Focused flow in the unsaturated zone after surface ponding of snowmelt

Surface ponding occurs if water flux from rain or snowmelt, increases the infiltration capacity of the soil. Such conditions are frequently observed during spring time in Nordic countries and may represent hazards to water resources if the area is exposed to pollution. During snowmelt water accumulates in local depressions due to frozen ground. At the end of the snowmelt period when the frozen soil thaws, the flux of water may be extremely high in the unsaturated zone because of the accumulated volume of water. In this study, flow velocities in the unsaturated zone were estimated by numerical flow simulations and cross validated by an independent tracer test. The observed transport velocities of conservative tracers were about ten times higher than the applied infiltration intensities and were explained by focusing of water flow in the vadose zone. The focusing effect was demonstrated by transient numerical simulations. Numerical simulations were run for infiltration velocities ranging from very low (5 mm/day) to extremely high (250 mm/day). Sensitivity analysis based on expected variation of the flow parameters illustrates the relative importance of the grain size distribution index, intrinsic permeability, air entry pressure, soil porosity, residual water saturation, and the ratio of horizontal to vertical permeability. The sensitivity analysis was performed for two different sedimentological architectures, first for horizontal layers and then for a gently dipping low pervious layer above the groundwater table. Opposite to what may be expected, the simulations indicate faster breakthrough in the presence of the low permeability layer because of the focusing effect.