Background Phosphorus (P) has high spatial heterogeneity and complex loss characteristics, which makes it difficult to prevent and control the non-point source pollution caused by soil erosion. As a major limiting element in the middle route of South-to-North Water Diversion Project, it is crucial to explore the law of P loss in the process of water and soil loss.

Methods The characteristics of runoff flux, runoff erosion power, total P loss and available P loss in Danjiang watershed were analyzed by located monitoring of natural rainfall. The runoff samples of 6 sections were selected in 2 rainfall events in 2016 and 3 rainfall events in 2019 to analyze the runoff and P loss processes from upstream to downstream. The relationship between runoff flux, runoff erosion power and P flux was explored and compared under the condition of natural rainfall.

Results The retardation time of runoff peak was 1-4 h; the P loss ranged from 0.01 to 0.43 mg/L. The average P concentration in runoff of 2016 and 2019 was 0.08 and 0.33 mg/L, respectively. The water quality improved with the increasing age of the small watershed management. If the rainfall intensity was lower, the total P loss in runoff was less, even though the precipitation was higher and during time was longer. This indicated that the P loss in runoff was more affected by rainfall intensity, rather than during time of rainfall and precipitation. In the same monitored rainfall, with the increase of rainfall duration, the concentration of total P loss at each cross section of the small watershed increased first and then decreased. The concentration of total P at each cross section showed a cumulative effect from top to bottom. When the rainfall intensity and I₃₀ (maximum 30-min rainfall intensity) was high, the runoff erosion power could better reflect the P loss in runoff, while the determination coefficient was 0.59 with higher rainfall intensity and 0.26 for lower rainfall intensity. But under the condition of rainstorm, the regression coefficient of runoff flux and phosphorus loss (>0.90) was larger than that of runoff erosion power (< 0.60). The M(V) curve of the two rainfall events in 2019 showed an upper convex type, that is, the peak value of P loss concentration appeared in the early rainfall period.

Conclusions Runoff flux is the key factor affecting P loss, especially that the effect of runoff flux on P loss under different rainfall intensities was significantly different. The priority control of P forms is different in the monitored natural rainfall of different times, due to the difference of rainfall intensity. The results of this study may provide a theoretical basis for the prevention and control of non-point source pollution caused by P loss in the process of water and soil erosion.