Data from: Feedbacks between shallow groundwater dynamics and surface topography on runoff generation in flat fields

In winter, saturation excess (SE) ponding is observed regularly in temperate lowland regions. Surface runoff dynamics are controlled by small topographical features that are unaccounted for in hydrological models. To better understand storage and routing effects of small scale topography and their interaction with shallow groundwater under SE conditions, we developed a model of reduced complexity to investigate SE runoff generation, emphasizing feedbacks between shallow groundwater dynamics and mesotopography. The dynamic specific yield affected unsaturated zone water storage, causing rapid switches between negative and positive head and a flatter groundwater mound than predicted by analytical agro-hydrological models. Accordingly, saturated areas were larger and local groundwater fluxes smaller than predicted, leading to surface runoff generation. Mesotopographic features routed water over larger distances, providing a feedback mechanism that amplified changes to the shape of the groundwater mound. This in turn enhanced runoff generation, but whether it also resulted in runoff events depended on the geometry and location of the depressions. Whereas conditions favourable to runoff generation may abound during winter, these feedbacks profoundly reduce the predictability of SE runoff: statistically identical rainfall series may result in completely different runoff generation. The model results indicate that waterlogged areas in any given rainfall event are larger than those predicted by current analytical groundwater models used for drainage design. This change in the groundwater mound extent has implications for crop growth and damage assessments.冬季，温带低地地区常会规律性观测到饱和过剩（Saturation Excess，SE）积水现象。地表径流动力学过程受小型地形特征调控，但此类特征未被纳入现有水文模型的考量范畴。为深入解析小型地形的蓄存与汇流效应，及其在饱和过剩条件下与浅层地下水的相互作用机制，我们构建了一款降复杂度模型以开展饱和过剩产流研究，重点关注浅层地下水动力学与中地形（mesotopography）之间的反馈关系。动态给水度（specific yield）会影响非饱和带（unsaturated zone）的含水量，致使水头（head）在正负值间快速切换，且形成的地下水丘（groundwater mound）形态较农业水文解析模型（analytical agro-hydrological models）的预测结果更为平缓。据此，实际饱和区域（saturated areas）面积大于模型预测值，局部地下水通量（local groundwater fluxes）则更小，最终引发了地表产流。中地形特征可将水流输送至更远距离，形成了可放大地下水丘形态变化的反馈机制，进而进一步促进了地表产流，但能否引发径流事件，还取决于洼地的几何形态与分布位置。尽管冬季可能存在大量利于产流的环境条件，但此类反馈机制会大幅降低饱和过剩产流的可预测性：统计特征一致的降雨序列，可能引发完全不同的产流结果。模型结果表明，在单次降雨事件中，实际积水区域面积大于当前用于排水设计（drainage design）的解析地下水模型的预测值。地下水丘范围的这一变化，会对作物生长与损害评估产生重要影响。