Paired-basin comparison of hydrologic response in harvested and undisturbed hardwood forests during snowmelt in central Ontario: II. Streamflow sources and groundwater residence times

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 (δ18O 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 ± 16 days) and forested (16 ± 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. Copyright © 2005 John Wiley & Sons, Ltd.