UAV-LiDAR Based Measurements of Canopy Structure and Its Impact on Land Surface Energy Simulations

The land surface is the main site of human activity and an important component of the climate system. The thermal, hydrological, and biological processes that occur on land and in the atmosphere are collectively referred to as land surface processes. The energy exchange of thermal processes is crucial for determining hydrological cycles, boundary layer development, weather, and climate in ecosystems. Due to the complexity and diversity of the underlying surfaces that make up the land surface, as well as the more complex dynamic and thermodynamic characteristics exhibited at different times, there are still many unknown processes that need further research. Model simulation is an important research method for land surface processes. Noah MP is a widely used new generation land surface model, but the current simulation has significant deviations. The canopy structure within the model grid land unit is usually represented by a basic one-dimensional model, and the canopy structure of vegetation has always been set as the "big leaf model". Due to the fact that vegetation in nature is usually composed of three-dimensional (3D) structures, and the vegetation composition and canopy geometry treated in one-dimensional (1D) in land surface models have significant differences in the representation of radiation transfer, resistance, and surface flux calculations. Therefore, in the study of land surface models, it is necessary to consider the canopy structure and corresponding physical processes. In order to study the three-dimensional effects of tree crowns, scholars extracted leaf area index and albedo parameters, and introduced concepts such as gap probability and aggregation index. Previous studies found that leaf area index, tree height, and canopy radius have significant impact on the land surface heat exchange process. Previous researchers in the field of remote sensing have greatly improved the ability to determine large-scale canopy structure variables using unmanned aerial vehicle infrared photography observation technology. Meanwhile, airborne LiDAR has the advantages of high accuracy, low impact from weather and environment, and can penetrate vegetation canopy. It can accurately obtain three-dimensional information of canopy structure, which has important practical significance for forest canopy parameter measurement.

This study utilized unmanned aerial vehicles (UAVs) equipped with LiDAR for photogrammetry, taking typical mixed forests in forest areas as the research area. Take photos using LiDAR and visible light imaging tools, and calculate the captured data to obtain representative forest canopy parameters such as leaf area index, tree height, and canopy radius for the forest area. Replace it with the original default value of the model and compare it with the validated benchmark observation dataset, etc., to evaluate the impact of LiDAR shooting parameters on the simulation and see if it better reflects the land surface heat exchange situation of the underlying surface. Through research, the following conclusions can be drawn: firstly, the accuracy of crown diameter and crown length obtained by unmanned aerial vehicle lidar is much higher than that of visible light shadow images. Moreover, the three-dimensional geometric structure of the tree crown extracted by the drone LiDAR is more accurate, and the reconstructed tree crown volume and surface area are closer to the directly estimated results. Secondly, the temperature and soil moisture simulation results under more precise geometric structure schemes perform better than the default scheme of the model. Although there are many temperature outliers and a small range of temperature disturbances, the correct solution of vegetation canopy diameter can help reduce the disturbance range and increase simulation accuracy. Finally, under more precise geometric schemes, the calculation of energy distribution between the Earth and the atmosphere can be more accurately characterized. The disturbance range of latent heat flux is relatively narrow, and the disturbance of sensible heat is also small.