UAV-Lidar Data Combined with Multispectral and Thermal Infrared Imagery
May Increase the Accuracy of Water Vapor Flux Estimates in Heterogeneous
Vegetated Areas.
Abstract
Advances in remote sensing technology, notably in UAV Light Detection
and Ranging (Lidar), may yield to better predictions of the implications
of land cover alterations on greenhouse gas (GHG) exchanges by
facilitating the acquisition of high-resolution information on surface
topography and vegetation structure. Although surface morphology is
fundamentally related to the magnitude of aerodynamic roughness length
(zo) and zero place displacement height (d), assigning appropriate
values to estimate energy and GHG fluxes remains challenging. In this
study, we evaluate the effectiveness of a workflow for processing
small-footprint point clouds from a UAV-Lidar system, multispectral and
thermal infrared data in order to obtain necessitated parameters for
calculation of water vapor fluxes over a mixed canopy, populated by
agricultural vegetation and evergreen trees in Denmark. Point cloud data
are classified into ground and vegetation using the progressive
triangulated irregular network densification algorithm, and are
interpolated with the kriging method to generate canopy height models
(CHMs) with 0.5 m pixel resolution. CHMs are then delineated using the
watershed algorithm to extract geometrical characteristics, orientation
and spacing of the low and high vegetation and assign them to four
morphometric roughness models that calculate zo and d. The rasterized
aerodynamic resistance maps are rectified with thermal and multispectral
orthomosaics to obtain the spatial distribution of available energy,
sensible heat and water vapor fluxes by incorporating these terms into a
surface energy balance model. The Lidar-derived geometric attributes are
validated with selected ground truth data and the modeled water vapor
fluxes are compared with eddy covariance measurements. Derivation of
more precise high-resolution aerodynamic parameters and reflectance
characteristics from UAV-based instrumentation can increase the accuracy
of water flux estimates of a canopy under surface heterogeneity
conditions and may confine the uncertainty in describing the propagation
of their long-term effects on ecosystem’s resilience.