Deriving Leaf Area Index Reference Maps Using Temporally Continuous In Situ Data: A Comparison of Upscaling Approaches

Abstract

To further progress the validation of global leaf area index (LAI) products, temporally continuous reference data is a key requirement, as periodic field campaigns fail to adequately characterise temporal dynamics. Progress in cost-effective automated measurement techniques has been made in recent years, but appropriate upscaling methodologies are less mature. Recently, the use of multitemporal transfer functions has been proposed as a potential solution. Using data collected during an independent field campaign, we evaluated the performance of both vegetation index-based multitemporal transfer functions and a radiative transfer model (RTM)-based upscaling approach. Whether assessed using cross validation or data from the independent field campaign, the RTM-based approach provided the best performance (r 2 ≥ 0.88, RMSE ≤ 0.41, NRMSE < 13%). For upscaling temporally continuous in situ data, the ability of RTM-based approaches to account for seasonal changes in sunsensor geometry is a key advantage over vegetation index-based multitemporal transfer functions.

Fiducial Reference Measurements for Vegetation Bio-Geophysical Variables: An End-to-End Uncertainty Evaluation Framework

“…However, if in situ reference measurements and high-spatial-resolution imagery acquired over multiple dates were to be used, atmospheric correction would be required. As an increasing number sites are equipped with permanent, automated instrumentation, this will become an important consideration, since consistent time-series of high-spatial-resolution imagery are needed to upscale the temporally continuous in situ data provided by such systems [69,70]. It is worth noting that the development of a L2 RUT for the Sentinel-2 mission is the subject of a recently initiated ESA-funded project [71].…”

Potential of Automated Digital Hemispherical Photography and Wireless Quantum Sensors for Routine Canopy Monitoring and Satellite Product Validation

“…Though already useful for validating decametric satellite products such as those from Sentinel-2, to make the temporally continuous in situ data provided by the systems described in this study useful for validating moderate spatial resolution satellite products, appropriate upscaling approaches are required. Recent progress using multitemporal transfer functions and radiative transfer modelbased approaches has been made [17]- [19], and future work will evaluate these methods over our study sites.…”