Critical Source Areas in Non-Point-Source Pollution: Four Decades of Global Research Trajectories, Benchmark Loads, and Emerging Challenges

Non-point-source (NPS) pollution is a major driver of watershed degradation, and its pronounced heterogeneity makes uniform control impractical. This review synthesizes four decades of global research on critical source areas (CSAs) to clarify research patterns, identify limitations, and inform practice. A systematic literature screening was combined with synthesis of reported pollutant loads and comparative assessment of modeling approaches. Results show that CSA research expanded rapidly after the early 2000s, with nitrogen and phosphorus dominating indicator use and process-based models—especially the soil and water assessment tool (SWAT)—most widely applied. Publication activity has increased steadily, with China emerging as the largest contributor, underscoring the growing global attention to CSAs. Reported loads vary greatly across regions and methods, yet synthesized ranges for sediment, nitrogen, and phosphorus provide quantitative benchmarks to support cross-study comparison and assessment. Thematic analysis indicates strong attention to methodological innovation and management translation, but future prediction and explicit uncertainty analysis remain underdeveloped. This review not only establishes benchmark values and traces the global evolution of CSA research but also consolidates dispersed evidence into a coherent framework. By clarifying unresolved gaps, it provides a stronger basis for linking CSA identification with watershed management and for designing more targeted, cost-effective strategies for NPS pollution control.