基于生态节水与水质净化相协调的台田湿地植被NDVI指数阈值研究

It has become a key point of wetland ecological protection under the severe water resource stress that how to analyze the eco-hydrological effects of the vegetation pattern changes in wetlands from an overall perspective of groundwater-soil-plant-atmosphere continuum (GSPAC) to achieve the water security goal of coordinating ecological water saving and water quality purification by regulating the vegetation pattern. In this project, platform field wetlands with plentiful reeds in Baiyangdian Lake are chosen as typical research areas. In the regional scale, NDVI index is selected as a key control indicator of the reed distribution pattern because of its sensitivity to the changes of hydrological factors and environmental factors. And in the field scale, the vegetation distribution density is chosen as a key control indicator for the vegetation pattern. During the research, “spatiotemporal changes of vegetation pattern -GSPAC system responses-water quality and water quantity changes in wetlands” is the core. The experimental monitoring, statistical analysis and the SWAP model are combined to explore how the GSPAC system responses to the changes of the vegetation pattern. After that, through combing the analysis of the relationship among water and nutrients in GSPAC and those in water resource system with the change of vegetation pattern, response mechanism of wetland water resource system under different spatial and temporal scales to vegetation pattern changes is revealed. On the basis of the above, NDVI index thresholds are analyzed and predicted under the water security goal of ecological water saving and water quality purification, which will provide theoretical basis and technical support for the development of the wetland ecological restoration mode and the water security maintenance technology under regional water stress.

如何从地下水－土壤－植被－大气系统（GSPAC）的整体角度分析湿地植被格局变化的生态水文效应，进而通过调控湿地植被格局来实现区域水安全目标，已成为水资源胁迫下湿地生态修复亟待突破的关键。本项目以白洋淀大量种植芦苇的台田湿地为研究区，在区域尺度下选取可直观反映湿地植被状况、且对水文要素和环境要素变化敏感的NDVI指数，田间尺度下选取植被密度来表征湿地植被芦苇的格局变化，以“植被时空格局变化－GSPAC系统响应－湿地水质水量变化”为核心，通过野外实验监测、统计分析和SWAP模型模拟相结合，探究不同植被格局下湿地GSPAC系统的响应，并通过耦合GSPAC系统水分、养分（氮、磷）运移与湿地水质水量指标的相关关系分析，揭示不同时空尺度下湿地水质水量对植被格局变化的响应机制，提出基于生态节水和水质净化相协调的NDVI指数阈值区间，为进一步发展水资源胁迫下湿地生态修复模式和水安全保障技术提供科学理论和技术支持。

针对变化环境下白洋淀湿地水资源胁迫问题突出的现状，本项目系统地研究了芦苇植被格局和地下水埋深对土壤－植被－大气系统（SPAC）水分运移耗散的影响，提出了基于生态节水和水质净化相协调的NDVI指数阈值，在通过湿地植被格局调控实现区域水安全目标的研究方面取得了重要进展。主要研究内容和成果包括：. （1）基于田间试验监测，研究了不同生长时期表层土壤含水量对芦苇密度的响应机理，揭示了芦苇密度和地下水埋深对土壤水分动态影响规律。结果表明：土壤储水量与芦苇密度呈显著负相关，不同密度条件下土壤储水量具有明显的季节性变化规律；地下水埋深对0-50cm深度土壤含水量存在显著的影响。. （2）将HYDRUS模型和双作物系数模型相结合，建立了一个改进的模型来研究植被密度变化和地下水位波动对SPAC系统水分运移和耗散的影响。确定了适用于白洋淀湿地的芦苇作物系数，揭示了田间尺度芦苇SPAC系统水分运移耗散对芦苇密度和地下水埋深变化的动态响应规律。结果表明：芦苇密度对蒸散发存在显著影响，地下水埋深与蒸散发呈负相关；地下水埋深变化在不同月份对土壤储水量、蒸散发影响程度存在差异。. （3）采用Vine Copula函数和InSAR技术等提取土地利用类型、芦苇密度、地下水埋深等空间分布数据，构建了区域尺度SPAC系统水分运移耗散分布式模型，揭示了多水位情景下芦苇SPAC系统水分运移耗散的时空分布规律。结果表明：2016-2018年整个淀区处于高水位，芦苇蒸散发量约占淀区总补给量的150%左右；在6月份适当地调整芦苇格局，降低20%芦苇密度，芦苇群落具有14%的节水潜力。. （4）考虑白洋淀湿地的水质和水量安全，以维持湿地水质安全为目标确定NDVI指数阈值下限，以减少不必要的蒸散耗水为目标确定NDVI指数阈值上限，确定了基于生态节水与水质净化相协调的植被NDVI指数阈值为[0.140, 0.206]。. 研究成果丰富和发展了湿地多尺度生态水文学研究的理论和方法，可为进一步发展水资源胁迫下湿地生态修复模式、实现区域水安全目标提供科学理论和技术支持。