Impacts of forest harvesting on groundwater properties, water flowpaths and streamflow response were examined 4 years after the harvest using a paired‐basin approach during the 2001 snowmelt in a northern hardwood landscape in central Ontario. The ability of two metrics of basin topography (Beven and Kirkby's ln(a/tan β) topographic index (TI) and distance to stream channel) to explain intra‐basin variations in groundwater dynamics was also evaluated. Significant relationships between TI and depth to potentiometric surface for shallow groundwater emerged, although the occurrence of these relationships during the melt differed between harvested and control basins, possibly as a result of interbasin differences in upslope area contributing to piezometers used to monitor groundwater behaviour. Transmissivity feedback (rapid streamflow increases as the water table approaches the soil surface) governed streamflow generation in both basins, and the mean threshold depths at which rapid streamflow increases corresponded to small rises in water level were similar for harvested (0·41 ± 0·05 m) and forested (0·38 ± 0·04 m) basins. However, topographic properties provided inconsistent explanations of spatial variations in the relationship between streamflow and depth to water at a given piezometer for both basins. Streamflow from the harvested basin exceeded that from the forested basin during the 2001 melt, and hydrometric and geochemical tracer results indicated greater runoff from the harvested basin via surface and near‐surface pathways. These differences are not solely attributable to harvesting, since the difference in spring runoff from the harvested basin relative to the forested control was not consistently larger than under pre‐harvest conditions. Nevertheless, greater melt rates following harvesting appear to have increased the proportion of water delivery to the stream channel via surface and near‐surface pathways. Copyright © 2005 John Wiley & Sons, Ltd.

Abstract:We used a paired-basin approach to examine the impacts of forest harvesting on event-pre-event water partitioning in streamflow and on shallow groundwater residence times during snowmelt in a northern hardwood landscape. The ability of topographic properties to explain spatial variations in groundwater residence times within forested and harvested basins was also explored. Reasonable agreement between isotopic (υ 18 O as tracer) and geochemical (Cl as tracer) hydrograph separation results and good agreement between residence times obtained using the two tracers suggested that Cl behaves as a conservative tracer in these basins. Geochemical hydrograph separations showed a greater total event-water flux and event-water contribution to peak streamflow from the harvested basin relative to the control 4 years after harvesting. This is consistent with hydrometric and hydrochemical results (Monteith et al., 2006, Hydrological Processes 20: this issue) that indicate surface and near-surface pathways contributed a greater proportion of runoff reaching the stream in the harvested basin, which may reflect increased daily melt rates following harvesting. However, there was no significant difference in mean groundwater residence times between the harvested (18 š 16 days) and forested (16 š 15 days) basins, and residence times did not exhibit significant differences with depth in either basin. Topographic metrics (topographic index, distance to stream) had a limited relationship with spatial patterns in groundwater residence times, as well as other groundwater properties (Monteith et al., 2006) in both basins. Spatial variations in factors such as near-surface hydraulic conductivity and till thickness should also be considered when explaining observed groundwater behaviour in a basin and its response to forest harvesting during snowmelt.