Background The relationship between runoff and sediment in small watershed is a complex physical process of the interaction between rainfall and underlying surface. Rainfall and human activities cause a significant impact on the relationship between water and sediment in small watersheds. Monitoring the occurrence and transmission direction of soil and water loss process is conducive to revealing the mechanism of runoff and sediment yield in small watershed.

Methods The inter-annual variation of runoff and sediment transport was analyzed by mathematical statistics, and the annual inner diameter flow and sediment transport were calculated using the runoff and sediment transport of each month for many years. Combined with Mann-Kendall rank correlation test, the trend of runoff and sediment transport in Liujiagou small watershed was explained, and the power function of fitting equation reflected the correlation of runoff and sediment.

Results 1) The average annual runoff, sediment transport and peak discharge of small watershed were 582 000 m³, 206.00 t and 12.49 m³/s respectively. The variation coefficient of annual sediment transport was 151.92%, which was much higher than that of annual runoff of 42.50%. The annual inner diameter flow and sediment transport were mainly concentrated from May to September, accounting for about 85% of the whole year. The inter-annual variation of runoff in the small watershed was large, and the inter-annual sediment transport fluctuated. The maximum runoff coefficient was 2.38 times of the minimum runoff coefficient, and the variation coefficient of annual sediment transport was greater than the annual runoff and annual rainfall. 2) The abrupt change points of runoff and sediment transport in small watersheds began to change from 2006 to 2007. The abrupt change points of sediment transport were more than that of runoff. The significant abrupt change points of runoff in small watersheds occurred in 2018, and the abrupt change points of sediment transport mainly occurred in 2006, 2012 and 2020. 3) The annual rainfall of small watershed had a greater impact on runoff (coefficient of determination 0.745 4) and less impact on annual sediment transport (coefficient of determination 0.365 8). The annual runoff, sediment transport and rainfall were positively correlated, indicating that the quality of monitoring data was valid. The inter-annual variation of rainfall and runoff in small watershed was small. The variation of runoff was affected not only by precipitation, but also by other factors. The ecological restoration project of soil and water conservation generally reduced the runoff and sediment yield of small watersheds.

Conclusions This paper studied the change of runoff and sediment characteristics and land use change under different time scales, analyzed the sediment status of inter-annual and annual inner diameter flow and fits the sediment correlation equation, which may provide scientific reference for the layout of soil and water conservation measures and the optimization and adjustment of land use type structure in small watershed.