黄河源区不同类型冻土土壤水分入渗特性

Abstract

冻土土壤水分运动由于受到冻融过程的影响而显示其独特性，而目前对于不同类型冻土土壤水分入渗特性尚缺乏足够的认识。为此，以黄河源区康穷小盆地多下坡年冻土和上坡季节冻土区为例，结合季节降雨变化，基于大气降水、冻土土壤水分、冻结层上水等野外监测数据分析，采用HYDRUS-1D软件冻融模块进行土壤水分入渗模拟，对比分析了融化期多年冻土和季节冻土土壤水分运移过程的差异性，研究结果表明：①在快速融化阶段，降雨以地表径流为主，表层土壤水分含量增加，土壤下渗有限，冻结层上水位上升幅度较小；在稳定融化阶段，土壤水分含量增加，土壤水分下渗增强，受冻土层阻隔影响，多年冻土区冻结层上水水位上升幅度较大，季节冻土区土壤水分则以深层渗漏或侧向流动为主。②受到降雨强度、土壤质地、蒸散发、植被覆盖等因素的影响，降雨损失主要以地表径流为主，下坡各层土壤水分随冻结土壤融化自上而下逐渐增加并达到饱和状态，但上坡表层土壤不易达到饱水状态。③区域河流贯穿融区地下水发育，导致上坡冻结层上水位小幅度上升，下坡冻结层上水位的变化除受到降雨入渗的影响外，还受到融区地下水的影响，引起下坡冻结层上水位的快速上升。研究结果有助于深入了解全球气候变化背景下的冻土退化及其水文效应，进而为定量评估流域水资源脆弱性与区域生态敏感性提供科学依据。;The water movement of frozen soil shows its uniqueness due to the influence of freeze-thaw process. However, there is a lack of in-depth study on soil moisture infiltration characteristics of different types of frozen soil at present. Therefore, a slope cross-section at the small basin of Kangqiong in Source Area of the Yellow River is studied. This zone on the upper slope is covered by seasonally frozen ground and that on the down slope is covered by the permafrost. Based on the field observation, the data has been collected from various sources including atmospheric precipitation, soil moisture, and suprapermafrost water from May 2017 to October 2017. In addition, the soil moisture infiltration is numerically simulated by the freeze-thaw module of HYDRUS-1D software package. Based on seasonal rainfall variation, the difference of soil moisture infiltration in permafrost regions and seasonally frozen regions is statistically investigated during thawing period. The results are summarized in the following: 1) in the rapid thawing stage, the rainfall infiltration is dominated by surface runoff. The water content of surface soil increases, but the infiltration of soil is limited. So, the suprapermafrost water flow rises slightly. During the stable thawing stage, soil water content increases and soil moisture infiltration increases. Influenced by the barrier of permafrost layer, the suprapermafrost water flow in permafrost regions rises greatly. While in seasonally frozen regions, the soil moisture infiltration is dominated by deep leakage or lateral flow. 2) Influenced by rainfall intensity, soil texture, evapotranspiration and vegetation coverage, the rainfall loss is mainly caused by surface runoff. Soil moisture content of each layer on the down slope increases gradually from top to bottom with the thawing of frozen soil and reaches the saturation state. Soil moisture content in surface layer on the upper slope has been rising with time, but it can not reach a stable state. 3) The existence of regional talik groundwater leads to a small increase of the suprapermafrost water level on the upper slope. In addition to rainfall infiltration, the change of the suprapermafrost water on the down slope is also affected by talik groundwater, which causes the rapid rise of the suprapermafrost water level on the down slope. The results are helpful to understand the degradation of frozen soil and its hydrological effects under the background of global climate change. At the same time, it will provide scientific basis for quantitative assessment of water resources vulnerability and regional ecological sensitivity.