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Publication Title: Hydro-biogeochemical coupling at the hillslope and catchment scale
Year: 2007 Publication Type: Thesis
H. J. Andrews Publication Number: 4299
Citation: van Verseveld, Willem J. 2007. Hydro-biogeochemical coupling at the hillslope and catchment scale. Corvallis, OR: Oregon State University. 216 p. Ph.D. dissertation.
Online PDF: http://andrewsforest.oregonstate.edu/pubs/pdf/pub4299.pdf
Abstract: The specific objectives of this dissertation are to determine subsurface flow behaviors across different antecedent wetness conditions from a top-down perspective and to mechanistically assess the hydrological controls on DOC and N transport at the hillslope and catchment scale. The study area is a small catchment where hillslopes issue directly to the stream without any riparian zone modulation. Subsurface flow is measured from a 10 m wide trench. Streamflow is measured at the catchment outlet. Tree regression of subsurface flow and soil matric potential with controlling variables rainfall history and antecedent wetness show three different subsurface flow behaviors. Furthermore, unsaturated zone dynamics that follow the Darcy-Richard’s equation are a dominant control on rainfall pulse propagation. DOC and DON concentrations in subsurface flow and in stream water decrease from the transition (Fall) period to the wet (Winter-Spring) period, suggesting supply-limited DOC and DON at the seasonal scale. Specific UV absorbance (SUVA), a tool to “fingerprint” sources, is always lower in subsurface flow compared to stream water, suggesting transient groundwater (high SUVA) mixes differently with seepage groundwater (low SUVA) at the hillslope and catchment scale, even when subsurface flow and stream water are ‘in sync’ with respect to DOC and N during the wet period. The dominant flushing mechanism at the hillslope and catchment scale is vertical transport of nutrients, by ‘preferential flow’ to the soil-bedrock interface and then laterally downslope with limited supply of nutrients in the organic horizon, and higher contributions of deep soil water/seepage groundwater during the falling limb compared to the rising limb of the hydrograph. Two dominant flowpaths: vertical flow and then lateral along the soil-bedrock interface, mass transfer between a small mobile zone and a large immobile zone, and dispersive mixing, in combination with supplylimited DOC in the organic horizon/shallow layer lead to a conceptual model that resolves the double paradox: rapid mobilization of old water but variable runoff chemistry. Overall these findings result in a mechanistically plausible conceptual model how DOC and N are transported at the hillslope and catchment scale.
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