The increasing water clarity of Tibetan lakes over last 20 years according to MODIS data

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Water clarity is a comprehensive indicator of the water environment status. Studies have shown that in recent years the clarity of inland and ocean water has decreased in many parts of the world owing to the influence of climate change and human activities. However, changes in inland water clarity exclusively resulting from climate change are largely ignored because most inland water is being utilized by human beings for applications such as waterworks or aquaculture. Therefore, understanding the trends and reasons of inland water clarity under natural conditions is important to recognize the inland water environment shifts under the global changes. Because of the harsh environment, the lakes on the Tibetan Plateau are less disturbed by human activities and are ideal objects for studying water clarity variations attributed to climatic changes. Here, we describe water clarity changes as measured using the Secchi Depth (SD) in 152 big lakes with an area greater than 50 km2 based on in-situ investigations and the retrieval results from a high accuracy retrieval model (r = 0.94, P < 0.01). Most of the lakes had an SD of 3–10 m, although there was marked spatial variation and a significant positive correlation with lake area (P < 0.01). The mean SD of all 152 lakes significantly increased at a rate of 0.033 m/year (95% CI, 0.021–0.045; P < 0.01) between 2000 and 2019. Variations in lake SD were linked with temporal changes in precipitation, and related to spatial changes of water optical components including suspended matter (0.13 < R2 < 0.40, P < 0.01 or P < 0.05), fDOM (R2 = 0.24, P < 0.01) and chlorophyll-a concentration (R2 = 0.12, P < 0.01).The reconstructed lake water clarity in our study will improve the understanding of inland water clarity changes and provide basic data to study the heat exchanges between lake water and atmosphere.