Effect of wheat straw mulch on slope hydrodynamic characteristics

Straw mulching has been one of the important measures of conservation tillage, particularly for the soil and water conservation. This study aims to systematically investigate the effect of straw coverage on the hydrodynamic characteristics of overland flow. A series of the indoor fixed-bed simulation experiments were carried in the Soil and Water Conservation Engineering Laboratory in Northwest A&F University, located in the southern Loess Plateau, Shaanxi Province, China (34°14′N-34°20′N, 107°59′E-108°08′E). The experimental conditions of five flow rates (3.0, 4.5, 6.0, 7.5, and 9.0 L/min), and four slope gradients (3°, 5°, 10°, and 15°). An analysis was made to determine the variations in the hydrodynamic parameters of bare slope and different straw cover (0, 1.5, 2.5, 3.5, and 4.5 t/hm2). Hydrodynamic parameters included the flow velocity, Reynolds number, Froude number, and Darcy-Weisbach resistance coefficient. The systematic investigation was also implemented to clarify the effects of straw mulch on the flow pattern, flow regime, and resistance characteristics of overland flow. The results showed that: 1) Straw mulching affects the flow pattern and flow regime of overland flow. Specifically, the flow pattern of overland flow was the rapid flow without straw coverage under different flow rate and slope. But, the tranquil flow was found under straw mulching. One the flow rate was ≤7.5 L/min, the flow pattern was the laminar flow. By contrast, when the flow rate was >7.5 L/min, the flow pattern was the transition flow. 2) The flow velocity showed a decreasing trend as a whole under the same flow rate and slope, with the increase of straw coverage. The flow velocity of slope flow decreased by 47.85%, 53.86%, 57.69%, and 60.11%, respectively. 3) The slope significantly dominated the flow index. The inconsistent change was observed in the flow index with the slope under different straw coverage. When the coverage was ≤2.5 t/hm2, the flow index increased first and then decreased with the increase of slope. When the coverage was >2.5 t/hm2, the flow index decreased gradually with the increase of slope. The flow index of bare slope was ranged from 0.086 to 0.178, whereas, the flow index of straw mulching slope was ranged from 0.247 to 0.766. 4) The Darcy-Weisbach resistance coefficient was negatively correlated with the Reynolds number under the bare slope. By contrast, there was the positive correlation with the Darcy-Weisbach resistance coefficient and Reynolds number under the straw mulching. The Darcy-Weisbach resistance coefficient presented a significant power function relationship with Reynolds number, with the bare slope and straw coverage of 1.5 t/hm2. The Darcy-Weisbach resistance coefficient of straw covering slope was 9.36-19.68 times higher than that of bare slope.Under different coverage conditions, the variation of Darcy resistance coefficient with slope is different. Furthermore, the Darcy-Weisbach resistance coefficient reached the maximum at 10° slope under the low coverage (≤2.5 t/hm2). In the high coverage (≥3.5 t/hm2), the Darcy-Weisbach resistance coefficient increased with the increase of slope. The Darcy-Weisbach resistance coefficient shared a power function relationship with the Froude number. This finding can provide a scientific basis for the dynamic mechanism of the slope water erosion process under different straw coverages.