Abstract
Abstract. Meltwater ion concentration and infiltration rate into frozen soil both decline rapidly as snowmelt progresses. Their temporal association is highly non-linear and a covariance term must be added in order to use time-averaged values of snowmelt ion concentration and infiltration rate to calculate chemical infiltration. The covariance is labelled enhanced ion infiltration and represents the additional ion load that infiltrates due to the timing of high meltwater concentration and infiltration rate. Previous assessment of the impact of enhanced ion infiltration has been theoretical; thus, experiments were carried out to examine whether enhanced infiltration can be recognized in controlled laboratory settings and to what extent its magnitude varies with soil moisture. Three experiments were carried out: dry soil conditions, unsaturated soil conditions, and saturated soil conditions. Chloride solutions were added to the surface of frozen soil columns; the concentration decreased exponentially over time to simulate snow meltwater. Infiltration excess water was collected and its chloride concentration and volume determined. Ion load infiltrating the frozen soil was specified by mass conservation. Results showed that infiltrating ion load increased with decreasing soil moisture as expected; however, the impact of enhanced ion infiltration increased considerably with increasing soil moisture. Enhanced infiltration caused 2.5 times more ion load to infiltrate during saturated conditions than that estimated using time-averaged ion concentrations and infiltration rates alone. For unsaturated conditions, enhanced ion infiltration was reduced to 1.45 and for dry soils to 1.3. Reduction in infiltration excess ion load due to enhanced infiltration increased slightly (2–5%) over time, being greatest for the dry soil (45%) and least for the saturated soil (6%). The importance of timing between high ion concentrations and high infiltration rates was best illustrated in the unsaturated experiment, which showed large inter-column variation in enhanced ion infiltration due to variation in this temporal covariance.
Highlights
Underneath a melting snowpack, the infiltrability of the stratum, whether it is frozen or unfrozen, determines the partitioning of meltwater into ponding water, overland flow, infiltration to the organic layer and/or infiltration to the mineral soil (e.g., Hillel, 1998; Zhao and Gray, 1999)
The overflow valve ensured a maximum constant head, which was comparable to natural conditions (10 mm), and minimized the influence of the high and highly variable precipitation rates on the infiltration rate
The experiments were set up to simulate field conditions. This was attempted by using in-situ soil for the top part of the soil columns, keeping the soil temperature within the range of many seasonally frozen soils during snowmelt, allowing infiltration to occur freely, as well as applying solutions to the surface that decreased in concentration in a similar way as snow meltwater
Summary
Underneath a melting snowpack, the infiltrability of the stratum, whether it is frozen or unfrozen, determines the partitioning of meltwater into ponding water, overland flow, infiltration to the organic layer and/or infiltration to the mineral soil (e.g., Hillel, 1998; Zhao and Gray, 1999). A sufficient heat flow into the soil due to frozen conditions underneath the melting snowpack (i.e. in cold regions) may cause ponding water to refreeze, forming a basal ice layer (Woo and Heron, 1981). This will result in an alteration of both ion pathway and concentration. The rate and volume of snowmelt runoff depends on the melt intensity and amount of water in the snowpack, and on the physical and thermodynamic properties of the underlying stratum
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