data release consists of: (1) measured loads at 24 sites, computed using the surrogate regression approach (Robertson et al., 2018; Koltun, 2020) that were monitored as part of the Great Lakes Restoration Initiative project, (2) estimated annual loads at point source facilities throughout the Great Lakes Basin obtained from the U.S. Environmental Protection Agency and state agencies, (3) loads subdivided into nonpoint and point source contributions, (4) extrapolation factors for each basin (nonpoint model load ratios, nonpoint yields, point source delivery factors), and (5) estimated annual loads for each tributary and direct drainage area to the Great Lakes during 2012–18. This data release consists of three tab delimited files: (1) ReferenceSite.txt, which contains all of the information (site data and phosphorus loads from SPARROW (Robertson and Saad, 2019) and measured during 2012–18 for the 24 reference (extrapolation sites) used in this study, (2) PointSource.txt, which contains all of the measured or extrapolated point source information for wastewater treatment plants in the Great Lake Basin for 2012–18; and (3) ExtrapolatedLoads.txt, which contains all of the nonpoint model load ratios, point source delivery ratios, measured wastewater treatment plant loads, and resulting annual loads for each tributary and direct drainage area in the Great Lakes Basin for the long-term average and 2012–18.