HYDROLOGIC VARIABILITY OF SMALL, NORTHERN MICHIGAN LAKES MEASURED BY THE ADDITION OF TRACERS

The hydraulic residence time (or flushing rate of water) is a key vari able for any aquatic ecosystem and is used in many types of models and calculations. Rather than being measured directly, the hydraulic resi dence time is usually inferred from estimates of watershed size, preci pitation, and water yield. Such estimates can be problematic in any en vironment but are especially so in environments in which flat or compl ex topography makes delineations of mapped watershed boundaries diffic ult to discern. We added lithium bromide, (LiBr) to three small seepag e lakes in the flat topography of the Upper Peninsula of Michigan to p rovide an independent estimate of the water residence time. Water resi dence time [volume/(outflow + evaporation)] averaged 921 +/- 381 (SD) days among lakes and years and ranged from 400 to 1661 days at the ext remes. This variation was not clearly related to year-to-year variatio n in precipitation, which was relatively constant [0.26 +/- 0.06 (SD) cm day (d)(-1)]. The addition of the tracer (along with measurements o f lake volume) enabled us to estimate, independent from other hydrolog ic information, the flow of water leaving the lakes in seepage plus su rface outflow. This value, in conjunction with measurement of precipit ation 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 i nput. This estimate was larger by 150% than that obtained by multiplyi ng precipitation (minus estimated evapotranspiration) times a mapped v alue of the watershed areas. Our analysis enables us to calculate the relative significance of groundwater and precipitation for solutes suc h as phosphorus, hydrogen ion, and dissolved organic carbon.