Water infiltration of snowpack plays an important role in the formation of wet avalanches. Several studies have examined water infiltration of snowpack on flat land. However, because water flow has a directional component parallel to the slope in snowpack on a slope, the effects of water infiltration into snowpack may differ between flat land and slopes, where avalanches are more likely to occur. From January 2012 to April 2012, we simultaneously observed snow pits on flat land and on a slope (40 degrees incline; NE aspect). The observations showed that MFr (the ratio of the total thickness of the layers composed of melt forms to the thickness of all layers of the snowpack) was, on average, 23% higher for the snowpack on the slope than for the snowpack on flat land, despite the slope's being a shaded side slope. The largest difference between the MFr of the slope and that of the flat land was observed in early March, when MFr was 99% at the slope and 46% at the flat-land site. We analyzed these observations using a one-dimensional multi-layer snowpack model proposed by Katsushima et al. (2009). The model included parameterization of the vertical water-channel process in snowpack. Based on the results, to represent the MFr at each site, the amount of water infiltrating through vertical water channels was estimated at 27.4 mm (2% of the total amount of water provided from the snow surface) for the slope and 797.2 mm (54%) for the flat land. Our results indicate that in wet-snow regions, differences in the water infiltration process can generate notable differences between snowpack on a slope and that on flat land. (C) 2014 Elsevier B.V. All rights reserved.