The seasonal and inter-annual variation in water export from a boreal forest headwater stream

Athukorala, Dilanka Niroshan (2020) The seasonal and inter-annual variation in water export from a boreal forest headwater stream. Masters thesis, Memorial University of Newfoundland.

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Abstract

Headwater streams are important locations representing access points to assess aquatic to terrestrial fluxes and source areas that control the water and solute fluxes to downstream aquatic environments. Boreal headwater streams are in post-glacial high latitude landscapes which show a wide range of topographical heterogeneity in landscapes containing high stores of organic matter highly susceptible to climate change. Boreal streams represent important sentinels for landscape changes relevant to global climate feedbacks. Catchment hydrology of boreal streams is controlled by the precipitation regime, particularly snow dynamics and the connection between the landscape elements and hydrology. Snowfall and snowpack dynamics are important features impacted by recent climate change over the last recent decades with potential to significantly impact catchment hydrology of boreal streams and thereby terrestrial to aquatic transport of water, solute, and nutrients in these high latitude landscapes. In this study, I investigated two key boreal headwater components (low gradient wetland and pond and upland forest regions) making up one third order stream watershed situated in western Newfoundland and Labrador in Canada. The second chapter describes a series of approaches evaluated for extending the rating curves required to obtain discharge estimates from continuously monitoring water level in remote, rocky small boreal streams beyond field measurement limited by access during key high flow periods. Results of this study suggest that the use of Ferguson flow resistant equations, which help to better inform the discharge-gauge height relationship through incorporation of stream geometry including stream cross-section, stream gradient, and stream bed grain size data, significantly improves discharge estimates. The third chapter focuses on the study of water flux dynamics of a third order stream in three years contrasting in winter and snowmelt dynamics. The goal of this work was to assess how annual and seasonal discharge and runoff ratios of this small stream varied with (1) differences in snowpack dynamics (i.e. little winter melt to complete snowpack loss in mid-winter); and (2) watershed attributes defined by two key boreal watershed elements (i.e. low gradient pond and wetland dominated and hillslope forest dominated regions). Results from this work indicate that reductions in spring snowmelt events can lead to reduced discharge on an annual basis and decrease the contribution of water from upland hillslope regions that rely on snowpack water storage in this catchment. This suggests reduction in hillslope lateral flow water inputs associated with a reduced spring snowmelt event may be associated with (1) reductions in overall stream water discharge, and (2) increases in the proportion of low gradient pond or wetland contributions to the aquatic environment in these boreal landscapes. Both of these may have important implications for the boreal terrestrial-aquatic fluxes as well as downstream aquatic ecosystems in this region in a future warmer climate.

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