Abstract:Snow accumulation and melt were observed at shrub tundra and tundra sites in the western Canadian Arctic. End of winter snow water equivalent (SWE) was higher at the shrub tundra site than the tundra site, but lower than total winter snowfall because snow was removed by blowing snow, and a component was also lost to sublimation. Removal of snow from the shrub site was larger than expected because the shrubs were bent over and covered by snow during much of the winter. Although SWE was higher at the shrub site, the snow disappeared at a similar time at both sites, suggesting enhanced melt at the shrub site. The Canadian Land Surface Scheme (CLASS) was used to explore the processes controlling this enhanced melt. The spring-up of the shrubs during melt had a large effect on snowmelt energetics, with similar turbulent fluxes and radiation above the canopy at both sites before shrub emergence and after the snowmelt. However, when the shrubs were emerging, conditions were considerably different at the two sites. Above the shrub canopy, outgoing shortwave radiation was reduced, outgoing longwave radiation was increased, sensible heat flux was increased and latent flux was similar to that at the tundra site. Above the snow surface at this site, incoming shortwave radiation was reduced, incoming longwave radiation was increased and sensible heat flux was decreased. These differences were caused by the lower albedo of the shrubs, shading of the snow, increased longwave emission by the shrub stems and decreased wind speed below the shrub canopy. The overall result was increased snowmelt at the shrub site. Although this article details the impact of shrubs on snow accumulation and melt, and energy exchanges, additional research is required to consider the effect of shrub proliferation on both regional hydrology and climate.

ABSTRACT

Modeling studies examining the effect of lakes on regional and global climate, as well as studies on the influence of climate variability and change on aquatic ecosystems, are surveyed. Fully coupled atmosphere-land surface-lake climate models that could be used for both of these types of study simultaneously do not presently exist, though there are many applications that would benefit from such models. It is argued here that current understanding of physical and biogeochemical processes in freshwater systems is sufficient to begin to construct such models, and a path forward is proposed. The largest impediment to fully representing lakes in the climate system lies in the handling of lakes that are too small to be explicitly resolved by the climate model, and that make up the majority of the lake-covered area at the resolutions currently used by global and regional climate models. Ongoing development within the hydrological sciences community and continual improvements in model resolution should help ameliorate this issue.It has been long understood that lakes and reservoirs can influence local and regional climate, as open water has significantly different radiative and thermal properties compared with soil or vegetated surfaces. It is not surprising then, that various attempts have been made over the years to include the effects of terrestrial surface water in global and regional climate modeling studies, though the effects considered are usually limited to the flux exchange of moisture, heat, and momentum with the overlying atmosphere. On the other hand, the effect of climate variability and change on thermal structure, water quality, and aquatic ecosystems-also long known to be important-is generally only evaluated in the context of individual lakes or reservoirs. Even though very elaborate models exist for examining these issues, they are generally not run fully coupled with global or regional climate models, presumably because of the computational expense or the complexity of such an exercise (or both). Yet to understand the role of lakes and reservoirs in the climate system, fully coupled models must be developed in which key lacustrine processes relevant on climate timescales are integrated within the climate model. This is especially clear given the recent (and growing) awareness of the importance of lakes and reservoirs in the global carbon balance (St. Louis et al. 2000;Tranvik et al. 2009;Williamson et al. 2009). Nutrient loading, biogeochemical cycling, food webs, and ecosystems will all need to be represented, in addition to the thermal structure, mixing regimes, and ice cover that are usually considered in climate modeling studies (i.e., if lakes and reservoirs are considered at all). Modeling techniques exist for all of these, yet it would appear that a fully coupled atmosphere-land surface-lake model that would meet the needs of both the terrestrial

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computed the mean annual basin temperature (T) to be -3.4°C, the mean annual basin precipitation (P) to be 421 mm and the mean annual basin evapotranspiration (E) to be 277 mm. A simple water balance was applied to test the consistency of the P and E fields with the observed basin discharge (Q). For the 24-year period 1972 to 1995, the mean annual residual (P-E-Q) for the water balance was -28.4 mm. This residual is a combination of errors in the three water balance components and the assumption of zero annual storage. It is within the estimation errors associated with the measurement and analysis methods used. Further analysis on the correlation of Q with the total basin net surface moisture supply (P-E) showed that P and (P-E) are most strongly correlated with Q with a 3-month lag, i.e., a discharge water-year of October-September corresponds best with a (P-E) year of July-June. In examining the seasonal correlation of T and E with Q, we found that summer T was significantly correlated with annual Q but there was no significant correlation between any seasonal E or annual Q. This suggests that although the Morton model estimates of E provided a reasonable magnitude for the long-term annual basin water balance, it cannot be considered reliable for year-to-year or shorter-term estimates of the basin evapotranspiration. In examining the 1994/95 water year, it was found to be the lowest discharge year (October-September) in the observed record. This is consistent with the 3-month lagged climate data year (July-June) which for 1994/95 has the largest (P-E) anomaly; it is also the driest year since 1950 having the warmest summer months on record and the third lowest precipitation on record.To demonstrate a practical application of the climate datasets generated by this study, we constructed a multilinear regression model for annual (October-September)