宁夏中部砂田土壤大孔隙特征及其对土壤水文过程的影响

Gravel-sand mulched field is a typical case of mulching gravel on soil surface to save water in northern China. Gravel-sand mulched field as a kind of method to prevent drought, mainly by artificially laying gravel to form a dry soil layer on soil surface to reduce evaporation and enhance infiltration.Gravel mulch not only has an influence on infiltration and evaporation by cutting off soil capillary, but also on the characteristic of soil macropore.Soil macropore is the pore that can offer path for preferential flow. Mulching gravel can enhance soil macropores and thus facilitates precipitation infiltration and the preferential flow in soil macropore and decreases runoff and evaporation. Therefore soil hydrological process and its functions were changed respectively. According to the weak research state and practical need, we take gravel-sand mulched field as object of study. By using dye tracer experiment in field and undisturbed soil column CT scanning experiment indoor, compare and analyze the influence of macropores characteristic and preferential flow caused by mulching gravel of different thickness and grain size; By using dye tracer experiment combined with soil profile photographing in field and image treatment experiment indoor, analyze the influence of soil macropores structure to preferential flow generating process; By using rainfall simulation experiment in field combined with soil column simulation experiment indoor, analyze the relationship among preferential flow infiltration and soil macropores structure and hydrological process, explore the mechanism of reducing evaporation and enhancing infiltration of mulching gravel. The above research result can offer theoretical proof and practical direction for raising water use efficiency, analyzing degradation level of gravel-sand mulched field, establishing rational irrigation technological parameters, defining suitable thickness and grain size of gravel mulch and developing water saving agriculture.

我国西北的砂田是利用砾石覆盖地表的典型节水范例。砾石覆盖对土壤物理特征和水文生态功能产生不可忽视的作用，尤其是对土壤大孔隙特征、渗透及蒸发等有重要影响。土壤大孔隙内优先流现象是砾石覆盖促进降水入渗、降低地表产流和抑制蒸发等土壤水文生态功能的关键。针对研究薄弱的现状和现实的需求，本项目以宁夏中部砂田为研究对象，通过田间染色示踪实验结合原状土柱CT扫描图像分析方法，对比分析不同粒径、厚度砾石覆盖对土壤大孔隙及优先流特征的影响；采用染色示踪剖面拍照结合图像分析方法分析土壤大孔隙结构对优先流发生过程的影响；通过田间降雨模拟实验结合室内土柱模拟实验，分析优先流入渗与土壤大孔隙形态结构、土壤水文过程的关系，探讨砾石覆盖对增加土壤入渗、抑制蒸发的机制，为提高砂田水分利用效率、分析砂田退化程度、制定合理农田灌溉技术参数、确定适宜的砾石覆盖粒径和厚度和发展节水农业提供理论依据和实践指导。

砂田是我国西北旱作节水农业的典范。随着对土壤结构及其水流运动研究不断深入，发现土壤水运动中存在优先流，大孔隙是其表现形式之一，是一种不遵循达西定律的运动形式。砾石覆盖对土壤物理特征和水文生态功能产生不可忽视的作用，尤其是对土壤大孔隙特征、渗透及蒸发等有重要影响。土壤大孔隙内优先流现象是砾石覆盖促进降水入渗、降低地表产流和抑制蒸发等土壤水文生态功能的关键。因此，本项目以宁夏中部砂田为研究对象，通过田间染色示踪实验结合原状土柱 CT 扫描图像分析方法，对比分析不同粒径、厚度砾石覆盖对土壤大孔隙及优先流特征的影响；采用染色示踪剖面拍照结合图像分析方法分析土壤大孔隙结构对优先流发生程的影响；通过田间降雨模拟实验结合室内土柱模拟实验，分析了优先流入渗与土壤大孔隙形态结构、土壤水文过程的关系。结果表明：1）不同厚度、粒径砾石覆盖均可显著增加土壤大孔隙的数量。但粒径越小，大孔隙的数量越多且分布深度越深；粒径越大，则表现为大孔隙分布集中于表层且数量相对较少；随着厚度增加，大孔隙数量增加趋势不是很显著。通过田间染色试验可以看出,砾石覆盖可显著增加土壤大孔隙。随着耕作年限延长，大孔隙的分布深度逐年加深，从而使染色面积增加。通过室内模拟试验可以得出，不同粒径砾石覆盖对土壤的蒸发都有抑制作用，粒径越大抑制作用越明显；对入渗有显著的促进作用，都可以增加累积入渗量；厚度越大对蒸发的抑制作用则越明显，对入渗的促进作用也越显著；同时，通过田间调查发现，由于耕作的影响，土石混合严重，导致大孔隙的数量减少，致使砂田的蓄水保墒、抑制蒸发、增加入渗的功能衰退。因此，有必要对老砂田进行土砂分离或者重新铺设。本项目的研究结果可为提高砂田水分利用效率、分析砂田退化程度、制定合理农田灌溉技术参数、确定适宜的砾石覆盖粒径和厚度和发展节水农业提供理论依据和实践指导。