Estimating Canopy Interception Water Storage with GNSS-Transmissometry

Storage of interception water in the canopy (S
c
) heavily affects measurements of vegetation optical depth (VOD) from rain, dew and fog, impeding the direct retrieval of tree physiological parameters such as biomass and plant water content. This study presents a time series decomposition of VOD from Global Navigation Satellite System-Transmissometry (GNSS-T) into biomass, plant moisture content (M
g
) and S
c
. The experiment was conducted at eddy covariance (EC) towers in two temperate forest types in Germany, over the entire vegetation period of 2023 and under fairly wet conditions. S
c
-values were 1.5 times (needleleaf) to two times (broadleaf) higher than the average diurnal M
g
cycle, allowing partitioning of interception water storage from plant water. Furthermore, we found indications that S
c
maxima did not linearly increase with precipitation, suggesting sensitivity of VOD to saturation effects when canopy interception storage reaches a maximum during strong precipitation events. Results indicate the sensitivity of VOD from GNSS-T to canopy wetness. This allows partitioning of canopy water storage from other VOD components and improves the usefulness of VOD as a remote sensing metric for forest canopy water relations. Moreover, it opens pathways to quantify S
c
and evaporation fluxes independently from EC measurements and field experiments.