土壤质地和滴头流量对地下滴灌土壤水分运移特征的影响

Influence of soil texture and drip emitter flow rate on soil water movement under subsurface drip irrigation

  • 摘要: 地下滴灌是一种局部灌溉方式, 灌溉水经由滴头在土壤中形成湿润体的过程对作物生长用水至关重要。根据土壤质地、滴头流量和土壤含水率等因素选择适宜的灌溉参数, 能够避免灌溉过量或不足。本研究以壤土、砂土和黏土3种质地土壤为研究对象, 设置地下滴灌滴头埋深为30 cm, 对0.39 L∙h−1、0.90 L∙h−1和1.38 L∙h−1流量下土壤湿润锋的变化规律进行观测。结果表明, 不同土壤类型含水量变化对不同滴头流量的响应存在差异, 壤土和砂土在高流量下更易形成规律的湿润体, 而黏土则受到滞水效应的影响, 在低流量时表现出明显的滞水特性。不同土壤质地对水分运移的影响也有所不同, 砂土中湿润锋水平和向下运移距离最大, 但向上运移距离为15~20 cm, 比壤土小, 运移能力较低; 黏土中水分运移所受阻力最大, 因此在各方向运移距离最短, 但变化持续时间最长; 壤土中水分运移距离在水平和向下方向上均处于居中水平, 向上运移距离最大, 说明壤土中水分向上的传输能力最强, 该土壤类型相对砂土和黏土更适合30 cm左右埋深的地下滴灌。对于同一质地而言, 砂土中水平和向下方向的湿润锋运移速率均随滴头流量的增加而增大, 但是向上方向运移速率为先减小后增大, 说明在1.38 L∙h−1的流量下, 流量对湿润锋运移速率的影响超过了向上的土壤阻力和重力的影响; 壤土中湿润锋在3个方向的运移速率均随滴头流量的增加而增大; 黏土水分湿润锋在水平和向下方向随流量在6%~10%之间变化, 相对其他土壤质地变化程度较小, 说明黏土水分湿润锋运移在这两个方向上受流量影响较小, 但其垂直向上方向运移速率随滴头流量增加先降低再增加, 且降低幅度远高于增加幅度, 说明较低流量更有利于黏土水分向上运移。在3种土壤质地和3种流量下, 随着入渗时间的延长, 湿润锋的运移速率总体呈逐渐减缓的趋势; 随滴灌流量的增加, 湿润锋在水平方向与垂直向上方向的运移距离之比相应增大, 说明流量增加相对于土壤水分向上运移来说更有利于促进其水平运移能力。此外, 湿润锋水平和垂直方向的运移速率与入渗时间遵循幂函数的规律。砂土垂直向上只运移到距地表10 cm左右处, 壤土在3种流量条件下垂直向上运移均达到地表, 黏土垂直向上只运移到距地表10.3 cm处。综上所述, 不同质地土壤进行地下滴灌时, 砂土的滴头埋深须在15 cm以内, 流量在0.39 L∙h−1左右可以保证作物苗期用水的同时减少渗漏; 对于壤土而言, 滴头埋深可增至30 cm, 流量需依据种植作物出苗时间、灌溉时间以及节水目标决定; 黏土滴灌不适合深埋和深根系植物。

     

    Abstract: Subsurface drip irrigation (SDI) has emerged as one of the most effective methods for achieving efficient water-saving cultivation in agriculture. Considering that SDI is a localized irrigation method, the formation of a wetting front in the soil created by water applied via drip emitters is crucial for effective crop water uptake. Selecting appropriate irrigation parameters based on soil texture, emitter discharge, soil moisture content, and other factors can help avoid over- or under-irrigation. However, previous studies have lacked systematic research on the combined effects of soil texture, emitter flow rate, and emitter depth. Therefore, this study focused on three soil textures — loam, sandy soil, and clay — using drip emitters at a depth of 30 cm, with three flow rates: 0.39 L∙h−1 (low flow rate), 0.90 L∙h−1 (medium flow rate), and 1.38 L∙h−1 (high flow rate), to observe changes in the soil wetting front. The results indicated that different soil types exhibited varying responses to changes in moisture content under different drip emitter flow rates. Loam and sandy soils tended to form regular wetting patterns at the high flow rate, whereas clay, because of its water retention properties, exhibited obvious water retention characteristics at the low flow rate. The influence of soil texture on water movement also varied. In sandy soil, the wetting front exhibited the greatest horizontal and downward migration distances, while the upward migration distance was approximately 15−20 cm, which was smaller than that in loam, indicating lower migration capacity. In clay, the resistance to water movement was the highest, resulting in the shortest migration distances in all directions, although the duration of water movement was the longest. In loam, the horizontal and downward migration distances were intermediate, whereas the upward migration distance was the greatest, indicating that loam had the strongest upward water transmission capability. This suggested that loam was more suitable for SDI with a burial depth of approximately 30 cm than sandy or clay soils. In sandy soil, the horizontal and downward migration rates of the wetting front increased with increasing emitter flow rate; however, the upward migration rate initially decreased and later increased. This indicated that at the flow rate of 1.38 L·h−1, the impact of flow rate on the wetting front migration rate surpassed the influence of upward soil resistance and gravity. In the loam, the migration rates of the wetting front in all three directions increased with the emitter flow rate. In clay, the horizontal and downward migration rates of the wetting front varied by 6%−10% with flow rate changes, showing a smaller change compared to other soil textures. This indicated that the wetting front migration in these two directions in the clay was less affected by the flow rate. However, the upward migration rate in clay initially decreased and then increased with the increase in emitter flow rate, with the decrease being much more pronounced than the increase. This suggested that lower flow rates were more conducive to upward water movement in the clay. As the drip irrigation flow rates increased, the ratio of the horizontal to upward migration distances of the wetting front increased accordingly. This indicated that increased flow rates were more favorable for enhancing the horizontal migration capacity than upward water movement. Additionally, the migration rates of the wetting front in both the horizontal and vertical directions followed a power function with infiltration time. In sandy and clay soils, the wetting front migrated vertically to 10.0 and 10.3 cm from the surface, respectively. In contrast, in the loam, the wetting front migrated vertically to the surface under all three flow conditions. In summary, for SDI, the emitter burial depth in sandy soil should be within 15 cm, with a flow rate of 0.39 L·h−1 to ensure water availability during the seedling stage and reduce leakage. For loam, the emitter burial depth should be increased to 30 cm, and the flow rate should be determined based on crop emergence time, irrigation timing, and water-saving goals. Drip irrigation in clay is not suitable for deep-buried and deep-rooted plants.

     

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