## 2 Data and Methods  
### Equipment and Methods  
The equipment used in the micro-spectrometer radiometric calibration is shown in Figure 1 and includes the following: the micro-spectrometer (STS-VIS, Ocean Optics company, Orlando, FL, USA), an optical fiber, a cosine receiver, a spectrometer (Trios, TriOS Mess, Rastede, Germany), a diffuse reflectance plate with different reflectance values and a target cloth, a micro-spectrometer linked to the optical fiber to measure radiance and a micro-spectrometer linked to the optical fiber and cosine receiver to measure irradiance. The diffuse reflectance plate and the target cloth provide stable and varying values of irradiance and radiance. The measured spectral ranges and spectral resolution of Trios and STS-VIS are shown in Table 1. The Trios can measure spectra from 200 nm up to 1100 nm with a spectral resolution of 3.3 nm, whereas the STS-VIS can measure spectra from 350 to 810 nm with a spectral resolution of 2.2 nm. There are three common methods of field measurements of remotely-sensed reflectance ratios in water bodies: the in-water approach [26], the above-water approach [27] and the skylight-blocked approach [28]. The in-water approach generally uses more expensive instruments, which are complex to operate and deploy, and are subject to a certain amount of self-shadowing and uncertainty in the data results. This method can generally only be used in water depths greater than 10 m, and thus, this method is very widely used in pelagic Class I waters, but rarely in inland waters. The skylight-blocked approach uses a mask to directly block the sky light from the field of view of the observation sensor, thus enabling the direct measurement of the off-water irradiance of a water body. This method avoids the geometrical errors in observation caused by the complexity of the field and the uncertainties associated with the skylight rejection method. However, it requires a continuous power supply, and therefore, cannot measure water spectra in the unmanned areas of the Tibetan Plateau. The above-water method has the advantages of a simple field operation and the low cost of field experiments, and is currently the most commonly used measurement method in the study of the spectral properties of water bodies; however, it is affected by water and weather conditions. The details of this method of measurement can be found in Figure 1 in Appendix A. The main zenith angle conditions met by the instrument are that the downward irradiance sensor should point vertically towards the sky, the radiance sensor for measuring sky light should be equal to 50\({}^{\circ}\) and the radiance sensor for measuring sky light should be equal to 140\({}^{\circ}\). The main azimuth angle conditions met by the instrument are that the angle between the left and right sunlight should be equal to 45\({}^{\circ}\). This helps avoid sun glint. This method is the easiest water body measurement method to implement in unmanned lakes on a plateau; therefore, the above-water method was selected for this paper to carry out spectral measurements of water bodies in unmanned areas on the Tibetan Plateau, and the formula for measuring R_rs_ is as follows (Equation (1)):  
\[\mathrm{R}_{rs}(\lambda)=\frac{L_{w}(\lambda)}{E_{\mathrm{d}}(\lambda)}=\frac {L_{u}(\lambda)-r_{sky}(\lambda)L_{sky}(\lambda)}{E_{\mathrm{d}}(\lambda)} \tag{1}\]  
R_rs_ is equal to the ratio of the off-water radiance to the downward irradiance, and in the above formula, \(L_{u}(\lambda)\) is the total off-water radiance, \(r_{sky}(\lambda)\) can be derived from the look-up table [29], \(L_{sky}(\lambda)\) is the skylight radiance and \(E_{\mathrm{d}}(\lambda)\) is the downward irradiance. They are measured simultaneously using a micro-spectrometer, using (a)-1 for sky radiance, (a)-2 for total off-water radiance and (a)-3 for downward irradiance, so as to avoid the effects of weather variations.