ABSTRACTGroundwater recharge and stream base flow are important elements in the Earth's water cycle. As such, addressing these elements is essential in the sustainable management of Earth's water resources. However, as with many environmental processes they are complex and not completely understood. Specifically, in Michigan, recently enacted laws limit groundwater withdrawals based on the base flow characteristics of these streams. However, most of these streams have no direct monitoring to determine base flow levels and thus must rely on a state-wide regression model that seeks to predict base flow levels based on characteristics of the climate and watershed. Preliminary analyses of the accuracy of this regression model show that the predicted base flows have significant errors, especially for small streams. The study involved the coupling of an unsteady recharge model and groundwater model to a 150 km2 watershed in Michigan's Hillsdale and Branch Counties that is drained by a groundwater-dependent stream that is one of Michigan's important designated and legally protected `coldwater fisheries'. The study utilized existing statewide databases with detailed data on land use and cover, soils and root zone depth, climate, glacial and bedrock geology and associated hydraulic conductivity, digital elevation model, and stream and lake topography and topology. Utilizing this data, the models simulated the processes of precipitation and snow melt, vegetative interception, surface runoff, evapotranspiration, infiltration (recharge), groundwater flow, and stream base flow. The developed modeling system was calibrated to static water level data obtained from the Michigan Department of Environmental Quality's Wellogic database.Annual base flow in the stream was shown to be significantly smaller than annual recharge-even without considering any groundwater withdrawals-primarily due to potentially significant surface seepage from streams to wetlands and lakes. Despite the seasonal variability of recharge, the variability in the base flow was less than expected because of the buffer effect of the aquifer. This buffer effect results in winter base flows being significantly lower than expected (based on classically expected recharge rates) and summer base flows being significantly higher than expected. This situation in which predicted summer base flows are higher than measured stream flows is potentially due to significant surface water evaporation and near-stream saturated soil evapotranspiration.
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