Response of runoff and sediment production on sand-covered loess slopes to slope length and sand covering thickness

Objective Aeolian sand-covered loess slope is 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 length and thickness of sand covering 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 was based on observations of runoff and sediment production in indoor simulated rainfall experiments with the slope length (between 1 and 3 m) and thickness of sand covering (2, 5 and 10 cm). The effects of slope length and thickness of sand covering were analysed against a control group without sand covering.

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

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