Thermohydraulic Processes during Water Infiltration into Frozen Soil with Implications for Geohazards under a Changing Climate

气候变化正在影响山区水文，气温高于全球平均水平，冬季降雨也更加频繁和严重。这些变化将大大增加灾难性自然灾害的数量，如广泛的地表径流和泥石流。发生这种情况的原因之一是水对（部分）冻土的渗透有限。如果降水积累，坡面会发生径流，由于水土流失可能发展为泥石流。水通过优先通道在冻结层之间渗透，会导致孔隙压力增加，从而引发边坡破坏。此外，地表径流限制了地下水补给，并消除了地下水水库的重要缓冲作用。在世界许多地方，山区的融雪和降水是对地下水资源的重要贡献，在欧洲也是如此。在本项目中，将采用先进的建模和实验方法，在实验室和现场研究零度以下温度下入渗水与土壤之间的热液耦合。土壤中的优先通道，如由植被或动物活动引起的大孔隙，对水的渗透至关重要，因为它们允许水更快地向更深的深度渗透，并且与土壤基质的小微孔（冰从孔隙内部生长）相比，它们表现出不同的冻结行为（从外部向内部冻结）。因此，了解大孔隙在入渗过程中对水的冻结和融化的影响对任何进一步的评估都是至关重要的。水在冻土中入渗过程中，其水力特性受入渗相的热态控制。在液态水渗入冻土的情况下，渗透水的温度高于冰点。另一方面，土壤颗粒和孔隙填充的温度低于冰点。不同相温度的热演化是由相之间的热传递来描述的，由于各种依赖关系，水的流动和热传递是强耦合的，并且在相变温度周围是高度动态的，因此描述和量化是具有挑战性的。因此，土壤和入渗水之间的温差被怀疑对土壤冻结或解冻的进程有重大影响。通过了解大孔隙的作用和系统的热水力演化，可以确定可能发生极端地表径流或边坡破坏的特定环境和气象条件。这方面的知识可能会影响危害缓解措施以及依赖山区水资源的地区的地下水管理。

Climate change is affecting mountainous hydrology with temperature increase above the global mean, as well as with more frequent and heavier rain events also during winter months. These changes will dramatically increase the number of disastrous natural hazards, such as extensive surface runoff and debris flow. One of the reasons for such events is a limited infiltration of water into (partly) frozen soil. If precipitation accumulates, surface runoff occurs on slopes, which might develop into debris flow due to soil erosion. Water infiltrating between frozen layers through preferential pathways can lead to increased pore pressures triggering slope failure. Further, surface runoff limits groundwater recharge and eliminates the important buffering effect of groundwater reservoirs. Snowmelt and precipitation in the mountains are a crucial contribution to groundwater resources in many parts of the world, also in Europe. In this project, the thermohydraulic coupling between infiltrating water and the soil at sub-zero temperatures will be investigated using advanced modeling and experimental methods in the laboratory as well as at a field site. Preferential pathways, such as macropores caused by vegetation or animal activity, in the soil are critical for water infiltration because they allow a faster infiltration of the water towards greater depth and show a different freezing behavior (freezing from the outside towards the inside of the macropore) than small micropores (ice growing from the inside of the pore), of the soil matrix. Understanding the influence of macropores on the freezing and melting of water during infiltration is, therefore, crucial for any further assessment. During water infiltration into frozen soil, the hydraulic behavior is controlled by the thermal state of the involved phases. In the case of liquid water infiltration into frozen soil, the infiltrating water has temperatures above the freezing point. On the other hand, the soil grains and the pore-filling have temperatures below the freezing point. The thermal evolution of the separate phase temperatures is described by heat transfer between the phases, which is challenging to describe and quantify due to various dependencies because water flow and heat transfer are strongly coupled and highly dynamic around the temperature of phase transition. The temperature difference between soil and infiltrating water is therefore suspected to have a major influence on the progression of freezing or thawing of the soil. With an understanding of the role of macropores and the thermo-hydraulic evolution of the system, specific environmental and meteorological conditions can be identified, at which either extreme surface runoff or slope failure might occur. This knowledge might influence hazard mitigation measures as well as groundwater management in regions that depend on mountainous water resources.