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

Abstract. 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 between 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 mid-day). 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.