Seasonal change in precipitation, snowpack, snowmelt, soil water and streamwater chemistry, northern Michigan
We have studied weekly precipitation, snowpack, snowmelt, soil water and streamwater chemistry throughout winter for over a decade in a small (176 ha) northern Michigan watershed with high snowfall and vegetated by 60 to 80 year-old northern hardwoods. In this paper, we examine physical, chemical, and biological processes responsible for observed seasonal change in streamwater chemistry based upon intensive study during winter 1996-1997. The objective was to define the contributions made to winter and spring streamwater chemical concentration and flux by processes as snowmelt, over-winter forest floor and surface soil mineralization, immobilization, and exchange, and subsurface flowpath. The forest floor and soil were unfrozen beneath the snowpack which permitted most snowmelt to enter. Over-winter soil mineralization and other biological processes maintain shallow subsurface ion and dissolved organic carbon (DOC) reservoirs. Small, but steady, snowmelt throughout winter removed readily mobilized soil NO3- which resulted in high over-winter streamwater concentrations but little flux. Winter soil water levels and flowpaths were generally deep which increased soil water and streamwater base cation (C(B)), HCO3-, and Si concentrations. Spring snowmelt increased soil water levels and removal of ions and DOC from the biologically active forest floor and shallow soils. The snowpack solute content was a minor component in determining streamwater ion concentration or flux during and following peak snowmelt. Exchangeable ions, weakly adsorbed anions, and DOC in the forest floor and surface soils dominated the chemical concentration and flux in soil water and streamwater. Following peak snowmelt, soil microbial immobilization and rapidly increased plant uptake of limiting nutrients removed nearly all available nitrogen from soil water and streamwater. During the growing season high evapotranspiration increased subsurface flowpath depth which in turn removed weathering products, especially C(B), HCO3-, and Si, from deeper soils. Soil water was a major component in the hydrologic and chemical budgets.
Seasonal change in precipitation, snowpack, snowmelt, soil water and streamwater chemistry, northern Michigan.
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