Average Annual Loads and Yields of Nitrate and Phosphorus from Illinois Watersheds (HUC8s) for Three Periods between 1997 and 2022

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; Illinois Environmental Protection Agency [IEPA] and others, 2015). This data release contains estimates of incremental nitrate (NO3) and total phosphorus (TP) loads and yields for 50 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 U.S. Geological Survey and IEPA water quality sites used for HUC8-based computations.