A sensitivity-informed correlation-based framework for detecting runoff pollutant hotspots in grazed catchments:...

Cattle grazing is a known source of water pollution. Identifying areas within grazinglands where cattle activity contributes most to downstream water quality impairment is important for effective management. This study presents a framework to detect these runoff pollutant hotspots. The framework is based on Pearson correlation coefficients between cattle presence across the catchment and water quality at the outlet. Cattle locations were recorded at 5-60 min time intervals using Virtual Fencing (VF) technology in three experimental catchments located in north-central Oklahoma, USA, two under rotational grazing and one continuously grazed. Escherichia coli (E. coli), total Kjeldahl nitrogen (TKN), and total phosphorus (TP) were measured during runoff events between March 2022, and June 2024. The spatial and temporal scales at which runoff quality was most affected by cattle presence were determined through a sensitivity analysis of grid cell size (GCS), time interval (ΔT) between consecutive cattle Global Positioning System (GPS) records, and pollutant half-life (HL) representing manure decay. The strongest grid cell-scale correlations between water quality parameters and cattle presence ranged from 0.48 to 0.83 for E. coli, 0.70 to 0.86 for TKN, and 0.67 to 0.90 for TP. These hotspots were detected using grid cell sizes ranging from 70 m × 70 m to 100 m × 100 m, and half-life values ranging from 3 to 12 days for E. coli, 3-23 days for TKN, and 3-20 days for TP. Compared to the continuously grazed catchment, hotspots in rotationally grazed, VF-managed catchments with cattle exclusion from riparian buffers were fewer, diffused, and located away from streams. Significant positive spatial autocorrelation was observed in five hotspot maps. Hotspot identification was influenced by pollutant persistence and grazing management, with rotational grazing showing fewer and more diffuse hotspots than continuous grazing.