QUANTIFYING VARIABLE EVAPOTRANSPIRATION ACROSS A RIPARIAN ZONE, MANISTEE NATIONAL FOREST, MICHIGAN

A hydrogeological monitoring network along the forested headwaters of the White River in Manistee National Forest, Michigan, collects long-term, high-resolution data to characterize groundwater-surface water interactions. As part of that investigation, quantification of temporally and spatially variable evapotranspiration may provide information on shifts in seasonality over time as a result of climate change. The study site is underlain by a thick sequence of well-sorted, fine to medium-grained glacial outwash sand. Three piezometers form an east-west transect from the upland to riparian zone adjacent to the White River, and all wells are equipped with instrumentation to record hourly groundwater levels. The unsaturated zone thickness varies from approximately 2.3 to 1.6 m in “excessively drained” sandy soils at water-table well sites, and one piezometer penetrates through approximately 2.4 m of saturated peat to the underlying artesian confined outwash sand. Water-table wells display responses to precipitation and evapotranspiration rates that are, in part, a function of unsaturated zone thickness. An 8.8 cm rainfall event generated a groundwater level increase of 23.7 cm in the “deep” water table well, and a 36.8 cm increase in the shallow well. Specific yield values for the unconfined sand, derived from water-table fluctuations, ranged from 0.12 to 0.17. Calculated evapotranspiration rates are as much as an order of magnitude greater at the shallow well site (0.29 - 0.88cm/day) than at the deeper well site (0.03 - 0.27cm/day). Spatially variable evapotranspiration rates may also be a function of vegetation zonation in this transitional ecosystem. Long-term environmental monitoring in Manistee National Forest serves to inform resource management on the nature of water fluxes through this forested headwater reach of the White River.