[1] Thirty-three snowpack models of varying complexity and purpose were evaluated across a wide range of hydrometeorological and forest canopy conditions at five Northern Hemisphere locations, for up to two winter snow seasons. Modeled estimates of snow water equivalent (SWE) or depth were compared to observations at forest and open sites at each location. Precipitation phase and duration of above-freezing air temperatures are shown to be major influences on divergence and convergence of modeled estimates of the subcanopy snowpack. When models are considered collectively at all locations, comparisons with observations show that it is harder to model SWE at forested sites than open sites. There is no universal ''best'' model for all sites or locations, but comparison of the consistency of individual model performances relative to one another at different sites (and vice versa). Calibration of models at forest sites provides lower errors than uncalibrated models at three out of four locations. However, benefits of calibration do not translate to subsequent years, and benefits gained by models calibrated for forest snow processes are not translated to open conditions.

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: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.

Abstract.One of the purposes of the Cold Regions Hydrological Modelling platform (CRHM) is to diagnose inadequacies in the understanding of the hydrological cycle and its simulation. A physically based hydrological model including a full suite of snow and cold regions hydrology processes as well as warm season, hillslope and groundwater hydrology was developed in CRHM for application in the Marmot Creek Research Basin (∼ 9.4 km 2 ), located in the Front Ranges of the Canadian Rocky Mountains. Parameters were selected from digital elevation model, forest, soil, and geological maps, and from the results of many cold regions hydrology studies in the region and elsewhere. Non-calibrated simulations were conducted for six hydrological years during the period 2005-2011 and were compared with detailed field observations of several hydrological cycle components. The results showed good model performance for snow accumulation and snowmelt compared to the field observations for four seasons during the period 2007-2011, with a small bias and normalised root mean square difference (NRMSD) ranging from 40 to 42 % for the subalpine conifer forests and from 31 to 67 % for the alpine tundra and treeline larch forest environments. Overestimation or underestimation of the peak SWE ranged from 1.6 to 29 %. Simulations matched well with the observed unfrozen moisture fluctuation in the top soil layer at a lodgepole pine site during the period 2006-2011, with a NRMSD ranging from 17 to 39 %, but with consistent overestimation of 7 to 34 %. Evaluations of seasonal streamflow during the period 2006-2011 revealed that the model generally predicted well compared to observations at the basin scale, with a NRMSD of 60 % and small model bias (1 %), while at the sub-basin scale NRMSDs were larger, ranging from 72 to 76 %, though overestimation or underestimation for the cumulative seasonal discharge was within 29 %. Timing of discharge was better predicted at the Marmot Creek basin outlet, having a Nash-Sutcliffe efficiency (NSE) of 0.58 compared to the outlets of the sub-basins where NSE ranged from 0.2 to 0.28. The Pearson product-moment correlation coefficient of 0.15 and 0.17 for comparisons between the simulated groundwater storage and observed groundwater level fluctuation at two wells indicate weak but positive correlations. The model results are encouraging for uncalibrated prediction and indicate research priorities to improve simulations of snow accumulation at treeline, groundwater dynamics, and small-scale runoff generation processes in this environment. The study shows that improved hydrological cycle model prediction can be derived from improved hydrological understanding and therefore is a model that can be applied for prediction in ungauged basins.

A model including slope effects on snow redistribution, interception and energetics was developed using the Cold Regions Hydrological Model platform, parameterized with minimal calibration and manipulated to simulate the impacts of forest disturbance on mountain hydrology. A total of 40 forest disturbance scenarios were compared with the current land cover for four simulation years. Disturbance scenarios ranged from the impact of pine beetle kill of lodgepole pine to clear-cutting of north-or south-facing slopes, forest fire and salvage logging. Pine beetle impacts were small in all cases with increases in snowmelt volume of less than 10% and streamflow volume of less than 2%. This small impact is attributed to the low and relatively dry elevations of lodgepole pine forests in the basin. Forest disturbances due to fire and clear-cutting affected much larger areas and higher elevations of the basin and were generally more than twice as effective as pine beetle in increasing snowmelt or streamflow. For complete forest cover removal by burning and salvage logging, a 45% increase in snowmelt volume was simulated; however, this only translated into a 5% increase in spring and summer streamflow volume. Forest burning with the retention of standing burned trunks was the most effective forest cover treatment for increasing streamflow (up to 8%) because of its minimizing of winter snow sublimation losses from interception and blowing snow. However, increases in streamflow volumes were almost entirely due to reductions in intercepted snow sublimation with decreasing canopy coverage. Peak daily streamflow discharges responded more strongly to forest cover disturbance than did seasonal streamflow volumes, with increases of almost 25% in peak streamflow from the removal of forest canopy by fire and the retention of standing burned trunks. Peak flow was most effectively increased by forest removal on south-facing slopes and level sites.