Water infiltration in soil at catchment scale: consequences for cold-climate regions

Soil erosion is a global problem because of its environmental consequences, including sedimentation and pollution in many areas of the world. An estimated 400 million hectares of land have been abandoned due to soil erosion over the past 50 years. The main biophysical factor influencing the quantity of overland flow is the infiltration rate. Infiltration during a runoff-generating rainfall event is regulated by the hydraulic properties of the Critical Zone and the antecedent soil moisture conditions. The process of water induced soil erosion is described by the detachment of individual soil particles and their transport by overland flow. Detachment of soil particles is mainly caused by rain splash and the erosive force of overland flow. The major control of the quantity of overland flow is the infiltration rate. Most rainwater falls, directly or indirectly through stem flow or leaf drainage, on the soil. A small part remains on the leaves (interception) and eventually evaporates. The water that reaches the soil surface will be stored in micro-depressions, infiltrates in the soil profile or will move downhill as overland flow. The amount of infiltration and the infiltration rate depend on the characteristics of the soil. Physical processes in the soil occurring at cm3 and m3 scale near the surface are important controls of the flow of water and can also affect the hydrology at a catchment scale (e.g., the response time). With prolonged rainfall, infiltrating wetting fronts in water repellent soils may become unstable, leading to the formation of high velocity flow paths, the so-called fingers. Finger formation is generally regarded as a potential cause of the rapid transport of water and contaminants through the Critical Zone. A specific situation is the cold-climate hydrology. Frozen soil, freezing-thawing cycles and snowfall/melt are dominated processes influencing the water flow through the Critical Zone and at catchment scale. Infiltration during snowmelt often occurs as focused recharge in local depressions on the surface. This may cause higher velocities (the fingers) through the unsaturated zone than during evenly distributed infiltration on the surface, hence causing less than optimal conditions for degradation of pollutants. The redistribution of melt water can also cause extreme runoff in areas where there is normally high infiltration capacity. Hence it is important to take the dynamics of infiltration capacity into account in estimates of produced runoff and soil loss from catchment areas to predict flooding-event time and size and sedimentation in downstream areas. This presentation shows results of studies on preferential flow-paths (fingers) and the effects on water quantity and quality and soil erosion on catchment scale with emphasize on the cold-climate region.