多重驱动条件下河套灌区土壤水盐时空动态平衡机理研究

The soil salinization has the trend of gradually increased and become a serious threaten to sustainable agriculture due to its special hydrogeology conditions and long-term inconsequence irrigation management in Hetao Irrigation District, Inner Mongolia. The formation of closed-form aquifer cause no pathway for underground runoff, strong evaporation, over irrigated land field add together lead to serious salinity accumulation problem in this area. Numerous existing research on soil salinity in Hetao Irrigation District is based on a single period, e.g., spring irrigation, irrigation during crop growth stage, autumn irrigation and freezing-thawing. And the exchange of salinity between vadose zone and shallow groundwater is not taken into consideration. It is difficult to describe the equilibrium mechanism of salt regardless of the continuity in spatial and temporal. The objectives of this research is to quantify the amount of salt and water from groundwater evaporation, irrigation management, seepage, transpiration through precise laboratory experiments and fields trails by taking all different transport period into consideration during a whole year without neglecting the vertical mitigation of brackish water by density driven in the shallow groundwater zone. The mechanism such as density driven convection, sedimentation, mixture, dispersion, exchange between vadose zone and saturated zone will be carefully analyzed. A new computer model coupled with soil water, salt and heat transport, crop growth and density driven transport of saline water in shallow groundwater together will be developed to simulate the water and salt dynamic in saturated and unsaturated zone. A distributed model with the support of hydrogeology data and irrigation management practices will also be developed on the purpose of long term irrigation water management and salinity control at a regional scale. The outputs of this research will also provide practical guidance for salinity control and theoretical basis for soil salinization regulation in Hetao irrigation area.

内蒙河套灌区受封闭的地下含水层构造和长期不合理的灌溉管理的影响，土壤盐渍化现象有逐渐加重的趋势，对灌区农作物生长和粮食安全构成了严重的威胁。现有的研究多集中在年内单个时期（春汇、生育、秋浇、冻融）、垂向不连续（仅包气带）条件下的土壤水盐运动规律，很难描述盐分在时空上的动态平衡过程。本研究拟从灌区土壤年内不同时期、不同区域（包气带-过渡区-潜水区）及水盐运动的多重驱动条件出发，通过精量的室内和田间实验，定量解析潜水蒸发、灌溉、渗漏及作物腾发等土壤水盐平衡的关键因素，量化密度驱动条件下潜水区咸淡水融和（沉降、弥散）规律；发展考虑水-盐-热过程、作物生长及潜水区密度驱动的土壤水盐动态耦合模型，开发基于灌区水文地质特征和灌溉管理构建分布式模型，探讨多重驱动条件下河套灌区区域土壤盐分的长期演变规律。研究成果可为河套灌区土壤盐渍化调控提供理论依据和科学指导。

内蒙河套灌区受封闭的地下含水层构造和长期不合理的灌溉管理的影响，土壤盐渍化程度有逐渐加重的趋势，对灌区农作物生长和粮食安全造成了潜在的严重威胁。为了阐明河套灌区土壤年际内不同时期、不同区域（包气带-过渡区-潜水区）水盐运动规律及多重驱动条件对盐分运动的影响机理,本项目以精量的室内和田间实验为基础，系统开展了饱和介质基于密度驱动的微咸水融和规律、大气驱动条件下灌区表土盐分累积规律、不同灌溉条件下土壤根区全周年水盐动态规律、灌区盐分动态平衡模拟等内容研究，主要的研究内容和取得的成果如下：（1）饱和介质基于密度驱动的微咸水融和规律表明：盐分在密度差驱动下不断向深层运移，并最终聚集在不透水底板处；表层含盐量越高，盐分密度驱动现象越明显；（2）大气驱动条件下灌区表土盐分累积规律表明：表土盐分累积与地下水水位间联动规律明显，潜水蒸发随地下水埋深的增加逐渐降低，日均蒸发量变化范围为0.3～2.1mm，潜水蒸发量越大则盐分累积越明显；土壤冻结过程中土壤含水率和盐分均呈增加趋势，荒地0-10 cm层土壤盐分增加最大，高达2倍；农田、树林和荒地0-100 cm土层的土壤盐分含量分别提高了24.2%、10.0%和68.7%；（3）当地下水埋深低于0.6m时，作物根区土壤含水率基本处于饱和状态，不利于作物生长而形成减产，秋浇脱盐率随地下水埋深的增加而呈现逐渐递减的趋势，脱盐率范围在28.2%-42.6%之间；当地下水埋深处于1.5-1.7m之间时，春玉米的作物水分利用效率可达到最大值；（4）将土壤水盐动态模型HYDRUS-EPIC与潜水区密度驱动咸水动态模型SEAWAT模型耦合，耦合模型对于不同密度差作用下盐分分布过程有较好的模拟效果，高矿化度水平且隔水底板埋深越深的情况下，盐分向深层运移效果越明显，拟合度越高，通过模拟研究阐明了潜水蒸发及灌溉对土壤盐分累积贡献及潜水区对盐分的接纳关系；利用集合卡尔曼滤波水盐数据同化方法，获得了长期节水灌溉条件下盐分的时空演变规律。