基于土地休耕与粮食安全的华北平原冬小麦空间优化布局及其水足迹效应评估

The North China Plain (NCP) has suffered severe groundwater over-exploitation problems, and the large amount of irrigation water consumed by winter wheat is known as the major cause. The land fallow policy was adopted by the central government to encourage the abandonment of winter wheat, for the purpose of inducing protection and recovery of groundwater environment. However, the NCP is also the major winter wheat producing area of China, with a function of guaranteeing food security. The optimal allocation of winter wheat to the southern part of the NCP, where the water use of winter wheat mainly originates from natural precipitation and the pressure on the environment is much lower than that in the northern part, conforms to the ongoing trend of agricultural land use change, since evidences of winter wheat expansion were spotted in southern part of the NCP, and will be a reasonable choice to fulfill the win-win target between effective groundwater management and stable wheat supply. To what extent can winter wheat potentially expand in the NCP? How to optimally allocate winter wheat across the NCP? How will crop water footprint change after the spatially optimal allocation of winter wheat in the NCP? To answer the above questions, we will monitor the changes of agricultural cropping structure in the NCP using remote sensing technology, pinpoint the major alternative crops for winter wheat and the potential areas for winter wheat expansion, and select typical areas for the specification of driving factors and their mechanisms of households’ land use decisions. Then, we will construct a spatially explicit agricultural land use model, to optimally allocate winter wheat across the NCP under the constraints of land abandonment and food security and to calculate the compensation values for winter wheat expansion areas, and estimate crop water footprint and its spatial changes using China-AEZ model. We will also provide scientific suggestions for policies relevant to adjustment of agricultural cropping structure, guarantee of food security and protection of water resource in the NCP.

华北平原地下水超采问题严重，冬小麦灌溉用水量大是其主要原因。近期，国家出台土地休耕政策，在华北地下水超采区退耕冬小麦，以保护和修复地下水环境。然而，华北平原是我国的小麦主产区，承担着保障国家粮食安全的重任。考虑到平原南部地区冬小麦耗水多源于降水，对地下水环境的压力远小于北部，且已出现冬小麦扩张现象，土地休耕背景下冬小麦向南部布局顺应了农地利用变化趋势，是实现地下水超采治理和粮食安全双赢目标的合理选择。华北平原冬小麦潜在扩张范围有多大？如何进行空间优化布局？优化布局前后作物水足迹如何变化？本项目拟遥感监测华北平原种植结构变化，识别冬小麦主要替代作物及其潜在扩张范围，研究典型区农户土地利用决策影响因素及作用机理，依此构建空间详尽化农地利用决策模型，进行冬小麦空间优化布局和扩张区农业补偿标准核算，并评估优化布局前后作物水足迹及其变化，为平原种植结构调整、粮食安全保障、水环境保护等提供政策建议。

本项目经过三年研究工作，取得的主要研究进展有：（1）利用多时相Landsat影像和决策树分类算法，提取了华北平原农业种植结构现状，并结合大学生问卷调查数据和县级农作物统计数据，识别了华北平原冬小麦替代种植区域差异及其潜在扩张范围；（2）基于案例区农户问卷调查数据，构建农户土地利用决策理论模型并结合多层次模型，揭示了华北平原农户种植制度决策的关键影像因素及其作用机理；（3）在上述研究基础上，通过对应变量或替代变量，以公里网格为基本单元，构建空间详尽化农地利用决策模拟模型，制定了不同情景华北平原冬小麦空间优化布局方案，并确定了相关补偿标准；（4）借助水足迹理论和China-AEZ模型，核算了华北平原冬小麦优化布局前后冬小麦水足迹，包括绿水足迹、蓝水-地下水足迹和蓝水-地表水足迹及其时空变化。.研究的主要结论有：（1）2000年以来华北平原冬小麦播种面积呈现“南增北减”变化格局，冬小麦-夏玉米一年两熟制和春玉米一年一熟制成为2016年华北平原的主要种植制度；灌溉条件下华北平原南部冬小麦潜在扩张范围散布在山东半岛丘陵区、豫东平原、苏北和皖北平原，雨养条件下则主要集中在34°N以南地区。（2）案例区农户土地利用决策受地块、农户、村庄三种层次因素的共同影响，以地块层次解释程度最高，关键解释变量包括土地质量、灌溉条件等，上述因素通过影响土地的投入或产出部分，影响边际土地生产率或边际劳动生产率，进而影响农户土地利用决策。（3）空间详尽化农地利用决策模型模拟效果与微观农户理论和实证分析结果一致；基于该模型核算的华北平原冬小麦潜在扩张区两熟制发生概率结果显示河南、江苏和安徽省内的冬小麦潜在扩张区两熟制发生概率普遍较高，建议冬小麦优先布局在上述区域，补偿标准采用机会成本法计算。（4）华北平原冬小麦优化布局前后作物总水足迹及各组分均显著降低，冬小麦扩张区总水足迹及各组分变化较小，可见冬小麦空间优化布局在基本不影响南部地区水资源利用的情景下，实现了华北平原水资源保护和粮食安全的“双赢”。