Focused flow in the unsaturated zone after surface ponding of snowmelt Nils-Otto Kitterød ⁎ Norwegian Institute for Agricultural and Environmental Research, Norway Received 4 December 2006; accepted 28 September 2007 Abstract 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Unsaturated flow; Focused flow; Ponding; Grain size distribution; Tracer test; Unsaturated flow parameters; Sensitivity analysis 1. Introduction The snowmelt period during spring is characterized by repeated cycles of melting and freezing governed by solar radiation and long wave energy out flux. In some years the repetitive melting and freezing of snow gives rise to a solid layer of ice below the snow cover, which reduces infiltra- tion capacity. In the ground itself frost also reduces the infiltration capacity (Stadler et al., 2000). The magnitude of the reduction depends on the soil water content, which is related to the spatial continuity of water in the vadose zone prior to freezing. Stoeckeler and Weitzman (1960) dis- tinguish between granular, porous, and solid ice in the soil, and they found increasing resistance to the water flux with increasing continuity of ice. Another physical reason for reduced infiltration capacity is the suction gradient at the freezing front. The pressure gradient is due to the phase transition from liquid to solid water which transports water to the frozen soil (Hansson et al., 2004). By theoretical calculations the pressure at the solid water interface is Available online at www.sciencedirect.com Cold Regions Science and Technology xx (2007) xxx – xxx COLTEC-01270; No of Pages 14 www.elsevier.com/locate/coldregions ⁎ Fax: +47 63 00 94 10. E-mail address: nils-otto.kitterod@bioforsk.no. 0165-232X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2007.09.005 ARTICLE IN PRESS Please cite this article as: Kitterød, N.-O., Focused flow in the unsaturated zone after surface ponding of snowmelt, Cold Regions Science and Technology (2007), doi:10.1016/j.coldregions.2007.09.005