Spatiotemporal Changes and Driving Factors of Riverine Nutrient Export at HUC12 Scale in the Mississippi/Atchafalaya River Basin
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Effective management of nutrient pollution requires a detailed understanding of how nutrient export evolves across spatial and temporal scales, particularly in response to hydrological variability and anthropogenic activity. In this study, we developed a high-resolution SPARROW (SPAtially Referenced Regressions On Watershed attributes) model for the Mississippi/Atchafalaya River Basin (MARB) to quantify annual total nitrogen (TN) and total phosphorus (TP) exports at the HUC12 scale from 1980 to 2020. Model calibration was conducted using nutrient loads estimated with WRTDS-K at monitoring stations throughout the basin, enabling source attribution analysis. To examine long-term trends, we compared SPARROW-estimated nutrient loads and yields across three periods: 1980–1996, 2001–2010, and 2011–2020. A bootstrap-based scenario analysis was employed to isolate anthropogenic and hydrological influences on nutrient yield changes. Results show widespread increases in local incremental TN and TP yields across all HUC2 sub-basins in the MARB. Within the MARB, TN contributions from agricultural sources rose from 40.4% to 43.8%, while urban contributions increased from 20.9% to 26.2%. TN yields increased by 0.45 and 1.67 kg-N/ha/yr for 2001-2020 and 2011–2020 compared to the 1980–1996 baseline, with hydrologic changes explaining over 50% of the increase. The Conservation Reserve Program may have contributed to reducing TN land-to-water delivery. For TP, within the MARB, urban sources rose from 30.4% to 34.9%, while agricultural contributions fluctuated between 30% to 35% of the TP load. TP yields increased by 0.12 and 0.27 kg-P/ha/yr for 2001–2010 and 2011–2020, with hydrology accounting for over half of the increase. Among anthropogenic drivers, urban-related sources, including wastewater treatment plant discharge and urban nonpoint sources, were the dominant contributors to TP export. These results provide fine-scale insights into nutrient transport and source contributions across the MARB, identifying critical hotspots and temporal shifts in nutrient yields. Our findings highlight the importance of implementing localized, data-driven conservation strategies tailored to evolving hydrological and anthropogenic conditions to effectively mitigate riverine nutrient loading in the MARB.
