Emerging Methods to Validate Remotely Sensed Vegetation Water Content

Satellite‐retrieved vegetation optical depth (VOD) has provided extensive insights into global plant function (such as, carbon stocks, water stress, crop yields) because of VOD's ability to monitor plant water stress and biomass at near daily temporal frequency under all‐weather conditions. However, arguably, the greatest challenge with broadly applying VOD is its lack of validation partly because of VOD's simultaneous sensitivity to plant water status and biomass changes, as well as intensive methods required to measure these properties in‐situ. Here, inspired by the recent Yao et al. (2024), https://doi.org/10.1029/2023GL107121 article, I argue that VOD estimated from global navigation satellite systems (GNSS) and land surface models with plant hydraulic schemes are two emerging methods that show promise for more widely validating satellite‐based VOD. I encourage wider adoption of these approaches to validate and further advance satellite‐based VOD research. Plain Language Summary Vegetation water content, or the amount of vegetation water mass over a given area, can be measured with satellites over large areas (36 km scales) globally at the microwave frequency. The satellite microwave index is called vegetation optical depth (VOD). Satellite VOD has provided new insights into the plant sciences such as into how grasses, crops, and forests respond to water stress. However, satellite VOD and relevant estimates of vegetation water content at 36 km scales are challenging to validate with ground‐based measurements. This is because vegetation water content changes with how saturated the plants are and how much dry plant biomass there is over a given area, both of which require labor‐intensive methods to measure. Recent technological advances are providing an opportunity to validate VOD more widely. First, field configurations using global navigation satellite systems (GNSS) can measure VOD using cost‐effective sensors that receive microwave signals from navigation satellites. Second, land surface models are now including methods to model vegetation water content and thus estimate VOD globally at the same spatial scales as satellite measurements. I recommend further development of these GNSS and model methods such that satellite VOD can be more comprehensively validated. Key Points Satellite‐based vegetation optical depth (VOD) is sensitive to plant water status and biomass, but is challenging to validate Global navigation satellite systems (GNSS) and plant hy