More than half of the annual acidic deposition to Canada's boreal forests is released during spring snowmelt. To evaluate the impact of this acidity upon the aquatic environment, the sources and hydrologic pathways must be defined. Two environmental tracers, oxygen 18 and silica, were used to meet this objective for spring snowmelt at Lac Lafiamme. Stream water could be partitioned into four distinct waters based upon event and flow path; however, similar division for lake water was not possible due to inconsistencies between tracer contents in the lake water. In terms of the major source of acidity, snow meltwater, stream water had an average meltwater content of 70% while near-shore lake water ranged from greater than 70% at spring melt commencement to less than 45% at the end. These high amounts of meltwater represent a severe hazard for the potential occurrence of "acid shock" of nearshore spawning grounds. INTRODUCTION The acidification of lakes is greatly affected by spring snowmelt, which represents the major hydrological event for many watersheds in northern latitudes [Bottomley et al., 1986; Gjessing and Johannessen, 1987; Roberge and Plamondon, 1987]. Chemical fractionation within the melting snowpack also contributes to the acidic "shock" of spring meltwaters; 50 to 80% of the solutes contained within the snowpack will be released upon melt of the first third of the snowpack [Gjessing and Johannessen, 1987; Davis et al., 1987]. The amount of acidity that eventually reaches the stream or lake waters, however, is dependent upon dilution by preevent groundwaters and upon the flow pathways. Meltwaters are altered by chemical reactions within the organicmineral matrix and delayed in arrival by the path length and permeability of the flow system [Bottomley et al., 1985; Anderson and Bowser, 1986; Hooper and Shoemaker, 1987]. Thus a definition of the relative amounts of preevent groundwaters and the flow paths taken by the meltwaters is essential for the evaluation of the acidification potential of a watershed. The amount of groundwater in streams is, in part, dependent upon the amount of event water (precipitation and/or meltwater) that infiltrates into the soil. The infiltration of snowmelt waters is controlled by ice layers at the snow-soil interface, soil properties (hydraulic conductivity, porosity), and the height of the water table. For forested watersheds with deep snowpacks, much of the initial meltwater will infiltrate if concrete ice or basal snow ice layers are absent [Price and Hendrie, 1983; Bottomley et al., 1986; Roberge and Plamondon, 1987]. High soil moisture contents (resulting from rainfall in autumn or snowmelt in midwinter) can result in the reduction of infiltration rates and the occurrence of large amounts of surface runoff in the spring snowmelt season [Kane and Stein, 1983; Price and Hendrie, 1983]. As melt proceeds, the water table rises toward the soil surface, resulting in increased groundwater discharge, interflow (lateral flow within the top soil horizons), and surface runoff [R...

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[1] The force-restore method (FRM) was originally developed for estimating diurnal fluctuations in the ground surface temperature. Because of its relatively simple parameterization, it is commonly applied in meteorological and other models for this purpose. Its application to the calculation of deeper soil temperatures, to frozen soils, and to soils under snow covers has heretofore not been possible. This study demonstrates an extension of the FRM that permits accurate estimates of seasonal variation in mean daily deep soil temperature. The extended FRM is shown to provide a lower boundary condition for the heat conduction method, permitting a combination of the two approaches that avoids some limitations of each. The combined approach provides representations of the mean daily soil temperature, soil temperature at depth in frozen soils, and ground surface temperature under a snow cover. Diurnal variations can also be calculated. The extended method and combined approaches are tested using field site measurements collected in cold weather periods in Saskatchewan, Canada, and are found to provide a reasonable representation of measurements.INDEX TERMS: 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 1823 Hydrology: Frozen ground; 1878 Hydrology: Water/energy interactions; 1863 Hydrology: Snow and ice (1827); KEYWORDS: soil temperature, force-restore method, frozen soil Citation: Hirota, T., J. W. Pomeroy, R. J. Granger, and C. P. Maule, An extension of the force-restore method to estimating soil temperature at depth and evaluation for frozen soils under snow,

The phenomenon of freezing point depression in frozen soils results in the co‐existence of ice and liquid water in soil pores at temperatures below 273.15 K (0°C), and is thought to have two causes: (a) capillary and adsorption effects, where the phase transition relationship is modified due to soil‐air‐water‐ice interactions, and (b) solute effects, where the presence of salts lowers the freezing temperature. The soil freezing characteristic curve (SFC) characterizes the relationship between liquid water content and temperature in frozen soils. Most hydrological models represent the SFC using only capillary and adsorption effects with a relationship known as the Generalized Clapeyron Equation (GCE). In this study, we develop and test a salt exclusion model for characterizing the SFC, comparing this with the GCE‐based model and a combined salt‐GCE effect model. We test these models against measured SFCs in laboratory and field experiments with diverse soil textures and salinities. We consistently found that the GCE‐based models under‐predicted freezing‐point depression. We were able to match the observations with the salt exclusion model and the combined model, suggesting that salinity is a dominant control on the SFC in real soils that always contain solutes. In modeling applications where the salinity is unknown, the soil bulk solute concentration can be treated as a single fitting parameter. Improved characterization of the SFC may result in improvements in coupled mass‐heat transport models for simulating hydrological processes in cold regions, particularly the hydraulic properties of frozen soils and the hydraulic head in frozen soils that drives cryosuction.