Comparison of Water Dynamics in Aspen and Conifer: Implications for Ecology Water Yield Augmentation
Differences in water dynamics between deciduous aspen (Populus tremuloides) and co-occurring evergreen conifer species in the Northern Rocky Mountains result from complex physical and biological interactions. A comprehensive evaluation of individual water transfer mechanisms was used to elucidate the relative importance of several components of the hydrologic cycles of aspen and conifer, and determine which water transfer mechanisms have potential to cause differences in net water yield. Adjacent aspen and conifer stands were monitored to determine snow accumulation and ablation (snow survey), soil moisture recharge (capacitance probes) , snowpack sublimation (sublimation pan), transpiration period (thermal dissipation probes), and evapotranspiration (soil water content). Snow accumulation was 34 and 44% higher in aspen during springs of 2005 and 2006, respectively. Ablation rates in aspen (9.58 mm daft) were nearly double that of conifer (4.9 mm daft). When changes in soil moisture (due to over winter snowmelt) were combined with snow accumulation in 2006, aspen had greater potential (42-83%) for runoff and groundwater recharge. Snowpack sublimation during the ablation period was not different between open, aspen, and conifer sites and comprised <5% of snowpack losses. Extended conifer transpiration in spring did not contribute to large differences in snowmelt water yield (<2.8 mm yr- 1 ) . Summertime ET rate was higher in an aspen stand (3.6 mm day 1 ) than in an adjacent conifer stand (2. 7mm day 1) amounting to -126 mm more water lost over the growing season, but largely reflected post-ablation differences in stored soil water. The net effects of these water transfer processes could result in more watershed water yield from aspen than conifer forests. However, the difference in water yield between these two forests will largely depend on the fate of snow lost from the conifer canopy. Snow intercepted by conifer branches can be removed by the processes of sublimation (reduces water yield) and redistribution ( does not affect water yield). Future studies should focus on partitioning the ratio of sublimation to redistribution to predict hydrologic response of vegetation conversions for water yield augmentation in snow dominated watersheds .
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