Hydrologic processes within mineral flat wetlands, along with their
connections to groundwater and downstream surface water in lowland agricultural
catchments are poorly understood, particularly under different land uses. In the three
field studies included in this thesis, we examined infiltration, wetland hydroperiod,
groundwater recharge dynamics, surface runoff generation, and water quality in
mineral flat wetlands using a combination of soil and hydrometric measurements,
stable isotope tracers, and water chemistry analysis. Our overarching objectives were
to examine, for mineral flat wetlands under native prairie, farmed grassland, and
restored prairie land cover: 1) how different land management influences infiltration
and wetland hydroperiod at the plot scale, 2) the effects of land use on seasonal
groundwater-surface water dynamics at the field scale, and 3) seasonal variation in
runoff sources and nutrient transport from native prairie and farmed wetlands at the
small catchment scale.
At the plot scale, our results suggest that edaphic factors, particularly those
related to soil structure, are strongly associated with wetland infiltration and overall
hydroperiod across least-altered prairie, farmed, and restored prairie mineral flat
wetlands. The hydroperiod metrics we examined were generally more sensitive to
level of site disturbance than land use alone. At the field scale, our results indicate
that, in spite of land use differences and slight variations in soil stratigraphy, many
similarities exist in overall wetland hydroperiod, water sources and evaporation rates
for mineral flat wetlands in the Willamette Valley lowlands. Isotopic evidence
suggests that the greatest degree of groundwater-surface water mixing occurs in the
upper 0.5 m of the saturated soil profile across sites under all land uses. Finally, at the
small catchment scale, farmed wetland runoff was isotopically similar to field surface
water for most of the wet season, indicating that saturation excess was an important
runoff generation process. Prairie wetland runoff was isotopically similar to upstream
water throughout the winter, and briefly similar to shallow groundwater and surface
water within the wetland in mid-spring. Throughout the wet season, elevated nitrate,
sulfate, and chloride concentrations were observed in groundwater and surface water
at the farm site, and deeper groundwater at the prairie site. Upstream-downstream
runoff chemistry remained similar throughout the wet season at the prairie site. Farm
site runoff chemistry reflected the dominant water source within the farm field
throughout the wet season. Our findings suggest that, while surface water pathways
dominate runoff from wetland flats under farm land use, large wetland flat fields have
a high potential to absorb, store, and process nutrients and agrochemicals from on-site
and nearby off-site chemical inputs.
Mineral flats that maintain wetland hydrology in spite of farm use represent a
unique balance between agricultural production and preservation of some of the water
storage and delay, and water quality-related ecosystem services once provided at a
much larger scale in the Willamette Valley lowlands. We anticipate that results of this
work will lead to better understanding of key site-scale edaphic and hydrologic factors
to consider when prioritizing and managing sites for restoration, and how site
disturbance under a variety of land uses may impact different hydrologic processes
and components of the wetland hydroperiod. Additionally, our results provide a
better understanding of how land use affects seasonal runoff generation processes in
mineral flat wetlands, and the water quality implications of modifying groundwater
and surface water connectivity between mineral flats and surrounding surface drainage
networks. / Graduation date: 2012
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/26310 |
| Date | 16 September 2011 |
| Creators | Marshall, Sarah M. (Sarah Marie) |
| Contributors | Tulos, Desireé D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Relation | Wetlands Explorer |
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