This dissertation integrates a process-based hydrological investigation with an
ongoing paired-catchment study to better understand how forest harvest impacts
catchment function at multiple scales. We do this by addressing fundamental questions
related to the stocks, flows and transit times of water. Isotope tracers are used within a
top-down catchment intercomparison framework to investigate the role of geology in
controlling streamwater mean transit time and their scaling relationships with the
surrounding landscape. We found that streams draining catchments with permeable
bedrock geology at the Drift Creek watershed in the Oregon Coast Range had longer
mean transit times than catchments with poorly permeable bedrock at the HJ Andrews
Experimental Forest in the Oregon Cascades. We also found that differences in
permeability contrasts within the subsurface controlled whether mean transit time
scaled with indices of catchment topography (for the poorly permeable bedrock) or
with catchment area (for the permeable bedrock). We then investigated the process-reasons
for the observed differences in mean transit time ranges and scaling behavior
using a detailed, bottom-up approach to characterize subsurface water stores and
fluxes. We found that the mean transit times in catchments underlain by permeable
bedrock were influenced by multiple subsurface storage pools with different
groundwater ages, whereas storage in the poorly permeable catchments was limited to
the soil profile and that resulted in quick routing of excess water to the stream at the
soil bedrock interface, leading to mean transit times that were closely related to
flowpath lengths and gradients. Finally, we examined how and where forest trees
interacted with subsurface storage during the growing season using a forest
manipulation experiment, where we tested the null hypothesis that near-stream trees
alone influenced daily fluctuations in streamflow. We felled trees within this zone for
two 2.5 ha basins and combined this with isotopic tracing of tree xylem water to test if
water sources utilized by trees actively contributed to summer streamflow. We
rejected our null hypotheses and found that diel fluctuations in streamflow were not
generated exclusively in the near-stream zone. We were unable to link, isotopically,
the water sources trees were utilizing to water that was contributing to streamflow.
Our results provide new process-insights to how water is stored, extracted, and
discharged from our forested catchments in Western Oregon that will help better
explain how forest removal influences streamflow across multiple scales and
geological conditions. / Graduation date: 2012
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/26625 |
| Date | 19 December 2011 |
| Creators | Hale, V. Cody |
| Contributors | McDonnell, Jeffrey J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Relation | Oregon Explorer |
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