Spatio-temporal Changes and Driving Factors of Riverine Nitrogen Export in an Agriculture-dominated Watershed: the Illinois River Basin
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Riverine nutrient load from the Mississippi/Atchafalaya River Basin (MARB) plays a crucial role in the development of the Gulf of Mexico's hypoxic zone. Agricultural land in the MARB predominately contributes to nitrogen (N) and phosphorus (P) loading in the river system. Quantifying the sources, fate, and transport of nutrients from headwaters to large river basins is critical to diagnose the trend of exported nutrient load and guide conservation planning for nutrient loss reduction. Here taking the Illinois River Basin as an example, this study aims to quantify the changes in nitrate and nitrite (NO3+NO2) loads and yields at HUC12 scale from 2001-2020. We analyzed riverine loads and yields of nitrate and nitrite at 40 USGS gauge stations and simulated high-resolution nitrate and nitrite export throughout the Illinois River Basin using the SPARROW (SPAtially Referenced Regressions On Watershed attributes) model for two time periods (2001-2005, and 2016-2020) to quantify the spatiotemporal pattern and driving factors of changes in nitrogen export from the Illinois River Basin. We found that the five-year averaged total loads of nitrate and nitrite in the Illinois River Basin increased by 28% from 2001 to 2020, along with a 47% increase of discharge flow. Our analysis also revealed a complex spatial pattern of incremental yield of nitrate and nitrite: decreasing yields in the upper Illinois River contrasted with increasing yields in the lower Illinois and Kankakee Rivers. The simulation from the SPARROW model allowed us to identify and quantify contributions from various sources and examine driving factors for the observed spatial and temporal variations in nitrate and nitrite dynamics within the basin. By identifying areas of increasing and decreasing yields, as well as the factors driving these changes, our study provides crucial information for targeted conservation strategies to reduce nutrient load in the Illinois River Basin.
