Reconstructing daily cycles of canopy water for the validation of (subdaily) microwave estimates

Monitoring rapid, subdaily vegetation water dynamics is key to address fundamental questions surrounding the role of vegetation in the water, carbon and energy cycles, and to provide essential information for detecting and monitoring droughts on local to global scales. Active and passive microwave remote sensing has been used to estimate vegetation water content (VWC), e.g. using vegetation optical depth (VOD), because of the sensitivity of microwave observables to plant dielectric properties. These estimates were used for applications such as fuel load estimation, soil moisture retrieval, crop monitoring and studies on drought propagation. The expected availability of subdaily observations from the next generation of satellites opens the opportunity to also monitor rapid vegetation water dynamics. However, one of the main challenges is the validation of subdaily microwave products.

VWC is commonly measured through destructive sampling, which is labor- and time-intensive, in particular when this has to be done multiple times per day. Here, we present a proof of concept for a more efficient validation method, using continuously measuring sensors. First, we present our latest study on reconstructing continuous records of VWC in corn, using hydrometeorological data and sparse destructive sampling [Vermunt et al., in prep.]. Second, we present the estimation of surface canopy water (dew, rainfall interception), and illustrate the value of both data sets by using them to analyse our tower-based observations of subdaily fully polarimetric L-band backscatter [1]. The results demonstrate the potential for radar to monitor rapid vegetation water dynamics.

[1] Vermunt, P. C., Khabbazan, S., Steele-Dunne, S. C., Judge, J., Monsivais-Huertero, A., Guerriero, L., & Liu, P. W. (2020). Response of Subdaily L-Band Backscatter to Internal and Surface Canopy Water Dynamics. IEEE Transactions on Geoscience and Remote Sensing.