Land cover controls on depression-focused recharge on the Oak Ridges Moraine, southern Ontario, Canada

The Oak Ridges Moraine (ORM) is a key hydrogeologic feature in southern Ontario. Previous research has emphasized the importance of depression-focused recharge (DFR) for the timing and location of water recharge to the ORM's aquifers. However, the significance of DFR has not been empirically demonstrated, and the ORM's permeable surficial deposits imply that rainfall and snowmelt will largely recharge vertically rather than move laterally to topographic depressions. The exception may be during winter and spring, when concrete soil frost limits infiltration and encourages overland flow. The potential for DFR was examined for closed depressions under forest and agricultural land covers with similar soils and surficial geology. Air temperatures, precipitation, snow depth and water equivalent, soil water contents, soil freezing, and depression surface-water levels were monitored during the winter and spring of 2012-2013 and 2013-2014. Recharge (R) was estimated at the crest and base of each depression using a 1-dimensional water balance approach and surface-applied Br- tracing. Both forest and agricultural land covers experienced soil freezing; however, forest soils did not develop concrete frost. Conversely, agricultural fields saw concrete frost, overland flow, episodic ponding, and subsequent drainage of rain-on-snow and snowmelt inputs in open depressions. Recharge at the base of open depressions exceeded that in surrounding areas by an order of magnitude, suggesting that DFR is a significant hydrologic process during winter and spring under agricultural land cover on the ORM. Closed topographic depressions under agricultural land cover on the ORM crest may serve as critical recharge hot spots during winter and spring, and the ability of the unsaturated zone beneath these depressions to modify the chemistry of recharging water deserves further attention.