## 2 Materials and Methods
### Design of Experiments  
Figure 2 outlines the experimental procedure. A DJI (Da-Jiang Innovations, Shenzhen, China) Maxic 3M multispectral UAV, equipped with a visible-light camera and a multispectral sensor (capturing green, red, red-edge, and near-infrared bands), was employed for data acquisition. Four diffuse reflectance standard plates (DRSPs) with known reflectance values (1.2%, 5%, 10%, and 25%) were used for radiometric calibration.  
On 12 October 2024, a synchronous measurement experiment was conducted to acquire UAV imagery and _Suaeda salsa_ biomass data using square quadrats (0.6 m \(\times\) 0.6 m). The experimental procedure comprised the following steps:  
1. Experimental Site Setup (Figure 3a): Prior to measurements, the area with the highest density of _Suaeda salsa_ was identified and designated as the initial quadrat, representing the maximum biomass. The quadrat was then secured, and DRSPs were positioned in an adjacent open area for calibration purposes.  
Figure 1: UAV imagery of the experimental area in Wafangdian City, northern China.  
* UAV Camera Parameter Configuration and Initial Imaging: To ensure consistent illumination throughout data collection, the visible-light camera's exposure settings--including aperture, shutter speed, and ISO--were fixed before flight. A single-frame shooting mode was employed to capture high-resolution imagery of the _Suada salsa_ quadrat.
* Sequential UAV Imaging and Biomass Removal (Figure 3b): The UAV ascended to a fixed altitude of approximately 25 m to capture an initial image of the quadrat at its maximum _Suaeda salsa_ biomass. Subsequently, portions of _Suaeda salsa_ within the quadrat were evenly removed to incrementally reduce biomass. After each removal, another UAV image was captured, and the removed plant material was sealed in labeled bags for subsequent biomass analysis. This iterative process of incremental _Suaeda salsa_ removal, UAV image acquisition, and sample storage continued until the quadrat was completely cleared.
* Laboratory Processing of Biomass Samples: Upon the completion of UAV imaging, all plant samples were promptly transported to the laboratory. Roots were thoroughly rinsed with clean water to remove soil residues. Each sample was then wrapped in aluminum foil and oven-dried to a constant weight, and its dry biomass was meticulously recorded according to the sample labels.
* Image Processing: For each UAV image, the quadrat-corresponding area was accurately extracted. The hue angle was subsequently calculated for this region, and the average hue angle was determined for further analysis.
* Model Dataset Construction: Based on the sequential biomass removal, the _Suada salsa_ biomass within the quadrat (expressed in kg/m\({}^{2}\)) was calculated for each UAV capture. This yielded a robust modeling dataset linking varying biomass levels with their corresponding mean hue angles.  
Figure 3: _Cont._  
Following the quadrat-based _Suaeda salsa_ biomass experiment, a comprehensive UAV aerial survey of the entire study area was conducted on October 14, 2024. Similarly to the previous setup, DRSPs were deployed, and the camera's exposure settings were fixed to ensure consistent lighting conditions. The acquired UAV imagery served two primary purposes: (1) to determine the optimal hue angle threshold for _Suaeda salsa_ identification and (2) to extract _Suaeda salsa_ biomass information across the entire study area.