强蒸发条件下农田土壤-水-盐相互作用机理研究

Soil salinity is one of the major constraints in the development of irrigated agriculture in arid and semiarid regions. Salt accumulated in soil surface or soil pores in the form efflorescence or sub-efflorescence due to the intensive evaporation in the arid region. Salt crystallization is highly affected by the pore structure and morphology of soil. On the contrary, evaporation-driven salt crystallization/precipitation strongly influences the pore structure and changes the soil physical properties such as the porosity, the permeability and the capillary pressure, thus affects soil water and salt dynamics. The purpose of the project is to investigate the interaction mechanism of soil, water and salt under the intensive evaporation from the pore scale. A series of evaporation experiments will be conducted in the laboratory under various conditions. Salt crystallization induced by evaporation of aqueous solution is studied from visualization experiments in capillary tube with different shapes. The precipitation kinetics and transport of ions is quantitatively analyzed and simulated during the evaporation process. X-ray computed tomography is applied to visualize the pore space and salt crystallization in the saline soil. An algorithm is developed to reconstruct three-dimensional pore structure of saline soil. The acquired tomography data are processed and quantitatively analyzed through integral geometry, mathematical morphological analysis, and pore network approaches. The interactions among the soil structure, water evaporation and salt crystallization are quantitatively analyzed. Mathematical models are developed from both pore scale and continuum approach to quantify the soil water and solute movement with the consideration of soil, water and salt interaction. Soil water and salt dynamics are simulated under different soil, salt, irrigation and drainage scenarios. On the basis of scenarios simulations, we will propose the appropriate strategies for control of soil water and salt in the arid and semiarid farmland.

农田土壤盐渍化已成为旱区农业可持续发展的关键制约因素，旱区强烈的蒸发条件导致土壤或者浅层地下水中的可溶性盐通过水分的垂直或侧向运动向表层土壤积聚，以结晶沉淀、结皮或结壳的形式聚积于土壤或地表，改变土壤孔隙结构和水动力学特征，从而影响土壤水盐过程。本研究从土壤-水-盐相互作用机理出发，通过室内精细试验和理论分析，应用X射线断层扫描等先进技术，着力从微观尺度研究强烈蒸发条件下土壤结构-水分蒸发-盐分结晶互馈机理；发展盐渍化土壤三维孔隙结构的重构和定量表征方法；构建考虑土壤-水-盐互馈关系的多尺度水盐过程模型，阐明强蒸发条件下农田土壤盐渍化的演变规律，量化农田水盐与土壤质地、盐分类型和含量以及灌排条件的关系，提出旱区农田控制盐分累积的水盐调控模式。

本项目围绕干旱半干旱区强蒸发条件引起的农田土壤盐渍化问题，通过室内外处理实验和理论分析，系统开展了强蒸发条件下土壤-水-盐相互作用微观机理、土壤三维孔隙结构的重构和定量表征、微观尺度土壤水盐过程量化模型等方面的研究。取得主要成果如下：（1）揭示了盐溶液浓度与离子组成、土壤结构对盐溶液蒸发与结晶过程的影响规律，阐明了不同盐分类型和浓度条件下，土壤蒸发速率与盐分浓度间的非线性关系，明晰了蒸发过程土壤表层盐分集聚特征及其对土壤蒸发速率的影响；（2）开发了基于二维CT数字图像处理技术的三维土壤孔隙结构分析软件；明晰了不同数字图像处理方法对多孔介质结构分析精度的影响；提出了颗粒形貌特征定量化表征方法，建立了刻画颗粒形貌特征的指标参数；发展了刻画多孔介质孔隙结构的定量方法，阐明了多孔介质水-热-溶质迁移动力学参数与孔隙结构拓扑空间变量的定量关系；（3）揭示了毛细管横截面形状、尺寸大小对水分蒸发和盐分结晶过程的影响，明晰了孔隙尺度上盐分自扩增结晶与爬行机制；（4）明晰了孔隙结构特征对其水分运动和溶质运移特性的影响，揭示了土壤微观结构-水分蒸发-盐分结晶之间的相互作用机理，阐明了盐分结晶与土壤水文过程的互馈机制；（5）发展了考虑土壤-水-盐-大气互馈关系的孔隙尺度水盐过程模型，实现了多孔介质盐溶液蒸发过程孔隙结构-水分蒸发-盐分结晶互馈机理的表征模拟；（6）阐明了不同有机改良剂对旱区农田的土壤结构和理化特征的影响，揭示了有机改良剂改善土壤结构和理化特征的作用机理。项目成果在国内外学术刊物上发表水平论文7篇；申请登记软件著作权1件；培养博士生3名，硕士研究生4名。