干湿循环下裂缝网络对农田入渗的影响机理及精量灌溉制度

Upon wetting-drying effects, farmland soil results in reduplicative expansions and shrinkages and ultimately complicated crack networks come into being in soil space. The presence of crack network exerts an influence on unsaturated infiltration, irrigation efficiency and the water quality of vadose zone. The research project aims (1) to reveal the dynamic variation of soil crack network during wetting-drying cycles and to construct the cracking-closure model in micro scale and dynamic variation model of crack network according to the indoor soil cracking-shrinking experiments and outdoor crack monitoring experiments; (2) to detect the effects of crack network on unsaturated infiltration law and preferential flow distribution during wetting-drying cycles, according to PVC soil column and field experiments; (3) to construct crack preferential flow multiple-domain model consisting of matrix domain, dynamic crack domain and permanent crack domain, according to the flow distribution experiments; (4) to establish the irrigation schedule based on the soil cracking-closing mechanism, infiltration model in crack network and crack preferential flow model. The research results will further reveal the unsaturated infiltration law in soils during wetting-drying cycles and provide theoretical basis for increase of irrigation efficiency and water pollution prevention in vadose zone and groundwater.

农田土壤在干湿循环下反复收缩膨胀形成错综复杂的裂缝网络，这种裂缝结构对土壤非饱和入渗、灌溉水利用效率及包气带水源质量产生重要影响。本项目旨在(1)通过室内裂缝开闭模拟与室外裂缝监测实验，揭示干湿循环效应下土壤裂缝网络动态变化规律，建立单裂缝微观开裂闭合模型与裂缝网络动态变化模型；(2)结合PVC土柱和田间尺度的干湿循环实验，揭示干湿效应下裂缝网络对非饱和入渗规律和优先流运移分布的影响，根据实验入渗结果推求基于裂缝空间网络及干湿循环级数的非饱和入渗公式；(3)同步依据优先流运移分布试验，构建由基质区域、裂缝增湿闭合区域和永久裂缝区域构成的裂缝优先流多域模型；(4)以裂缝开闭机理、非饱和入渗规律及裂缝流运移模型为基础，构建干湿循环效应下控制无效入渗的精量灌溉制度。研究成果将进一步揭示干湿循环土壤非饱和入渗基本规律，为提高灌溉效率、防治包气带和地下水污染等提供理论依据。

农田土壤在干湿循环下产生裂隙网络，为水分、溶质及污染物迁移提供优先通道，造成灌水效率降低和地下水污染。本项目在室内及田间开展了裂隙开闭模拟和裂隙流试验，分析了干湿循环下裂隙开闭及裂隙流机理，构建了裂隙率预测模型和优先流多域模型，提出了裂隙影响下的最优灌水模式。研究成果如下：(1)裂隙几何特征在干湿循环内存在水力滞后现象，裂隙率在4级循环后渐趋稳定；裂隙形态在干湿循环间存在较高相似率，重现度达62.9%至81.5%；裂隙动态发育速率随干湿级数提升3倍；(2)采用CT技术量化了干湿循环中裂隙演化，揭示了初始含水率及干湿级数对裂隙深度记忆区影响；(3)根系及盐分对裂隙形态影响表明，根系抑制裂隙发育，增加了裂隙弯曲度；盐分增加了裂隙面积率和网络连通性，促进开裂能量释放过程；(4)根据孔隙转化原理构建了基于收缩特征曲线和几何因子的裂隙率预测模型，有效模拟了裂隙率变化；(5)裂隙流吸渗试验表明，裂隙形成的毛管断裂效应致使入渗率初期存在振动现象，振幅随干湿级数增大，根系添加后振幅显著减小；分析湿润锋弯曲度可知裂隙闭合使裂隙流向基质流转变；(6)基于干湿循环入渗试验，构建了引入初始含水率、裂隙体积和干湿级数的修正Horton入渗模型，模型精度较高(R2>0.95)；(7)田间染色试验表明裂隙流模式由基质流(<10cm)和优先流(>10cm)构成，裂隙流呈现出倒楔形，裂隙内水流呈层流沿边壁下移并伴有水平吸渗；(8)研究了地面灌(SU)、微喷灌(MSP)和滴灌(DR)下裂隙愈合性、裂隙流模式和灌水质量，其愈合性由大到小为SU>MSP>DR，灌水质量由高到低为MSP>SU>DR；(9)推演了变形土水分运移方程，构建了考虑裂隙湿润闭合的裂隙流多域模型，该模型RMSE误差较恒固相双渗透、变固相双渗透、恒固相离散双域和恒固相裂隙流双域模型降低了83.7%、85.3%、85.8%和72.4%；(10)分析了初始含水率θ(%)和灌水强度R(cm/min)旋转组合下的裂隙流指数和灌水质量，表明灌水质量在(R,θ)组合值下由(0.02,0.35)至(0.1,0.2)斜向呈梯度由0.92至0.57减小，受到灌水强度更为明显，提出了考虑优先流指数和灌水质量的最优灌水模式。研究成果揭示了干湿循环下裂隙干湿开闭及其优先流理论，为农田土壤水管理与灌溉决策提供理论依据。