PRECIPITATION, SNOWPACK, STREAM-WATER ION CHEMISTRY, AND FLUX IN A NORTHERN MICHIGAN WATERSHED, 1982-1991

Input-output budgets, snowpack ion content and loss, and change in str eam-water chemistry during snowmelt were quantified for 10 years in a small watershed in upper Michigan. Annual precipitation (centimetres) and H+ and SO42- inputs declined during the decade. Precipitation SO42 - concentration decreased, with two thirds of the decline occurring in winter. Winter precipitation and H+, NH4+, NO3-, and SO42- inputs dec lined during the decade. Reduced precipitation SO42- concentration acc ounted for a third of the reduction in winter SO42- input. Up to snowp ack peak water equivalent, snowmelt totalled one third and ion loss to talled 37-58% of cumulative precipitation inputs. Midwinter snowmelt w as attributed to unfrozen soils and intermittent thaws. About 90% of s nowmelt entered unfrozen soil, where soil processes significantly alte red its chemistry. The watershed retained >99% of snowmelt H+, >95% of NH4+, and >80% of NO3-. At peak snowmelt, stream-water acid neutraliz ation capacity declined by 360 mu equiv. L(-1), with >90% resulting fr om dilution. Stream-water NO3- concentration declined >85% during snow melt and SO42- concentration changed <10%. Soil SO42- desorption was p robably the major process buffering change in stream-water SO42- conce ntration.