Infiltration and soil water mixing on forested and harvested slopes during spring snowmelt, Turkey Lakes Watershed, central Ontario

Forest harvesting increases snow accumulation and water delivery to the soil surface relative to undisturbed forest; however, implications of these changes for water infiltration and storage in the soil profile are not well known. These processes were studied using a combined hydrometric and isotopic tracer approach in a harvested area and adjacent hardwood forest stand on a slope at the Turkey Lakes Watershed (TLW) in central Ontario. The slope had a mean gradient of 0.29 and a 0.5-1 m thick ablation till layer overlying compacted basal till. Pre-melt snow accumulation and daily water inputs in the harvested area and forest were measured during the 2000 and 2001 snowmelts. Soil water content in the ablation till was monitored at harvested and forest soil pits using time domain reflectometry. Isotopic signatures of input and soil water at the pits were sampled throughout melt. Total water inputs to the harvested soil surface exceeded that in the forest: 280 mm in the harvested area vs. 212 mm in the forest in 2000 and 526 mm in the harvested area vs. 409 mm in the forest in 2001. Near-surface soil water content, relative saturation of the ablation till and duration of soil saturation at 0.2 m depth were also greater in the harvested area compared to the forest for both years. Despite the absence of between-pit differences in soil properties influencing infiltration into the profile, a thinner ablation till layer at the harvested area pits would have contributed to greater soil saturation at 0.2 m depth in the harvested area. Greater soil wetness in the harvested area appeared to restrict mixing between inputs and pre-melt soil water relative to conditions in the forest during the 2000 melt, leading to longer water residence times at depth in the ablation till. This was partly supported by results from the 2001 melt. We suggest that harvesting of hardwoods on slopes in TLW may promote increased subsurface flow above the basal till contact and saturation overland flow during spring snowmelt, with implications for point-scale soil water chemistry as well as the quantity and quality of slope runoff to receiving streams.