Abstract
Abstract. Snowmelt is a major source of groundwater recharge in cold regions. Throughout many landscapes snowmelt occurs when the ground is still frozen; thus frozen soil processes play an important role in snowmelt routing, and, by extension, the timing and magnitude of recharge. This study investigated the vadose zone dynamics governing snowmelt infiltration and groundwater recharge at three grassland sites in the Canadian Prairies over the winter and spring of 2017. The region is characterized by numerous topographic depressions where the ponding of snowmelt runoff results in focused infiltration and recharge. Water balance estimates showed infiltration was the dominant sink (35 %–85 %) of snowmelt under uplands (i.e. areas outside of depressions), even when the ground was frozen, with soil moisture responses indicating flow through the frozen layer. The refreezing of infiltrated meltwater during winter melt events enhanced runoff generation in subsequent melt events. At one site, time lags of up to 3 d between snow cover depletion on uplands and ponding in depressions demonstrated the role of a shallow subsurface transmission pathway or interflow through frozen soil in routing snowmelt from uplands to depressions. At all sites, depression-focused infiltration and recharge began before complete ground thaw and a significant portion (45 %–100 %) occurred while the ground was partially frozen. Relatively rapid infiltration rates and non-sequential soil moisture and groundwater responses, observed prior to ground thaw, indicated preferential flow through frozen soils. The preferential flow dynamics are attributed to macropore networks within the grassland soils, which allow infiltrated meltwater to bypass portions of the frozen soil matrix and facilitate both the lateral transport of meltwater between topographic positions and groundwater recharge through frozen ground. Both of these flow paths may facilitate preferential mass transport to groundwater.
Highlights
Snowmelt infiltration and soil freeze–thaw processes are important for modulating hydrological behaviour in cold regions
Soil freeze–thaw conditions at the study sites are classified in terms of three soil temperature states designated as frozen, partially frozen, and thawed
This study highlighted the effects of preferential flow in frozen soils and infiltration–refreezing mechanisms on the hydrologic functioning and winter water balance of prairie grasslands
Summary
Snowmelt infiltration and soil freeze–thaw processes are important for modulating hydrological behaviour in cold regions. Water movement in frozen soils is strongly affected by meteorological and soil moisture dynamics during the preceding seasons and by the coupling of soil water flow and heat transfer processes (Stähli et al, 1999) This coupling, in addition to water and energy transfer at the ground surface, creates complex subsurface flow dynamics during snowmelt (Ireson et al, 2013; Lundberg et al, 2016). In many cold regions, focused infiltration of snowmelt under topographic depressions is an important mechanism of groundwater recharge (Baker and Spaans, 1997; Hayashi et al, 2003; French and Binley, 2004; Gerke et al, 2010; Greenwood and Buttle, 2018). Frozen ground limits infiltration under the uplands surrounding de-
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