再生水灌溉对亚热带典型土壤干缩裂缝演变特征的影响

为揭示再生水灌溉对亚热带土壤干缩裂缝及其发育过程的影响,该研究选取红壤、潮土、紫色土、水稻土作为供试土壤,采用再生水原液(RW)及稀释2倍(RW-2)、4倍(RW-4)、6倍(RW-6)4种不同浓度再生水进行持续模拟灌溉,并进行脱湿开裂试验,提取干缩裂缝参数。结果表明:1)再生水灌溉抑制红壤及促进其余3种土壤干缩裂缝形成与发育,除RW-4处理促进作用存在异常外,其余处理呈再生水浓度增高,抑制或促进作用增大;2)再生水灌溉下,红壤干缩开裂过程更为平缓,其余3种土壤干缩裂缝面积发育更集中于前期,低浓度再生水集中作用更大;3)土壤类型与再生水浓度对干缩裂缝具有交互作用,土壤类型对面积密度及其发育过程减缓段变化速率影响较大,再生水浓度对面积发育过程初始段变化速率、减缓段长度、加速段变化速率、长度发育过程变化速率峰值影响较大;4)土壤裂缝面积密度发育程度与含水率的关系可用三直线模型拟合,长度密度及连通性系数发育程度与含水率的关系可用log-logistic模型拟合。研究结果可为亚热带地区再生水水质标准制定及其灌溉制度设计提供参考。

Effects of reclaimed water irrigation on the evolution characteristics of desiccation crack of typical subtropical zone soils

The requirement for water resources is ever-increasing with the rapid development of the economy in China. Reclaimed water can be used to greatly alleviate the contradiction between supply and demand for fresh water. The unconventional water sources can also contribute to the structure of water supply, water pollution and ecological security in sustainable agriculture. But, long-term irrigation with reclaimed water may cause the change of soil physical and chemical properties, and then result in the development of desiccation cracks. This study aims to reveal the effects of reclaimed water irrigation on soil desiccation crack and the developmental process in the subtropical region. Four types of subtropical soils were selected as the test materials, including the red, Fluvo-aquic, purple, and paddy soil. Four concentrations of reclaimed water were also used for continuous irrigation, including original reclaimed water (RW) and diluted concentrations of 2(RW-2), 4(RW-4), 6(RW-6) times. An evaporation test was then carried out, where the digital image processing was combined to extract the parameters of soil desiccation cracks. The result showed that the reclaimed water inhibited the formation and development of desiccation cracks in the red soil. The inhibitory effect became more obvious, as the concentration of reclaimed water increased. Specifically, the water content of the initial crack, the area density, and the length density of the desiccation crack dramatically decreased by 57.9%, 54.3%, and 35.4%, respectively. By contrast, the reclaimed water promoted the formation and development of desiccation crack in the Fluvo-aquic, purple, and paddy soil. There were most promoting effects on desiccation crack areas of Fluvo-aquic soil, and desiccation crack length and connectivity of the purple soil, whereas, the least promoting effect was found in the paddy soil. Furthermore, the promoting effect became much stronger, as the concentration of reclaimed water increased, except in the reclaimed water-diluted concentration of 4 times. In addition, there was a balanced process of desiccation crack in the red soil, whereas, that concentrated in the early stage of Fluvo-aquic, purple, and paddy soil, indicating obvious concentration effect of reclaimed water with low concentration. It was also found that there were great influences of soil type on the developmental rate of crack area density in the initial stage, particularly on the developmental rate of connectivity index. Similarly, the concentration of reclaimed water presented a great influence on the length in the deceleration stage, the developmental rate in the accelerated and initial stage, and the maximum developmental rate of crack length density. A three-linear model was utilized to fit the relationship between the developmental degree of soil crack area density, and water content under reclaimed water irrigation. The water content of the initial, accelerated, and decelerated stages were 22.67% to the water content of initial crack, 17.20%-22.67%, and 0-17.20%, respectively, while the developmental rates of each stage were 0.017, 0.037, and 0.020, respectively. A four-parameter log-logistic equation was selected to fit the relationship between the developmental degree of length density and connectivity, as well as the water content. The water content of the accelerated and decelerated stages during the crack length development were 28.06% to the water content of initial crack and 0-28.06%, respectively, where the maximum developmental rate was 4.56, while the water content of the accelerated and decelerated stages during the development of crack connectivity were 26.27% to the water content of initial crack and 0-26.27%, respectively, where the maximum developmental rate was 4.50. The finding can provide a sound reference to develop the quality standards of reclaimed water for irrigation systems in the subtropical zone.