Modelling enhanced infiltration of snowmelt ions into frozen soil

Abstract

Abstract:A model is proposed in which the cumulative load of an ion infiltrating into frozen unsaturated soil can be estimated as a function of meltwater ion concentration and infiltration rate. Assumptions of the model are that the meltwater solution released to the soil surface is conservative, fully mixed within each time step, and that mass and energy are conserved. Infiltration and meltwater concentration are estimated using relationships developed by Gray and Stein respectively. The model suggests that the relationship between ion concentration and volume of infiltration is non-linear with a positive covariance. Infiltration of snowmelt ions is therefore a function of the products of the mean concentration in the meltwater and the cumulative volume of water that infiltrates, plus the covariance between instantaneous values of ion concentration and infiltration rate. This covariance effect is termed enhanced infiltration. Meteorological observations and soil parameters from four sites in western Canada were used to assess the sensitivity of the model to conditions at a prairie site, a boreal forest site, a mountain forest site, and a shrub tundra site. Model results showed the greatest cumulative infiltration of ion load for the Prairie site; the general ranking was Prairie > Mountain Forest > Boreal Forest > Tundra. However, the greatest impact of enhanced infiltration was found for the Tundra site. At this site, enhanced infiltration caused up to 50% more ion load to infiltrate within the initial third of the melt period compared to infiltration estimates not accounting for this effect. Over the whole melt period, enhanced infiltration caused 55-160% more ion load to infiltrate than estimates based solely on the mean depth of infiltration and ion concentration. Sensitivity analysis showed that enhanced infiltration varies most strongly with initial snow water equivalent, average melt rate over the whole melt period, and snowpack ion elution concentration factor (CF).

The contribution of rain‐on‐snow events to nitrate export in the forested landscape of south‐central Ontario, Canada

“…They suggested that NO 3 ‐N in runoff from ROS events may be transported relatively conservatively to the stream due to fast runoff rates and limited interaction with subnivean soil. This conservative transport of chemicals during ROS events has also been reported in situations where the soil is partially or completely frozen, limiting infiltration of meltwater (Jones and Pomeroy, 2001; Lilbaek and Pomeroy, 2007). The importance of ROS events to NO 3 ‐N budgets may be especially pronounced in catchments where physiographic characteristics enhance this limited interaction.…”

Hydrology of hillslope‐wetland streams, Polar Bear Pass, Nunavut, Canada

“…This would limit infiltration (e.g. Lillbaek and Pomeroy, 2007) and help to enhance runoff. Summer precipitation (snow + rain) was extremely large in 2008, and in comparison to the dominant and deeply thawed upper bowl area of LSC (maximum thaw depth = 0·71 m), these thinly thawed ground ice‐rich sites (maximum thaw depth = 0·4 m) would likely shed this water rather than storing it.…”

Laboratory evidence for enhanced infiltration of ion load during snowmelt

“…Using Zhao and Gray's (1999) parametric relationship for the cumulative mass of water that infiltrates a frozen mineral soil, F [kg m −2 ], and Stein et al's (1986) expression for meltwater ion concentration as a function of time, C i (t) [meq m −3 ], Lilbaek and Pomeroy (2007) showed that the temporal association between infiltration rate, f (t) [kg s −1 m −2 ], and meltwater ion concentration, C i (t), is highly non-linear, even though both decline rapidly with time. The study showed that the cumulative infiltration of snowmelt ions is enhanced by initially higher ion concentration in meltwater and infiltration rate.…”

“…The model by Lilbaek and Pomeroy (2007) assumes a limited infiltration regime as defined by Granger et al (1984) in which there are neither substantial macropores nor impeding basal ice layers as well as that the meltwater solute is conservative and fully mixed at all times. The use of Zhao and Gray's equation to estimate infiltration to mineral soils under an organic layer was assumed appropriate when the organic layer rapidly transfers water to the mineral soil surface .…”

“…A reasonable assumption where the organic layer is thin and porosity is high or organic layer macropores are abundant. The covariance term was labelled enhanced infiltration by Lilbaek and Pomeroy (2007) and represents the additional ion load that infiltrates during snowmelt due to the combination of initially rapid infiltration rate and higher ion concentration in meltwater. Its magnitude was found to be governed by initial snow water equivalent, SWE [kg m −2 ], average melt rate, M [kg s −1 m −2 ], and the meltwater ion concentration factor, CF [(meq m −3 ) (meq m −3 ) −1 ].…”