黄土高原北部植被-土壤斑块系统水分分配及过程模拟

Recent studies found that there was a “source-sink” relationship on the redistribution of water between vegetation patch and neighbor soil patch in arid and semiarid areas, however, knowledge was rather poor about the process of water migration and redistribution in vegetation-soil patches which was often affected by microtopography, in particular phytogenic mounds of vegetation patch. This process was essential to the formation and ecological stability of vegetation-soil patches. In this study, based on the field experiment observation and distributed hydrological model simulation, and the following contents will be studied in northern Loess Plateau, including: (1) water migration path in vegetation-soil patches; (2) soil moisture distribution characteristic in vegetation-soil patches driven by overland flow; (3) simulation of water distribution process in vegetation-soil patches based on CREST model. The objectives of this study was to fully understand the water distribution process between vegetation and soil patches, reveal the effect of microtopography and phytogenic mound on the water distribution process and provide scientific evidence for the evolution of “source-sink” relationship between patches. The expected results could have important implication to maintain and enhance the stability of vegetation-soil patches, and support references for vegetation restoration and desertification control in northern Loess Plateau.

尽管目前的研究发现，干旱和半干旱地区植被斑块和土壤斑块之间存在水分的“源-汇”关系，但水分在斑块之间的运移和分配过程及其对微地形的响应尚不明确，而这对于植被-土壤斑块系统的形成和维持稳定至关重要。项目拟在黄土高原北部，通过野外试验观测和分布式水文模型模拟等手段，主要研究：（1）植被-土壤斑块系统内水分运移路径；（2）地表径流驱动下斑块系统土壤水分分布特征；（3）基于CREST模型的斑块系统水分分配过程模拟。项目旨在揭示植被-土壤斑块系统水分运移规律和水分分配过程，明确地表微地形对斑块系统水分分配的影响，为回答斑块间的“源-汇”关系是否发生演变提供科学证据。预期研究结果对于维持和增强斑块状植被生态系统的稳定性具有重要意义，也为黄土高原北部植被恢复和荒漠化防治提供理论依据。

项目在黄土高原北部，通过野外试验观测采样和水文模型拟等手段，明确了植被-土壤斑块系统内水分运移路径和地表径流驱动下斑块系统土壤水分分布特征，同时对斑块系统水分分配过程进行模拟。主要结果如下：（1）研究区典型植被斑块和土壤斑块的土壤容重和颗粒组成差异不显著，植被斑块土壤有机质含量显著高于土壤斑块，斑块之间的土壤水分含量差异并不显著，并因植物种类而发生变化。植被斑块下的细根根长密度（RLDfine）、细根根体积密度（RVDfine）和根重密度（RMD）均大于土壤斑块，但差异不显著；（2）植被斑块的初始和稳定入渗率及土壤饱和导水率（Ks）均大于土壤斑块。各典型植被的斑块系统在10cm以下土层处，优先流发育程度较高，主要表现为侧向流和指流，但是对于不同植被群落和斑块类型而言，各优先流参数和土壤水分空间分布的差异并不显著，这主要是因为土壤斑块下也存在较丰富的根系，为水分的流动和分配提供了优先路径；（3）研究区植被斑块下土堆多呈圆帽形态、台阶性形态和与人工整地形成的外凹内凸形态，土堆的高度和面积大小与植被斑块的尺寸大小相关，与周边的土壤斑块形成了不同的微生境；（4）野外径流小区放水实验表明大多数小尺寸植被斑块的坡面水流流速、雷诺数和弗劳德数高于大尺寸植被斑块，而达西阻力系数较小，这与土堆高度和面积大小有关；（5）具有典型土堆的植被斑块边缘处土壤水分含量高于斑块内部和土壤斑块，而具有复合外凹内凸形态土堆的斑块内部的土壤水分含量却低于斑块边缘和土壤斑块。基于实测计算数据并考虑地表微地形的水文模型模拟结果表明，植被斑块下发育良好的土堆切断坡面漫流的连续性，导致水分在植被斑块边缘和土壤斑块入渗，难以达到植被斑块内部，因此植被斑块水分的汇集能力即“沃岛效应”减弱，植被斑块和土壤斑块间的“源-汇”关系发生改变。研究结果对于维持和增强斑块状植被生态系统的稳定性具有重要意义，也为黄土高原北部植被恢复和荒漠化防治提供理论依据。