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PhD defense - Andrew Birch

Friday, May 08, 2020, 11:00AM - 1:00PM


Andrew Birch


Virtual event - contact Sue for Zoom link

Andrew Birch will defend his PhD, titled "The influence of land cover on runoff generating mechanisms and biogeochemical processes in central Panama."


The humid tropics are undergoing land use and climate change at a rate which outpaces many other parts of the world. Despite this, the impacts of these environmental changes to many hydrological and biogeochemical processes remain poorly understood. To address outstanding knowledge gaps limiting our ability to anticipate how future changes will impact water resources in these regions, this dissertation investigated hydrological and biogeochemical processes across a gradient of tropical land covers in central Panama. By analyzing relationships between stream chemistry and physical hydrological processes in three catchments of varying land cover (mature forest, young secondary tropical forest, and cattle pasture), several outstanding questions in tropical hydrology and biogeochemistry were addressed.

A combination of geochemical mixing models was used to identify dominant hydrologic flowpaths in each catchment, and the hydrologic conditions under which they became active. Pasture land cover produced infiltration excess overland flow during large wet season storm events, resulting in massive exports of event water to streams and comparatively higher new- runoff efficiencies. Contrarily, lateral subsurface flow through macropores in the upper 30cm of the soil column was the dominant hydrologic flowpath in the forested catchments, producing lower new-water runoff efficiencies and allowing for greater vertical connectivity in the soil column. The activation of these flowpaths in each catchment were found to be driven by the exceedance of rainfall magnitude and intensity thresholds, which varied according to land cover. Differences in flowpaths between the catchments produced differences in seasonal runoff dynamics between them, with the forested catchments producing only a fraction of the total new-water driven runoff produced by pasture.

To study how hydrological and biological differences between the catchments impacts biogeochemical processes like weathering and nutrient cycling, concentrations-runoff relationships and exports of eight major solutes were quantified and compared between the catchments. Despite a large disparity in total runoff production between the different land covers, their export of bedrock and atmospherically derived solutes was largely the same. The catchments produced differing loads of biologically active solutes and nutrients, a function of both differences in their biota and in the hydrologic flowpaths connecting these solutes to the stream.


Free and open to the public.