黄土高原植被恢复的区域耗水规律及其对土壤水资源平衡的影响

Vegetation cover on the Loess Plateau was significantly improved after the implementation of “Grain for Green” project, which were helpful for controlling severe soil erosion. However, serious imbalance of water supply and demand appears during vegetation restoration because of water shortage and strong water consumption by evapotranspiration. Thus, deep-layer soil water is prone to be overused and this further results in soil desiccation. Based on the facts of climate warming and vegetation restoration, this project aims to investigate the regional scale water consumption rule of vegetation restoration and its impact on soil water balance over the whole Loess Plateau by integrating the approaches of land-atmosphere coupling model and vegetation dynamics. Through analysis of land cover databases and remote sensing images, the proposed project will derive dynamic Land Use/Cover Change (LUCC) and variations of relevant vegetation and land surface parameters over the periods of 2001-2015. Subsequently, the dynamic LUCC, vegetation parameters, and land surface parameters will be used to initialize a land-atmosphere coupling model, and in turn to simulate the regional climate and land surface water-energy exchange processes in the study region. The impact of vegetation restoration on water cycle and land-atmosphere coupling strength will be considered in this work. Finally, the water consumption rules of vegetation restoration, the relationships between evapotranspiration and soil water recharge, and the impact of vegetation restoration on soil water storage will be explored and illustrated. Based on the above efforts, this project would put forward a suitable mode of vegetation restoration over the Loess Plateau to reduce effectively excessive evapotranspiration and alleviate soil desiccation caused by overuse of soil water. This project is expected to provide insights into facilitating sustainable development of the late stage of “Grain for Green” project on the Loess Plateau and help ensure water security of the Yellow River basin.

黄土高原退耕还林（草）工程实施以来，大量坡耕地被转化为草地和林地，植被恢复效果显著，水土流失得到了有效控制。但是，由于水资源短缺和植被恢复蒸散耗水强烈，导致部分区域水分供需矛盾进一步加剧，容易出现以过度消耗深层土壤水资源为代价的土壤干燥化现象，产生新的生态问题。本研究立足于气候变化和植被恢复的实际，采用陆气双向耦合模式与植被动态过程相结合的方法，分析退耕还林（草）工程实施后黄土高原土地利用类型、不同植被和地表参数的时空变化趋势，在考虑植被恢复对流域水循环和陆气交互作用强度影响的基础上，模拟黄土高原区域气候与地表过程，阐明大规模植被恢复的区域水分效应及其对土壤水资源的影响，明确土壤水补给量和植被恢复耗水量间的定量关系，揭示区域尺度植被恢复耗水规律，提出基于土壤水资源平衡的植被恢复策略。该研究对黄土高原退耕还林（草）工程可持续性评价和黄河流域水资源综合管理有着重要的理论与现实意义。

区域气候趋于暖干化背景下，缺水的黄土高原进行大规模植被恢复，必将影响其区域水分状况。本研究通过分析大规模植被恢复对土壤水资源消耗补给平衡关系的影响，阐明区域尺度植被恢复耗水规律，并结合植被恢复现状合理性评价结果，以适宜的植被恢复策略指导生产实践，缓解由土壤水资源过度消耗而引起的土壤干燥化。紧扣研究目标，我们首先通过构建考虑植被动态变化的区域尺度PT-JPL蒸散发模型，研究了植被恢复对黄土高原蒸散发的影响，在此基础上通过PT-JPL蒸散发模型估算的ET产品，计算开展和未开展退耕还林（草）工程两种情景下黄土高原16个子流域的土壤水变化量（ΔS），发现植被恢复情景下的ΔS小于未开展植被恢复情景下的ΔS，而且植被生长状况越好的区域，ΔS对流域水量平衡的影响越小，然后利用Budyko框架在不同时间尺度对流域各降水分区进行了模拟，最终明确了气候变化背景下，在较为干旱缺水的植被恢复区开展水量平衡关系探究的最佳时间尺度。然后利用全局优化算法（SCE-UA）矫正了对植被动态过程具有显著影响的参数，使用集合卡尔曼滤波数据同化算法将遥感叶面积指数融合到陆气耦合模型的动态模拟过程中，再将优化的参数与数据同化相结合，从而更准确模拟黄土高原陆面水文过程。接着基于改进后考虑植被动态变化的WRF-Noah陆气双向耦合模式，通过对比分析开展和未开展植被恢复的两组陆气双向耦合模拟试验，发现大规模植被恢复引起的蒸散发和水汽通量增加对黄土高原局地降水量的增加具有积极作用，然而蒸散发增加幅度小于降水，导致地表水平衡发生变化，引起地表产水量的增加，从而有利于对土壤水的补给。最后基于改进的Shuttleworth-Wallace模型对黄土高原不同植被情景下的蒸散发进行模拟，结合分析模型估算降水和蒸散发的变化，对黄土高原植被可持续恢复的理论上限进行估算，并分析其变化对水碳循环的影响。以上研究结果可为黄土高原及类似地区的植被恢复规划提供科学指导和理论支持。