HYDROLOGICAL FLOWPATHS IN FORESTED AND DEFORESTED WATERSHEDS IN THE SOUTHWESTERN AMAZON: A MULTIVARIATE END-MEMBER MIXING ANALYSIS USING PRINCIPAL COMPONENTS

A multivariate, principal component-based analysis (PCA) of storm-induced stream water flow is used to elucidate the contribution to storm flow by various hydrological flowpaths at two adjacent forest and pasture watersheds (~ 1 ha) in the Southwestern Amazon Basin, in the Brazilian state of Rondônia. Episodic streams that stop flowing during the dry season drain both basins. Weirs, continuous stage monitors and precipitation collectors were deployed at both sites. Stage-triggered automatic samplers collected stream storm flow for a period of four hours, starting at every five minutes and every twenty minutes by the end of each event. Overland flow was collected, when generated, at one to three locations at each site. Canopy throughflow was collected at twenty locations at the forest site after each rain event. Soil solution was collected with suction lysimeters buried at 20 and 100 cm, at 20 sites (10 each) at each location every 10 days. Riparian groundwater was collected weekly at three wells proximate to drainage points at each basin. All samples were analyzed for an array of natural conservative ionic solutes and all forms of dissolved inorganic nitrogen (DIN). We address the question of how hydrological flowpaths (and N transformations) change in response to land clearing for pasture. Preliminary results from our mixing analysis suggest that a combination of riparian groundwater, soil solution and overland flow play a dominant role in storm flow generation at different hydrograph stages, hydrological conditions and rain intensities.