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

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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.