盐分及干湿循环对壤土蒸发特性与裂隙发育影响的机理探究

    Mechanism explorations of salt and drying-wetting cycle affecting evaporation and crack development of loam soil

    • 摘要: 为揭示盐渍土在干湿循环作用下的蒸发特性和裂隙发育特性,进而探究其内在影响机理,该研究设置5种盐度(0、0.3%、0.6%、1%、2%)的Na2SO4、CaCl2、NaCl型盐土,开展恒温蒸发试验,同时对Na2SO4型盐土进行干湿循环试验,结合数字图像处理技术对蒸发过程中土壤干缩裂缝网络几何形态特征进行定量分析,并进一步系统揭示水分蒸发与收缩开裂互作以及盐分(包括盐分类型和含盐量)和干湿循环对水分蒸发和收缩开裂的作用机制。结果表明:(1)不同处理蒸发过程具有相似性,即蒸发过程均包含一个线性阶段和一个非线性阶段。(2)盐分抑制了土壤水分蒸发,随着土壤含盐量的增加,抑制作用也不断增强;盐分类型和干湿循环能够改变土壤减速段的蒸发速率。(3)盐分类型对裂缝发展影响显著。对于Na2SO4型盐土,土壤盐分可以抑制表面裂缝的形成和发展,盐渍土的裂隙面积密度、裂隙总长度和平均宽度分别降低了4.5%~9.4%、0.01%~7.9%和10.5%~21.3%;而对于CaCl2和NaCl型盐土,随土壤含盐量的增加,裂隙面积密度分别增加了2.8%~5.5%和3.5%~8.3%,裂隙总长度分别增加了17.7%~35.0%和11.9%~36.9%,而裂隙平均宽度分别降低15.5%~22.1%和8.8%~21.5%。(4)干湿循环对裂隙指标的影响表现为低盐时抑制、高盐时促进,并且随着干湿循环次数的提高,这种影响也不断增强。机理分析表明,不同类型盐土的溶质势和晶体形态是影响土壤蒸发特性和裂隙发育的重要因素;钠质土壤相较钙质土壤具有更大的扩散双层,降低了土壤的抗拉强度;Na2SO4通过促进微团聚体间胶结和堵塞土壤孔隙等途径抑制地表开裂;干湿循环通过膨胀引起的裂纹愈合促进表面开裂。

       

      Abstract: In arid and semi-arid areas, soil shrinkage and swelling has become natural phenomena that manifest as changes in soil volume in response to water content, and finally the soils generate cracks. Desiccation cracks will also destroy the internal structure of the soil and provide preferential circulation channels for rainwater and irrigation water, causing many serious consequences to agricultural production, such as nutrient loss and groundwater pollution. Meanwhile, cracks typically occur in shrink-swell soils with high contents of clay minerals, which makes it more complex to predict the fluid transport in shrink-swell soils, since the porous medium turns into a variable-solid-skeleton soil and the cracks are typically randomly distributed. Specially, for the saline soils, salts may accumulate around cracks due to water loss at cracks, and the partial salt accumulation change hydraulic properties, destabilize soil particles, and destroy soil structure. Besides, the salts flow more easily into deep soil or groundwater through preferential channels formed by cracks. Therefore, the potential threats are more serious under the influences of cracks resulting from multiple drying and wetting (D-W) cycles under cyclical and regional changes in environmental conditions. In order to reveal the evaporation and crack development characteristics of saline soil under dry-wet cycles and to explore the intrinsic influence mechanism, this study conducted constant temperature soil evaporation experiments on the Na2SO4, CaCl2, and NaCl-type saline soils with five salt contents (0, 0.3%, 0.6%, 1%, and 2%), as well as dry-wet cycles experiments on the Na2SO4-type soils. Combining with digital image processing technology, the geometrical characteristics of soil drying and shrinkage crack network during evaporation were quantitatively analyzed, and the interactions between water evaporation and shrinkage cracking as well as the mechanisms of soil salt (including salt type and content) and dry-wet cycle were further systematically revealed. The results showed that (1) the evaporation process of different treatments is similar, that is, the evaporation process contains a linear stage and a nonlinear stage. (2) Soil salt inhibits the water evaporation, and the inhibition effect increases with the increase in soil salt content; the type of salt and the dry-wet cycles can change the evaporation rate of the soil deceleration section. (3) The salt type has significant influence on crack development. Soil salt inhibited the formation and development of surface cracks for the Na2SO4-type saline soil. The crack area density, total length of cracks, and average width of cracks of the saline soils decreased by 4.5%~9.4%, 0.01%~7.9%, and 10.5%~21.3%, compared to the non-saline soil, respectively. However, with the increase of soil salt content in CaCl2-type saline and NaCl-type saline soils, compared to the non-saline soil, the crack area density increased by 2.8%~5.5% and 3.5%~8.3%, and total length of cracks increased by 17.7%~35.0% and 11.9%~36.9%, while the average width of cracks decreased by 15.5%~22.1% and 8.8%~21.5%. (4) The effect of dry-wet cycles on crack indexes is inhibited at low salinity and promoted at high salinity, and this effect increases with the increase in the number of dry-wet cycles. The mechanism analysis showed that solute potential and crystal morphology of different types of saline soils are important factors affecting soil evaporation characteristics and crack development; Sodium soil has larger diffusion bilayer than calcium soil, which reduces the tensile strength of soil. The Na2SO4 in soil inhibits surface cracking by promoting cementation between microaggregates and plugging soil pores. The dry-wet cycle promotes surface cracking through the swelling-induced crack healing in the case of hydrophilic clay minerals in contact with water.

       

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