Tracing Flow Pathways of Nutrients in Dairy Farms—An End Member Mixing Analysis Approach

Understanding dominant transport pathways of nutrients is crucial for designing and implementing effective mitigation strategies to reduce downstream water quality impacts. This study applied a novel high frequency nutrient and major ion analyser and end member mixing analysis (EMMA) using conservative tracers to identify dominant flow pathways and their influence on nitrogen and phosphorus transport from two dairy farms in the Macalister Irrigation District, Victoria, Australia. Analyses show that farm drain water at both farms was consistently a mixture of event water (rainwater and irrigation water) and soil water, which represent surface runoff and shallow subsurface soil flow, respectively, with limited contribution from groundwater. The dominance of end members varied between farms and events and was controlled by surface roughness, microtopography, and antecedent soil moisture. Nutrient‐to‐chloride ratios revealed that nitrate and phosphate have a pronounced “first flush” behaviour during events when surface runoff is dominant, indicating rapid mobilization of nutrients from manure. In contrast, events when shallow subsurface soil flow was dominant are associated with low nutrient export. These results showed the role of surface runoff in driving nutrient export from low‐relief dairy farms and highlight the importance of irrigation management, surface roughness preservation, and reuse systems for reducing nutrient losses from flood‐irrigated dairy regions.