Abstract:After a programme of integrated field and modelling research, hydrological processes of considerable uncertainty such as snow redistribution by wind, snow interception, sublimation, snowmelt, infiltration into frozen soils, hillslope water movement over permafrost, actual evaporation, and radiation exchange to complex surfaces have been described using physically based algorithms. The cold regions hydrological model (CRHM) platform, a flexible object-oriented modelling system was devised to incorporate these algorithms and others and to connect them for purposes of simulating the cold regions hydrological cycle over small to medium sized basins. Landscape elements in CRHM can be linked episodically in process-specific cascades via blowing snow transport, overland flow, organic layer subsurface flow, mineral interflow, groundwater flow, and streamflow. CRHM has a simple user interface but no provision for calibration; parameters and model structure are selected based on the understanding of the hydrological system; as such the model can be used both for prediction and for diagnosis of the adequacy of hydrological understanding. The model is described and demonstrated in basins from the semi-arid prairie to boreal forest, mountain and muskeg regions of Canada where traditional hydrological models have great difficulty in describing hydrological phenomena. Some success is shown in simulating various elements of the hydrological cycle without calibration; this is encouraging for predicting hydrology in ungauged basins.

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The paper summarizes the results of 5 years of study of the interaction between snowmelt infiltration (INF), snow-cover water equivalent (SWE), and soil moisture content at the time of melt (0,) for soils in the Brown and Dark Brown zones of the Canadian Prairies. It is shown that in uncracked soils the depth infiltrating water percolates into the soil during the melt sequence is, on the average, approximately 30 cm and that 0, of the frozen layer at the soil surface (0-30 cm) is the dominant factor governing the amount of snowmelt infiltration, independent of soil texture and vegetative cover. Empirical expressions, in the form of power equations and graphs, are presented describing the relationship between the variables. Because of their simplicity they have direct practical application to a number of water management problems of the region. A conceptual model for describing the snowmelt infiltration phenomenon in operational water management schemes is presented. It divides the infiltration potential of Prairie soils into three broad categories: unlimited, limited, and restricted, based on the water entry, transmission, and storage properties of the frozen ground.Les auteurs prksentent les rksultats d'une ttude de 5 ans des relations entre I'infiltration (INF) provenant de la fonte des neiges, l'kquivalent en eau (SWE) de la couverture de neige et le degrC d'humidite (0,) du sol au dCbut de la saison de fonte pour les sols bruns et bruns-foncks des Prairies canadiennes. 11s dCmontrent que pour les sols intacts (non-fissurks) I'eau #nktre le sol gel6 i une profondeure moyenne de 30 cm et que 8, de cctte couche de surface (0-30 cm) s'avkre le facteur prkdominant limitant le taux d'infiltration provenant de la fonte. cela indkpendamment de la texture du sol ou de sa couverture vkgCtale.Les relations entre les variables son[ illustrkes A I'aide d'expressions empiriques et de graphiques. Vue leur simplicitt, celles-ci se p6tent directement ; l la solution d'un nombre de pmbltmes d'amknagement des eaux dans la region. Un modkle conceptuel du phCnomtne dc I'infiltration dans les sols gc1i.s est kgalement prCsentC B cette fin. Selon ce modele le potentiel d'infiltration dans les sols des Prairies est rCparti entre trois categories selon les caractkristiques de la #nCtration, de la transmission et de I'emmagasinage de l'eau dans le sol gelC, soit: illimitk, reduit ou restreint.

Abstract:Measurements were conducted in coniferous forests of differing density, insolation and latitude to test whether air temperatures are suitable surrogates for canopy temperature in estimating sub-canopy longwave irradiance to snow. Air temperature generally was a good representation of canopy radiative temperature under conditions of low insolation. However during high insolation, needle and branch temperatures were well estimated by air temperature only in relatively dense canopies and exceeded air temperatures elsewhere. Tree trunks exceeded air temperatures in all canopies during high insolation, with the relatively hottest trunks associated with direct interception of sunlight, sparse canopy cover and dead trees. The exitance of longwave radiation from these relatively warm canopies exceeded that calculated assuming canopy temperature was equal to air temperature. This enhancement was strongly related to the extinction of shortwave radiation by the canopy. Estimates of sub-canopy longwave irradiance using either two-energy source or two thermal regime approaches to evaluate the contribution of canopy longwave exitance performed better than did estimates that used only air temperature and sky view. However, there was little evidence that such corrections are necessary under cloudy or low solar insolation conditions. The longwave enhancement effect due to shortwave extinction was important to sub-canopy longwave irradiance to snow during clear, sunlit conditions. Longwave enhancement increased with increasing solar elevation angle and decreasing air temperature. Its relative importance to longwave irradiance to snow was insensitive to canopy density. As errors from ignoring enhanced longwave contributions from the canopy accumulate over the winter season, it is important for snow energy balance computations to include the enhancement in order to better calculate snow internal energy and therefore the timing and magnitude of snowmelt and sublimation.

Abstract:An algorithm for estimating areal snowmelt infiltration into frozen soils is developed. Frozen soils are grouped into classes according to surface entry condition as: (a) Restricted-water entry is impeded by surface conditions, (b) Limited-capillary flow predominates and water entry is influenced primarily by soil physical properties, and (c) Unlimited-gravity flow predominates and most of the meltwater infiltrates. For Limited soils cumulative infiltration over time is estimated by a parametric equation from surface saturation, initial soil moisture content (water C ice), initial soil temperature and infiltration opportunity time. Total infiltration into Unlimited and Limited soils is constrained by the available water storage capacity. This constraint is also used to determine when Limited soils have thawed.The minimum spatial scale of the infiltration model is established for Limited soils by the variabilities in surface saturation, snow water equivalent, soil infiltrability, soil moisture (water C ice) and depth of soil freezing. Since snowmelt infiltration is influenced by other processes and factors that affect snow ablation, it is assumed that the infiltrability spatial scale should be consistent with the scales used to describe these variables. For open, northern, cold regions the following order in spatial scales is hypothesized: frozen ground ½ snowmelt ½ snow water equivalent ½ frozen soil infiltrability ½ soil moisture (water C ice) and snow water.For mesoscale application of the infiltration model it is recommended that the infiltrability scale be taken equal to the scale used to describe the areal extent and distribution of the water equivalent of the snowcover that covers frozen ground. Scaling the infiltrability of frozen soils in this manner allows one to exploit established landscape-stratification methodology used to derive snow accumulation means and distribution.Scaling of soil infiltrability at small scales (microscale) is complicated and requires information on the association(s) between the spatial distributions of soil moisture (water C ice) and snow water.A flow chart of the algorithm is presented.