Objective This study aims to investigate the current status of runoff and sediment evolution in the Qiaozigou paired small watersheds of the “Hilly Third Area” on the Loess Plateau, and to better predict future runoff and sediment changes in this region, thereby providing guidance for soil and water conservation and management.

Methods Based on paired experiments, methods including comparative analysis, correlation analysis, and regression modeling for runoff and sediment prediction were employed to investigate the spatial changes in land use in the Qiaozi East Gully (managed) and Qiaozi West Gully (unmanaged) watersheds, as well as the variations in precipitation, runoff, and sediment over a 33-year period (1991−2023). By comparing model applicability, quadratic polynomial stepwise regression models for runoff and sediment were constructed at different scales.

Results 1) As of 2022, the management degree of the Qiaozi East Gully and Qiaozi West Gully watersheds was 92.65% and 88.07%, respectively. The land use structure of the Qiaozigou watershed showed a trend of gradual improvement. The effectiveness of the managed gully watershed was significantly higher than that of the unmanaged one, and the degree of management increased more during the period of 2001−2006. From 1991 to 2023, the precipitation amount and precipitation duration in the Qiaozigou watershed fluctuated but showed an overall increasing trend, with the average precipitation in the interannual and flood seasons being 508.99 mm and 415.31 mm, and the average precipitation duration being 93 d and 62 d. The proportion of precipitation amount during the flood season to annual precipitation amount showed an overall decreasing trend, while the proportion of precipitation duration during the flood season to annual precipitation duration showed an overall increasing trend, with 2014 marking the turning point for these proportions. The runoff volume and sediment load both increased over time, exhibiting a “single-peak” pattern, with the peak period occurring between 2003 and 2013. 2) The quadratic polynomial stepwise regression model could scientifically describe the relationship between sediment load in the Qiaozigou paired small watersheds and various factors of precipitation and runoff (R_i² ≥ 0.9449). It could make scientific predictions of the future changes of runoff and sediment in the region. 3) Under annual precipitation conditions, the main single factors influencing the annual sediment load in the Qiaozi East and West Gully watersheds were the annual runoff volume (x₃, x₄), with the interaction factors being annual precipitation amount × annual runoff volume (x₁x₃) and annual precipitation duration × annual runoff volume (x₂x₄), respectively. Under the conditions of precipitation during the flood season, the main single factors were still the annual runoff volume (x₃, x₄), with the interaction factors being annual runoff volume × precipitation duration during the flood season (x₃x₆, x₄x₆). The model validation of the annual sediment load showed good performance (R² ≥ 0.9431, SSE ≤ 0.2282), and correlations among runoff and sediment indicators were significant or highly significant, except for no significant correlation of the interannual precipitation duration in the Qiaozi East Gully watershed with the modulus of erosion and sediment load.

Conclusions This study can provide a reliable theoretical basis for predicting runoff and sediment evolution and for guiding soil and water conservation management in the “Hilly Third Area” of the Loess Plateau.