不同长度小麦秸秆覆盖下黄土耕地坡面流水动力学特性

Hydraulic characteristic of overland flow on loess farmland slope under mulch with different wheat straw lengths

为了系统研究秸秆长度对坡面流水动力学特性的影响,利用室内人工模拟冲刷定床阻力试验,在4个坡度、5个流量条件下分别对3~5、8~10和13~15 cm长度秸秆覆盖坡面流水动力学参数变化特征进行分析,阐明不同长度秸秆覆盖坡面水流阻力与雷诺数的关系。结果表明,秸秆长度对坡面流水动力学参数影响显著。不同长度秸秆覆盖条件下,坡面流雷诺数变化范围为166~558,当流量≤7.5 L/min时,坡面流态为层缓流;流量为9.0 L/min时,坡面流态为过渡缓流。秸秆覆盖条件下,坡面流具有较小的流速和较大的水深、Darcy-Weisbach阻力系数及曼宁糙率系数。随着秸秆长度增加,坡面流流速随之增加,而水深、Darcy-Weisbach阻力系数及曼宁糙率系数随之降低。随着秸秆长度和水力坡度增加,流态指数值总体呈现降低趋势。当秸秆长度由3~5 cm增加至13~15 cm时,流态指数平均由0.716降至0.501。研究可为秸秆覆盖条件下水土流失阻控机理研究提供科学依据。

Straw mulching is one of the important conservation tillage measures, and has a positive effect on reducing soil erosion on slopping farmland. In order to study the impact of straw length on the hydrodynamic characteristics of overland flow, indoor artificial fixed bed erosion resistance experiments were carried in the Soil and Water Conservation Engineering Laboratory in Northwest A＆F University. The study area was located in the southern Loess Plateau, Shaanxi Province, China （34~14＇-34~20＇N, 107~59＇-108~08＇E）. By the scour trough, the variations of hydrodynamic parameters of overland flow such as flow velocity, runoff depth, Reynolds number, Froude number, Darcy-Weisbach resistance coefficient and Manning roughness coefficient were determined at 4 slope gradients （3~, 5~, 10~ and 15~）, 5 flow rates （3.0, 4.5, 6.0, 7.5 and 9.0 L/rain） and 3 wheat straw lengths （3-5, 8-10 and 13-15 cm）. The slope without straw mulch was considered as the control （CK）. The results showed that under the experimental condition, the wheat straw length greatly affected the hydrodynamic parameters. Under straw mulching with different straw lengths, Reynolds number ranged from 166 to 558, the overland flow was primarily laminar and subcritical flow when the flow rate was less than or equal to 7.5 L/min, and transformed into transition and subcritical flow while the flow rate was 9.0 L/min. Under the conditions of straw mulching, compared with bare slope, the overland flow ran with smaller flow velocity and higher runoff depth, Darcy-Weisbach resistance coefficient and Manning roughness coefficient. Straw length affected the runoff depth. Compared with the bare slope, the runoff depth of overland flow on slope with the straw length was increased by 107.69%-178.64%. Straw mulching could decrease the flow velocity by 48.30%-60.64% than the CK. The runoff depth decreased as the straw length increased, and the flow velocity increased with the increase in straw length. The flow state index of overflow with straw mulching was reduced with increased of straw length and slope gradient. When the straw length increased from 3-5 cm to 13-15 cm, the average flow state index was decreased from 0.716 to 0.501. The Darcy-Weisbach resistance coefficient was 0.14-1.47 on bare slope, 2.67-8.00 on slope with 3-5 cm straw mulch, 2.39-6.36 on slope with 8-10 cm straw mulch and 1.31-4.28 on slope with 13-15 cm straw mulch, respectively. It averagely increased by 20.89, 15.69 and 8.17 times respectively compared with the bare slope, indicating that the straw mulch could strongly resist the flow movement. Darcy-Weisbach resistance coefficient was decreased with the increase in straw length. The Manning roughness coefficient was 0.013-0.044 on the bare slope, 0.071-0.132 on slope with 3-5 cm straw mulch, 0.067-0.118 on slope with 8-10 cm straw mulch and 0.049-0.083 on slope with 13-15 cm straw mulch, respectively. It increased by 4.38, 3.64 and 2.33 times respectively compared with the bare slope. Manning roughness coefficient was reduced with straw length increased. This study could provide reliable information for clarifying the mechanism behind soil erosion with straw mulching on farmland slope.