隧道排水诱发地下水位下降对地表植被影响的TSPAC分析方法

山岭隧道建设可能会改变周边地下水渗流场，严重时会疏干地下水，威胁地表植被生存。针对现有研究难以定量分析隧道排水对地表植被影响的问题，提出以土壤水基质势作为植被生存状态指标，建立隧道-土壤-植被-大气连续体（tunnel-soil-plant-atmosphere continuum, TSPAC）分析模型。通过Richards方程描述植被根系区的土壤水分运移，将大气、地下水渗流模型作为上下边界输入方程，计算植被根系区基质势分布，据此衡量植被凋萎过程并判断其生存状态。将该分析方法应用于大别山区明堂山隧道工程，分析隧道建设对地表植被的影响，结果表明： 1）植被根系区土壤水基质势对地下水位和大气变化的响应存在滞后性和非均匀性，植被凋萎是渐进、动态的过程； 2）大气条件在该区域对植被生存起控制性作用； 3）隧道建设引起的地下水位变化对周边地表植被影响较小，且对不同植被类型影响程度不一。TSPAC分析方法的创新性在于： 提出了物理概念明确、易于工程使用的植被生态危险状态判断指标与依据，在SPAC分析模型基础上引入隧道地下水渗流模型，将隧道排水的工程因素与环境因素相结合，分析隧道排水—地下水位下降—地表植被影响的整体过程，并在实际工程中进行应用，可为隧道建设的环境保护提供一种新思路。

TSPAC Analysis Method for Impact of Groundwater Drawdown Inducedby Tunnel Drainage on Terrestrial Vegetation

The mountain tunnel drainage may change the ambient groundwater seepage field and even drain groundwater resources, which threatens terrestrial vegetation. In order to quantitatively analyze the impact of tunnel drainage on terrestrial vegetation, the soil water matric potential is selected as an indicator for plant survival state to build an analysis model of tunnel soil plant atmosphere continuum (TSPAC). In the model, the Richards equation is utilized to describe the moisture movement in the root zone. The atmosphere and the groundwater seepage model are input to the Richards equation as top and bottom boundary conditions. Then, the soil matric potential distribution of the root zone is secured by solving the Richards equation. Based on matric potential distribution within the root zone, the plant dynamic wilting process can be evaluated and its survival state can be judged. The method is applied to Mingtangshan Tunnel in Dabieshan Mountains to analyze its drainage impact on terrestrial vegetation. The applicable results show that: (1) The response of matric potential in the plant root zone to variations in groundwater levels and atmospheric conditions is lagged and nonuniform, thus the plants wilt gradually instead of withering immediately after the groundwater table descends. (2) The atmospheric conditions play the dominant role in vegetation survival in this region. (3) The drainage impact of the tunnel on surrounding plants is relatively small and different for various vegetation. The TSPAC analysis method innovatively introduced tunnel groundwater seepage model to traditional SPAC models. By integrating factors that represent tunnels and environment, the approach analyzed the whole process of “tunnel drainage groundwater recession vegetation withering”. The method integrates tunnel, soil, vegetation and atmosphere as a whole to analyze the eco environmental impacts of tunnel drainage. The approach can provide a novel thought of facilitating tunnel engineering environmentally friendly.