TITLE: Using a pair of GPS sensors to measure vegetation optical depth
(VOD) and relate it to the evaporation of intercepted rainfall in a
subalpine forest.

AUTHORS: Sean P. Burns *[1,2], Vincent Humphrey [3,4], Ethan
D. Gutmann [2], Mark S. Raleigh [5], David R. Bowling [6], and Peter
D. Blanken [1]

[1] Department of Geography, University of Colorado, Boulder, USA
[2] NSF National Center for Atmospheric Research (NCAR), Boulder, Colorado, USA
[3] Federal Office of Meteorology and Climatology MeteoSwiss, Zurich Airport, Switzerland
[4] Department of Geography, University of Zurich, Zurich, Switzerland
[5] College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA
[6] School of Biological Sciences, University of Utah, Salt Lake City, USA

ABSTRACT: Between summer of 2022 and summer 2023, we deployed a pair
of Global Navigation Satellite System (GNSS) receivers at the Niwot
Ridge Subalpine Forest AmeriFlux site (US-NR1) to estimate the
vegetation optical depth (VOD) and determine the timing of the
evaporation of intercepted water over the diel cycle.  We compared the
changes in canopy water mass inferred by the VOD measurements and
concurrent tree sway measurements and found that they both suggested
canopy evaporation lasted around 10-14 hours following a precipitation
event.  We also compared the VOD and tree sway data with the US-NR1
long-term above-canopy (21.5 m) and subcanopy (2.5 m) eddy-covariance
water vapor flux measurements and highlight regions of agreement and
disagreement between these independent measurements as-related to the
timing and amount of canopy evaporation.