Objective  Aeolian sand-covered loess slopes are a special geomorphic landscape with a unique erosion pattern formed by multi-dynamic forces within the wind-water erosion crisscross region of the Loess Plateau. Objectives of this study are to investigate the response of runoff and sediment production processes to slope lengths and thickness of the sand covers on the aeolian sand-covered loess slopes, which can provide essential explanation for preventing and predicting soil erosion in this region.

Method The quantitative analysis is based on observations of runoff and sediment productions in indoor simulated rainfall experiments with the slope length (between 1 m and 3 m) thickness of sand covers (2 cm, 5 cm, and 10 cm). The effects of slope length and thickness of sand covers are analysed against a control group without sand covers.

Result (1) Compared to the loess slope without a sand cover, the time to runoff generation on the sand-covered slope is significantly extended by 3 to 30.72 times, the average runoff rate is reduced by 21% to 84%, the average sediment yield rate is increased by 2.99 to 10.66 times, and the sediment concentration is increased by 3.38 to 18.07 times, all of which are intensified as the thickness of the sand cover increases. (2) The 1-m slope with a 10-cm sand layer exhibits a significantly effect on reducing the runoff rate, while the average runoff rate with a 3-m slope demonstrates minor variations among different thickness of sand covers. Whether covered by sands or not, the average sediment yield rate and sediment concentration from the 3-m slope are significantly higher than those from the 1-m slope. (3) The increases in the slope length and thickness of the sand covers enhance the variability of instantaneous runoff and sediment yields during rainfall events. The instantaneous runoff rate of the 1-m slope without sand covers is found to be higher than that with a sand cover during rainfall. Notably, both runoff and sediment yields from 3-m slopes with a thicker sand cover show a distinct peak, and some instantaneous runoff coefficients exceeded 1 during the rainfall events. (4) The structural equation model reveals that the slope length has the greatest influence on runoff rate (path coefficient = 0.65), and the sand thickness had the greatest influence on the sediment yield rate (path coefficient = 0.71). The slope length exhibits an indirect positive effect (path coefficient = 0.40) on the sediment yield through runoff production.

Conclusion The slope length increases both runoff and sediment yield rates, while the thickness of sand covers reduces the runoff rate and increases sediment yield rate. The synergy of the slope length and thickness of sand covers enhances the variability of runoff and sediment production processes, which makes the runoff and sediment production more changeable during rainfall.