Water Table Fluctuations And Runoff Generation In Three Catchment Types In A Coastal Temperate Rainforest

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Abstract
The northern Pacific perhumid coastal temperate rainforest (PCTR) extends along the Pacific coast from central British Columbia through south-central Alaska. Soil hydrology is a dominant ecosystem control in the PCTR, affecting soil formation, vegetation distribution, biogeochemical cycling, and carbon storage. Despite the importance of soil hydrology to the ecosystem, there have been few studies investigating water table dynamics and runoff generation in the PCTR. In this study, we applied two methods to investigate this interaction across a replicated set of three common sub-catchment types spanning a range of landscape units. Over the summer and fall of 2013 we monitored sub-catchment discharge, water table position, and precipitation in order to measure catchment moisture balance and the interaction between water table and runoff. There was a strong non-linear response in the wet soil catchments (fen and forested wetland), with > 80% of runoff occurring above a threshold water table position. The hydrology of the upland sites appears to be controlled by shallow rock horizons. Despite cool summertime temperatures and frequent precipitation, catchments experience a moisture deficit during the summer months that is reflected in the water table positions and catchment runoff ratios. Because of the strong dependence of runoff on water table position, changes in seasonal moisture balance that affect the water table have the potential to cause large, non-linear changes in runoff generation and biogeochemical cycling. We also made use of an existing five year dataset from the same catchments to model catchment storage / discharge relationships as ordinary, non-linear, first-order differential equations. Performance of the models varied across sub-catchments, simulating discharge reasonably well in some (Nash-Sutcliffe efficiency 0.20 - 0.45) and poorly in others (Nash-Sutcliffe < 0). This difference in model performance appears to be the result of un-accounted for subsurface flow. Despite the failure of the model to recreate catchment functioning across all sites, an initial comparison between modeled storage / discharge relationships and observed water table and discharge showed a correlation between modeled and physical behavior.
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2014-08-18
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Temperate rainforest; Dynamical modeling; Water table thresholds
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Walter, Michael Todd
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Stedinger, Jery Russell
D'Amore, David V
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Agricultural and Biological Engineering
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M.S., Agricultural and Biological Engineering
Degree Level
Master of Science
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Government Document
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dissertation or thesis
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