极端降雨条件下秸秆覆盖坡面水流流速空间分布

Spatial distribution of overland flow velocity along straw-mulched slope under extreme rainfall

坡面流流速是地表水文和水土保持研究的重要参数。极端降雨条件下,秸秆覆盖坡面水动力过程复杂,探明其流速分布规律对坡耕地土壤侵蚀预报具有重要意义。该研究的模拟降雨试验在黄土高原土壤侵蚀与旱地农业国家重点实验室降雨大厅内进行。试验土槽宽0.25 m,长8 m,深0.3 m,底部填装黏壤土(31.8%黏粒,43.3%粉粒,24.8%砂粒),土壤表面均匀铺设长5~7 cm小麦秸秆。试验雨强为160 mm/h^(2),共设置3种坡度(5°、10°和20°)和3种秸秆覆盖率(0、2和8 t/hm^(2)),分别在沿着坡长方向距坡顶1.15、3.15、4.15和5.15 m处设置采集点;采用电解质示踪法测量坡面水流流速,分析秸秆覆盖率对坡面流流速及其空间分布规律的影响。结果表明,极端降雨条件下质心法适用于秸秆覆盖坡面流速测量,秸秆覆盖下测量流速为0.017~0.117 m/s;秸秆覆盖可削弱雨滴对坡面流的扰动,增大下垫面的粗糙度,流速随覆盖率的增大而减小。流速随坡度与坡长的增加而增大;沿坡长方向流速变化率逐渐减小,流速最终趋于稳定;由于秸秆对水流的阻碍作用,高秸秆覆盖率坡面达到最大稳定流速的距离减小。秸秆覆盖坡面与裸坡面的水流流速存在显著的线性关系。秸秆覆盖影响坡面流流态,层流的临界雷诺数取值应小于500。研究结果对理解极端降雨条件下覆盖坡面土壤侵蚀机理具有重要意义。

The velocity of overland flow is one of the important parameters to simulate the hydrological processes and soil erosion on sloping farmlands.In this study,a solute centroid model was selected to determine the velocity distribution,according to the salt transport of overland flow,in order to clarify the erosion mechanism on the straw mulching slope under extreme rainfall.A systematic experiment was conducted under an artificial rainfall intensity of 160 mm/h at the State Key Laboratory of Soil Erosion and Dryland Agriculture on the Loess Plateau.The velocity measurements were performed at the slope lengths of 1.15,3.15,4.15,and 5.15 m under the slope gradients of 5°,10°,and 20°,and the mulching rate of 2,4 and 8 t/hm^(2).The experimental soil flume was in the size of 0.25 m wide,8 m long,and 0.3 m deep,which was filled with the clay loam that composed of 31.8%clay,43.3%silt,and 24.8%sand,and sampled from Yangling,China(34°16′50″N,108°3′13″E).The wheat straw was air-dried and chopped to 5-7 cm before being laid over the surface soil.A downspout rain simulator was also selected,where the rainfall uniformity was 85%,and the rainfall height was 18 m.A flow velocity was determined using an electrolyte centroid.After that,some hydraulic parameters were calculated,including the unit width discharge,water depth,Reynolds coefficient,Froude number,and Darcy-Weisbach resistance coefficient.The measurements showed that the mean velocity was ranged in 0.071-0.352 and 0.017-0.117 m/s at the mulching rate of 0 and 8 t/hm^(2),respectively.There was a smooth curve of electrolyte transport under the condition of low straw coverage rate(2 t/hm^(2)),whereas,no smooth curve without the straw mulching treatment.The smoothness of electrolyte transport under the 100%straw coverage was between those of no and low straw coverage.The straw mulching greatly weakened the disturbance of raindrops on the overland flow,but increased the roughness of the eroded underlying surface,leading to a lower average flow velocity.Once the mulching rate reached a critical point,there was an intensive disturbance of mulching straw on the overland flow.The distance to the maximum velocity of overland flow on the slopes with a high mulching rate was much shorter than that with a low mulching rate.In addition,the mean flow velocity was ranged in 0.017-0.352,0.027-0.323,and 0.033-0.272 m/s under the slope gradients of 5°,10°,and 20°,respectively.The flow velocity increased significantly,with the increase of slope gradient and length.But the change rate of flow velocity decreased,with the increase in the slope length,until the velocity gradually tended to be stable.There was a strong linear relationship of flow velocities between the straw mulching and bare slope,where the proportional coefficient was 0.285.The flow regime of overland flow depended mainly on the mulching straw and the critical value of Reynolds number,which was less than 500 in the laminar flow.Once the overland flow was stable,the flow rate and velocity of overland flow were stable without changes with time.The water depth of the bare soil slope varied from 0.8 to 1.4 mm,whereas,the water depth of the covered slope varied from 2.0 to 5.1 mm.Consequently,the straw mulch can be widely expected to obstruct the water flow,further reducing the flow velocity for the low water level,compared with the bare soil slope.The disturbance effect of straw mulch on the water flow was also strengthened in the downstream section of the slope,with the increase of catchment areas.The finding can greatly contribute to clarifying the effects of soil erosion on the straw-mulched slope under the conditions of extreme rainfall.