Controls on depression-focused recharge during spring snowmelt on the Oak Ridges Moraine, southern Ontario, Canada

Abstract

The Oak Ridges Moraine (ORM) is a key hydrogeologic feature in southern Ontario. Previous work has emphasized the value of groundwater recharge (R) estimates and information on the timing and location of R to incorporating groundwater protection into land-use planning on the ORM, and has suggested that ponding and depression-focused recharge (DFR) occurs in closed topographic depressions on the ORM during winter and spring. However, the influence of land cover, topography, and soil type on the occurrence and extent of DFR is unclear. Sixteen closed topographic depressions on the crest and flanks of the ORM were chosen for study. Observations of ponding and measurements of pond extent during the snowmelt period were related to depression land cover (percent coverage of open vs. forested cover), depression geometry (i.e., contributing area, average slope, and relief ratio) and soil texture (% sand, silt and clay). Ponding occurred at 10 of 16 depressions, all of which were in predominantly open agricultural cover. Ponded and non-ponded depressions differed significantly in terms of percentage of open area and average slope. Ponded depressions were gently-sloped with high proportions (>85%) of open coverage, while non-ponded depressions were steep and largely forested. Extent of ponding was not related to any of the depression characteristics. However, pond volume showed a significant positive, non-linear relationship with average slope for a subset of sites with similar land cover and soil type. Results suggest that ponding and DFR during winter and spring on the ORM is restricted to open, agricultural areas and that this phenomenon may be more pronounced in steeper depressions. The relationship between land cover and DFR presented here could have implications for the quantity and quality of water recharging the ORM's aquifers, and further work is recommended to better define this relationship.