Refining Estimates of Vegetation Water Storage From SMAP L-Band VOD Using GEDI Canopy Height Observations

Vegetation water storage (VWS), defined as the water mass contained in plant tissues per unit ground area, is a small but dynamic component of the terrestrial water cycle. Microwave vegetation optical depth (VOD) can retrieve VWS via a linear scaling parameter β. β has been well documented for grasses and crops, but remains uncertain in forests, where non-foliar biomass strongly influences water storage capacity. To address this gap, Soil Moisture Active Passive (SMAP) ancillary VWS estimates were rescaled with canopy height maps from the Global Ecosystem Dynamics Investigation (GEDI) LiDAR instrument. Height rescaling increased the spatial heterogeneity of VWS across and within ecosystems. For International Geosphere-Biosphere Programme (IGBP) vegetation classes, new β values were calculated from the SMAP Dual Channel Algorithm (DCA) and Multi-Temporal Dual Channel Algorithm (MT-DCA) L-band VOD products, and the new height-scaled VWS estimates. The updated β values are lower in forests and woody savannas compared to previous estimates, but greater for grasslands and croplands. Resulting estimates of the global vegetation water pool proved sensitive to both the choice of β parameterizations and VOD algorithm, ranging from 403 to 609 km3. Compared to estimates using the SMAP algorithm β parameterizations, the updated β values in this study shift a larger fraction of the global vegetation water pool towards evergreen broadleaf forests, and reduce the proportion attributed to grasslands and savannas. These changes highlight the need for spatial patterns of VWS to be aligned with those of vegetation structure.