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，气温上升超过全球平均水平，冬季降雨也更加频繁和猛烈。这些变化将大大增加灾难性自然灾害的数量，如大面积地表径流和泥石流。造成这种情况的原因之一是水有限地渗入（部分）冻土。如果降水累积，坡面径流会发生，由于水土流失，可能发展成泥石流。水通过优先通道在冻结层之间渗透可导致孔隙压力增加，从而引发边坡失稳。此外，地表径流限制了地下水的补给，消除了地下水库的重要缓冲作用。山区的融雪和降水对世界许多地方的地下水资源至关重要，欧洲也是如此。在这个项目中，渗透水和土壤在零度以下的温度之间的热工水力耦合将使用先进的建模和实验方法在实验室以及在现场进行调查。土壤中的优先途径，例如由植物或动物活动引起的大孔，对于水渗透至关重要，因为它们允许水更快地渗透到更大的深度，并且显示出与土壤基质的小微孔（冰从孔的内部生长）不同的冻结行为（从大孔的外部向内部冻结）。因此，了解渗透过程中大孔隙对水的冻结和融化的影响对于任何进一步的评估都至关重要。冻土水分入渗过程中，各相的热状态控制着冻土的水力学行为。在液态水渗入冻土的情况下，渗入的水具有高于冰点的温度。另一方面，土壤颗粒和孔隙填充物的温度低于冰点。分离相温度的热演化通过相之间的热传递来描述，由于各种依赖性，描述和量化是具有挑战性的，因为水流和热传递是强耦合的并且在相变温度附近是高度动态的。因此，土壤和渗透水之间的温差被怀疑对土壤的冻结或解冻的进展有重大影响。了解大孔隙的作用和系统的热水力演化，可以确定特定的环境和气象条件，在极端的地表径流或边坡破坏可能发生。这方面的知识可能会影响减灾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.