An improved method of calculating air-water surface reflectance of skylight from water surface measured spectra

When calculating water leaving radiance or remote sensing reflectance from water surface measured spectra, the results are affected by the reflected skylight, which are the product of skylight radiance and the air-water surface reflectance of skylight. The skylight radiance can be measured over water surface by spectrometer. But the air-water surface reflectance is affected by multi factors, and is hard to be determined. Presently, there are some methods to estimate the value of air-water surface reflectance of skylight over ocean waters. However, when calculating the air-water surface reflectance of skylight over turbid inland waters, these methods all have some problems. In this paper, we develop an improved method to calculate air-water surface reflectance of skylight over turbid inland waters. This method is based on the spectral characteristics of turbid inland waters that water leaving radiance in shortwave infrared wavelength region can be assumed zero, which is verified by the data acquired in Taihu Lake on April 25 to 27, 2007. In this method, two atmospheric windows (1190 ~ 1330 nm and 1500 ~ 1700 nm) in shortwave infrared wavelength region are used. The main strategy of this method is to use the data in 1190 ~ 1330 nm wavelength region to calculate air-water surface reflectance of skylight first. When no reasonable result is obtained, the data in 1500 ~ 1700 nm wavelength region is used. With this method, the air-water surface reflectances of skylight on the 31 sampling stations are calculated. Then, water leaving radiance and remote sensing reflectance are calculated. The results of air-water surface reflectance of skylight and remote sensing reflectance on the 31 sampling stations are reasonable. Then, the advantages and limitations of the improved method are analyzed.