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.

No abstract

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.

Abstract:Observations of land surface and snowpack energetics and mass fluxes were made over arctic shrub tundra of varying canopy height and density using radiometers, eddy covariance flux measurements, and snow mass changes from snow surveys of depth and density. Over several years, snow accumulation in the shrubs was found to be consistently higher than in sparse tundra due to greater retention of snowfall by all shrubs and wind redistribution of snowfall to tall shrubs. Where snow accumulation was highest due to snow redistribution, shrubs often became buried by the end of winter. Three classes of shrub-snow interactions were observed: tall shrubs that were exposed over snow, tall shrubs that were bent over and buried by snow, and short shrubs buried by snow. Tall shrubs buried by snow underwent 'spring-up' during melt. Though spring-up was episodic for a single shrub, over an area it was a progressive emergence from early to mid melt of vegetation that dramatically altered the radiative and aerodynamic properties of the surface. Short shrubs were exposed more rapidly once snow depth declined below shrub height, usually near the end of melt. Net radiation increased with increasing shrub due to the decreased reflectance of shortwave radiation overwhelming the increased longwave emission from relatively warm and dark shrubs. Net radiation to snow under shrubs was much smaller than that over shrubs, but was greater than that to snow with minimal shrub exposure, in this case the difference was due to downward longwave radiation from the canopy exceeding the effect of attenuated shortwave transmission through the canopy. Because of reduced turbulent transfer under shrub canopies and minimal water vapour contributions from the bare shrub branches, sublimation fluxes declined with increasing shrub exposure. In contrast, sensible heat fluxes to the shrub surface became more negative and those to the underlying snow surface more positive with increasing shrub exposure, because of relatively warm shrub branches, particularly on clear days. From well-exposed tall shrubs, both a large upward sensible heat flow from shrub to atmosphere and a downward flow that contributed substantially to snowmelt were detected. As a result of radiative and turbulent transfer in shrub canopies, melt rates increased with shrub exposure. However, shrub exposure was not a simple function of shrub height or presence, and the transition to shrub-exposed landscape depended on initial snow depth, shrub height, shrub species and cumulative melt, and this in turn controlled the melt energetics for a particular site. As a result of these complex interactions, observations over several years showed that snowmelt rates were generally, but not always, enhanced under shrub canopies in comparison with sparsely vegetated tundra.

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Abstract:Storage heterogeneity effects on runoff generation have been well documented at the hillslope or plot scale. However, diversity across catchments can increase the range of storage conditions. Upscaling the influence of small-scale storage on streamflow across the usually more heterogeneous environment of the catchment has been difficult. The objective of this study was to observe the distribution of storage in a heterogeneous catchment and evaluate its significance and potential influence on streamflow. The study was conducted in the subarctic Canadian Shield: a region with extensive bedrock outcrops, shallow predominantly organic soils, discontinuous permafrost and numerous water bodies. Even when summer runoff was generated from bedrock hillslopes with small storage capacities, intermediary locations with large storage capacities, particularly headwater lakes, prevented water from transmitting to higher order streams. The topographic bounds of the basin thus constituted the maximum potential contributing area to streamflow and rarely the actual area. Topographic basin storage had little relation to basin streamflow, but hydrologically connected storage exhibited a strong hysteretic relationship with streamflow. This relationship defines the form of catchment function such that the basin can be defined by a series of storing and contributing curves comparable with the wetting and drying curves used in relating tension and hydraulic conductivity to water content in unsaturated soils. These curves may prove useful for catchment classification and elucidating predominant hydrological processes.