Trends in Precipitation, Snowpack, Snowmelt, Soil,
and Streamwater Chemistry in a Northern Michigan Watershed
Robert Stottlemyer1 and David Toczydlowski2
-
+
The Lake Superior Basin receives moderate atmospheric inputs of H+, NH 4 , NO3 , and SO2- . The
4
snowpack can temporarily store up to 50% of annual precipitation. In snow-dominated ecosystems,
establishing the cause of season change in streamwater chemistry is not simple. A major source of
variation is the degree to which snowmelt enters the soil. We have studied weekly precipitation,
snowpack, snowmelt, forest floor percolate, soil water, and streamwater chemistry throughout win-
ter for over a decade in the small (176 ha) northern Michigan Calumet watershed vegetated by 60- to
80-year-old northern hardwoods. In this paper, we examine physical, chemical, and biological pro-
cesses responsible for observed seasonal change in streamwater chemistry based upon intensive
study in water year 1997. Soils were unfrozen beneath the snowpack. Small, but steady, snowmelt
occurred throughout winter. Uniform cool snowpack temperatures, and late winter increases in pre-
-
+
cipitation Ca2+, NH 4 , NO3 , and SO2- concentrations and snowpack water equivalent (SWE)
4
resulted in a high snowpack ion content late in the season followed by rapid snowmelt. Cumulative
winter snowpack Ca2+ and Cl content likely reflects increased regional use of road salt. Snowmelt
ion concentrations were twice snowpack levels. Most snowmelt (90%) entered the forest floor and
surface soil. During snowmelt, soil water levels rose rapidly to the surface, concentrations of base
-
+
cations (CB) declined, and NO3 , NH 4 +, SO2- , and DOC increased. Soil lysimeter CB concentra-
4
-
+
tions increased with depth, but H+, NH 4 , and NO3 concentrations were <10% snowmelt levels.
Linkages between soil water and streamwater ion concentrations were apparent. In shallow soils,
rapid lateral movement of meltwater coupled with lesser amounts of readily weathered CB material
-
reduces soil water and streamwater CB and HCO3 concentrations. At peak streamflow, dilution
-
accounted for >90% of the decline in acid neutralization capacity. Increases in streamwater NO3
concentration before peak snowmelt and in shallow soil water during snowmelt indicate significant
-
centrations in shallow soil water and streamwater at maximum discharge are additional indicators of
high over-winter forest floor and surface soil organic minerali-zation. Streamwater SO42 concentra-
tion declined <10% during snowmelt. Soil desorption is the only process with the capacity to quickly
provide enough SO42 to offset a large streamwater con-centration decline from dilute snowmelt.
+
The watershed retained >99% of snowmelt H+ by soil exchange, and >99% of NH 4 and >90% of
-
NO3 by soil microbial and above-ground uptake while SO2- output exceeded input. Mass balance
4
analyses suggest little relationship of streamwater ion concentration and flux to precipitation or
snowmelt inputs. Seasonal change in streamwater chem-istry primarily reflected hydrology, over-
winter organic mineralization products, snowmelt/soil water movement of readily flushed ions and
DOC from shallow soils, and mineral soil weathering when soil water was deep.
1
U.S. Geological Survey, 240 W. Prospect Road, Fort Collins, Colorado 80526 USA
2
Department of Biological Science, Michigan Technological University, Houghton, Michigan 49931 USA
72
Previous Page
