Water property in high-altitude Qinghai Lake in China

Qinghai Lake in China, which is located at an altitude of 3260 m, is the highest inland lake among the 50 largest lakes in the world. Therefore, satellite measurement of water quality is the most important and feasible tool for routinely assessing the water property and its variations in Qinghai Lake. We derive the normalized water-leaving radiance spectra nL(w)(lambda) in Qinghai Lake from measurements of the Visible Infrared Imaging Radiometer Suite (VIIRS), after accurately computing the top-of-atmosphere Rayleigh radiances in the satellite Level-1 to Level-2 ocean color data processing. Satellite-derived nL(w)(lambda) spectra, chlorophyll-a (Chl-a) concentration, and the water diffuse attenuation coefficient at 490 nm, K-d(490), in the period of 2012-2018 are used to evaluate and understand the dynamics of water properties in Qinghai Lake. Results show that VIIRS-measured nL(w)(lambda) spectra match quite well with the in situ data, e.g., the mean and median ratios of the VIIRS-derived and in situ-measured nL(w)(lambda) at 443 nm are 0.9741 and 0.9898, respectively. Enhanced nL(w)(lambda) at 486 nm and 551 nm can be observed from satellite-derived nL(w)(lambda) spectra for the entire lake. VIIRS-derived nL(w)(lambda) spectra show significant seasonal cycles. High nL(w)(lambda) values occur in July while low values occur in April. Results also show the seasonal and interannual changes of Chl-a and K-d(490) in Qinghai Lake. Low Chl-a and K-d(490) are normally observed in April, and they typically peak in the early summer. Peculiarly, however, anomaly high values in Chl-a and K-d(490) occurred in June 2016. The analysis of empirical orthogonal functions (EOFs) shows the dominant modes for Chla and K-d(490) are interannual modes that account for about 79.1% and 31.5% of the their total variances, respectively. The second EOF mode for Chl-a and K-d(490) shows the seasonal changes, and contributes to about 6.1% and 14.0% of their respective total variances.