WHAT MIGHT GROUNDWATER IN THE CRITICAL ZONE TELL US ABOUT CLIMATE CHANGE? DECADAL SHIFT IN THE ONSET OF EVAPOTRANSPIRATION IN A NORTHERN RIPARIAN WOODLAND

Phenology, studying the timing of seasonal ecosystem behaviors, is clearly relevant for understanding impacts of climate change. Spring leaf-out of deciduous vegetation is a phenological change that is shown to respond to climate change. Quantifying leaf-out is a challenge due to time and labor demands for direct observations, and the need for satellite imagery and data for remote sensing interpretations. “Phenocams,” fixed-location cameras to continuously record canopy character, are also used to document timing of leaf-out. Integrated phenology investigations combine satellite data, webcams, and CO2 sensors to monitor and interpret timing of leaf-out. High resolution groundwater monitoring along a headwater reach of the White River in Manistee National Forest, Michigan, was put in place a decade ago in response to ecosystem threats from bottled water extraction. That monitoring effort has become well-positioned to capture ecosystem changes that may arise from climate change. In this, and other linked upland-aquatic systems, shallow groundwater displays dynamic and pronounced diurnal declines that result from evapotranspiration (ET) by phreatophytic vegetation. Preliminary observations in the riparian corridor along the White River from 2008 – 2017 suggest that ET discharge from groundwater has started approximately 17 days earlier. Coincident with this earlier ET onset, was a measurable increase in May temperatures, the month of leaf-out for this location (43.5 ° N). Maximum, average, and minimum temperatures for the month of May during this decade increased 0.219, 0.230, and 0.240 ° C per year, respectively. The observed onset of diurnal ET fluctuations in groundwater may serve as a proxy for leaf-out of local and regional vegetation communities, as ET onset coincides with the onset of photosynthesis. Preliminary conclusions from this decade of high-resolution hydrogeological monitoring is consistent with other observations of seasonal leaf-out change, observed climate changes, and predicted responses to increased atmospheric CO2 concentrations.