Using GNSS-based vegetation optical depth, tree sway motion, and eddy covariance to examine evaporation of canopy-intercepted rainfall in a subalpine forest

Recent advances in the measurement of water content within a forest have led to new possibilities to study canopy evaporation. We used a pair of Global Navigation Satellite System (GNSS) receivers (one above the canopy and one near the forest floor) to calculate the vegetation optical depth (VOD) during the warm season in a Colorado subalpine forest. One goal in our study was to compare VOD to the concurrent tree sway motion and subcanopy/above-canopy eddy covariance evapotranspiration (ET) measurements. We found that VOD increased and tree sway frequency decreased during wet periods; furthermore, both measurements exhibited a linear relationship with each other and suggested that it took around 14 h after rainfall ceased for the intercepted rainwater to fully evaporate from the canopy. On dry days, we found that tree sway was more sensitive to diel changes in internal tree water content than VOD. The ET measurements provided quantitative estimates of canopy evaporation (0.02 mm h-1 at night to 0.08 mm h-1 during midday). Following rainfall, nighttime VOD, tree sway, and ET all showed a steady (nearly constant) drying of the canopy. Variability in the VOD and tree sway measurements, comparisons with water content from the CLM4.5 land surface model, and challenges with ET measurements are also discussed.