Into the deep: using stable isotopes of trees to examine groundwater dynamics in bedrock outcrops Abstract

abstract

  • The temporal dynamics of soil water and groundwater is poorly understood, especially in the transient snow zone where both the timing and quantity of moisture inputs is extremely heterogeneous. Here, we present an approach to characterizing annual and spatial variability in relatively deep, yet likely transient groundwater using vegetation growing on bedrock outcrops to 'naturally' sample into the rock profile. In late June and early July of 2011 and 2012, we sampled and analyzed the isotopic composition (18O and 2H) of xylem water from trees growing on bedrock outcrops across a montane catchment within the Boulder Creek Critical Zone Observatory in Colorado. The isotopic composition of shallow soil water, groundwater from wells, and streamwater was also analyzed. Snow inputs to the catchment were highly variable between the two years, with 2011 having above average snowpack that persisted longer into the growing season and 2012 representing the opposite extreme, in terms of snow melt timing and amount. In 2011, we found that trees were taking up a water source that was isotopically comparable to typical snow pack values and was not representative of soil water within the upper 30 cm or of groundwater sampled from deep wells. We hypothesize that this water is deep percolation of snow melt from earlier in the season, potentially stored in bedrock outcrop fractures. In contrast, we found trees obtained over 85% of their moisture from groundwater during the dry conditions of 2012. Trees growing on five different rock outcrops all exhibited similar behavior. We used aerial LiDAR data to examine topographic characteristics that might help explain potential lateral subsurface flow paths. We characterized the study site with respect to vegetation, slope, and topographic wetness index. Results of topographic wetness index indicated that rock outcrops occurred most frequently in areas of intermediate levels of wetness. Examining the relationship between vegetation and subsurface hydrology through stable isotopes furthers understanding of both subsurface hydrology and forest water relations.

authors

publication date

  • 2012

presented at event