Hydrologic Variability of Small, Northern Michigan Lakes Measured by the Addition of Tracers |
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Authors: | Jonathan J Cole Michael L Pace |
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Institution: | (1) Institute of Ecosystem Studies, Cary Arboretum, Millbrook, New York 12545, USA , US |
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Abstract: | The hydraulic residence time (or flushing rate of water) is a key variable for any aquatic ecosystem and is used in many
types of models and calculations. Rather than being measured directly, the hydraulic residence time is usually inferred from
estimates of watershed size, precipitation, and water yield. Such estimates can be problematic in any environment but are
especially so in environments in which flat or complex topography makes delineations of mapped watershed boundaries difficult
to discern. We added lithium bromide, (LiBr) to three small seepage lakes in the flat topography of the Upper Peninsula of
Michigan to provide an independent estimate of the water residence time. Water residence time volume/(outflow + evaporation)]
averaged 921 ± 381 (SD) days among lakes and years and ranged from 400 to 1661 days at the extremes. This variation was not
clearly related to year-to-year variation in precipitation, which was relatively constant 0.26 ± 0.06 (SD) cm day (d)−
1]. The addition of the tracer (along with measurements of lake volume) enabled us to estimate, independent from other hydrologic
information, the flow of water leaving the lakes in seepage plus surface outflow. This value, in conjunction with measurement
of precipitation and evaporation, enabled us to calculate complete water budgets for these lakes. Among lakes and years, the
groundwater input averaged 0.48 ± 0.36 cm d−
1 and accounted for 57%± 19% of total water input. This estimate was larger by 150% than that obtained by multiplying precipitation
(minus estimated evapotranspiration) times a mapped value of the watershed areas. Our analysis enables us to calculate the
relative significance of groundwater and precipitation for solutes such as phosphorus, hydrogen ion, and dissolved organic
carbon.
Received 17 February 1998; accepted 19 February 1998. |
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Keywords: | : hydraulic residence time hydrologic variability lakes Michigan tracers watershed |
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