Climate warming is projected to affect forest water yields but the effects are expected to vary. We investigated how forest type and age affect water yield resilience to climate warming. To answer this question, we examined the variability in historical water yields at long-term experimental catchments across Canada and the United States over 5-year cool and warm periods. Using the theoretical framework of the Budyko curve, we calculated the effects of climate warming on the annual partitioning of precipitation (P) into evapotranspiration (ET) and water yield. Deviation (d) was defined as a catchment's change in actual ET divided by P [AET/P; evaporative index (EI)] coincident with a shift from a cool to a warm period – a positive d indicates an upward shift in EI and smaller than expected water yields, and a negative d indicates a downward shift in EI and larger than expected water yields. Elasticity was defined as the ratio of interannual variation in potential ET divided by P (PET/P; dryness index) to interannual variation in the EI – high elasticity indicates low d despite large range in drying index (i.e., resilient water yields), low elasticity indicates high d despite small range in drying index (i.e., nonresilient water yields). Although the data needed to fully evaluate ecosystems based on these metrics are limited, we were able to identify some characteristics of response among forest types. Alpine sites showed the greatest sensitivity to climate warming with any warming leading to increased water yields. Conifer forests included catchments with lowest elasticity and stable to larger water yields. Deciduous forests included catchments with intermediate elasticity and stable to smaller water yields. Mixed coniferous/deciduous forests included catchments with highest elasticity and stable water yields. Forest type appeared to influence the resilience of catchment water yields to climate warming, with conifer and deciduous catchments more susceptible to climate warming than the more diverse mixed forest catchments.
Abstract:Quantification of the relationships between snow and forest cover, including its removal through logging, insects or disease and its regrowth, is a prerequisite to assessing the effects of forestry practices on streamflow from montane and boreal forest watersheds. Over a 3 year period, a juvenile and a juvenile-thinned lodgepole pine (Pinus contorta Dougl.) stand, a mature mixed Engelmann spruce (Picea engelmannii Parry), subalpine fir (Abies lasiocarpa (Hook.) Nutt) and lodgepole pine stand, and a clearcut were intensively surveyed to quantify differences in snow water equivalent (SWE). Daily snowmelt, weather conditions, and the energy balance were measured during the first year of this study. The 1 April SWE was 32% and 14% less under the mature and juvenile forests respectively than in the clearcut. No significant differences in peak SWE were measured between the juvenile and juvenile-thinned stands. Continuous snowmelt lysimeter measurements showed that snowmelt began earlier, accumulated more rapidly, and disappeared 2 to 4 days earlier in the juvenile-thinned stand than in either the unthinned juvenile stand or the clearcut. When the snowpack had disappeared from the clearcut and juvenile stands, 30% of the SWE on 1 April remained in the mature forest. The results not only show that snow accumulation and melt differ significantly between clearcut, juvenile, and mature stands, but also that snowmelt patterns vary among juvenile stands with distinct structural differences. This is due to the difference in the energy balances, dominated by radiant heat fluxes, of the four sites.
[1] This study evaluates the performance and internal structure of the distributed hydrology soil vegetation model (DHSVM) using 1998-2001 data collected at Upper Penticton Creek, British Columbia, Canada. It is shown that clear-cut snowmelt rates calculated using data-derived snow albedo curves are in agreement with observed lysimeter outflow. Measurements in a forest stand with 50% air crown closure suggest that the fraction of shortwave radiation transmitted through the canopy is 0.18-0.28 while the hemispherical canopy view factor controlling longwave radiation fluxes to the forest snowpack is estimated at 0.81 ± 0.07. DHSVM overestimates shortwave transmittance (0.50) and underestimates the view factor (0.50). An alternative forest radiation balance is formulated that is consistent with the measurements. This new formulation improves model efficiency in simulating streamflow from 0.84 to 0.91 due to greater early season melt that results from the enhanced importance of longwave radiation below the canopy. The model captures differences in canopy rainfall interception between small and large storms, tree transpiration measured over a 6-day summer period, and differences in soil moisture between a dry and a wet summer. While the model was calibrated to 1999 snow water equivalent (SWE) and hydrograph data for the untreated control basin, it successfully simulates forest and clear-cut SWE and streamflow for the 3 other years and 4 years of preharvesting and postharvesting streamflow for the second basin. Comparison of model states with the large array of observations suggests that the modified model provides a reliable tool for assessing forest management impacts in the region.
The effect of forest litter on snow surface albedo has been subject to limited study, mainly in the hardwood-dominated forests of the northeastern United States. Given the recent pine beetle infestation in Western North America and associated increases in litter production, this study examines the effects of forest litter on snow surface albedo in the coniferous forests of south-central British Columbia. Measured changes in canopy transmittance provide an indication of canopy loss or total litterfall over the winter of 2007-2008. Relationships between percent litter cover, an index of albedo, snow depth, and snow ablation during the 2008 melt season are compared between a mature, young, and clearcut coniferous stand. Results indicate a strong feedback effect between canopy loss and subsequent enhanced shortwave transmittance, and litter accumulation on the snow surface from that canopy loss. However, this relationship is confounded by other variables concurrently affecting albedo. While results suggest that a relatively small percent litter cover can have a significant effect on albedo and ablation, further research is underway to extract the litter signal from that of other factors affecting albedo, particularly snow depth.
The Upper Penticton Creek watershed experiment has collected 28 years of streamflow data from 2 small snow-dominated watersheds on the Okanagan Plateau of British Columbia, where the effects of timber harvesting on streamflow regime are of broad concern. We apply 3 empirical analysis techniques to these data to evaluate changes in streamflow regime following clear-cut logging of 47% of the 241 Creek watershed, with the adjacent 240 Creek watershed serving as an unlogged control.While logging had only a small effect on annual yield (5% increase), the results of all 3 analysis techniques confirmed a dramatic change in the timing and magnitude of April through June streamflow. A paired watershed analysis showed that during the first 7 years post-logging, average April and May water yield increased by 29% and 19%, respectively, while June and July water yield decreased by 23% and 17%, respectively. This pattern of change was confirmed by significant increases in standardised April-May monthly total water yield. Changes in the daily flow duration curves for each month also show a 67% increase in daily yields exceeded ≤10% of the time in April and a 15% increase in May. Daily yields exceeded ≤10% of the time decreased by 24% in June and 17% in July. These streamflow shifts increase the risk of channel destabilization and damage to aquatic habitat during the snowmelt season, and water shortages in the Okanagan region early in the irrigation season (June through July).
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