Modelling vertical and lateral water flow in frozen and sloped forest soil plots

Abstract

Snowmelt runoff from forest land is influenced by the spatial distribution of soil frost which depends on climatic conditions and stand structure. Snow and frost dynamics as well as near-surface runoff and liquid water content in the soil were measured for two years in two small plots located on a slope within a subalpine spruce stand. These measurements were used to calibrate the one-dimensional water and heat transport model SOIL by only fitting three snow- and two soil-related parameters. The simulated snow and frost dynamics agreed well with field data. The partitioning of snowmelt water into lateral runoff and vertical percolation at the snowpack bottom was best represented assuming that downward water flow through the frozen layers occurred not only in the liquid phase between soil particles and pore ice (low-flow-domain) but also as microscopic bypass flow in the previously air-filled macropores (high-flow-domain). Simulated near-surface runoff is very sensitive to the impedance factor accounting for a reduced upward water flow in frozen soil layers and also to the heat transfer coefficient controlling the refreezing of water infiltrating through the empty voids. The runoff dynamics differed only slightly when the model was driven by daily instead of hourly weather variables.