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Figure 6: Distribution of ice-free duration for the large Eurasian lakes and reservoirs
3.4. Regulating factors of lake surface temperature
3.4.1. Climatic zone and air temperature
LWST varies according to climate zones (Figure 7). The temperature of most lakes in the dry climate zone varied between 17 and 31 °C, while for most lakes in the equatorial climatic zone LWST was between 23 and 27°C. In the warm temperate climatic zone, LWST was in the range of 20 to 27 °C. In the cold temperate zone, for most lakes (620 lakes or 56.5% of Eurasian lakes) LWST was between -0.6 and 17 °C. The average summer daytime LWST was -0.71 °C in the polar climate zone.
The water temperature of lakes on the Tibetan Plateau are generally lower than in lakes at the same latitude but in other climatic regions. As shown in Table 1, the elevation of the three Tibetan lakes selected was >4000 m, while the elevation of the three selected lakes in the Indo-Pakistani region was below 600 m (although at the same latitude). So, lakes elevation is a critical factor explaining difference in LWST. Based on our results, for every 100 meters increase in elevation, the LWST drops by about 0.6 °C. The higher the altitude, the lower the lake water surface temperature.
All of the large water bodies in the South Asian subcontinent are man-made water reservoirs (Figure 7). In the Deccan Plateau (India), the altitude of the three selected reservoirs exhibiting highest and lowest LWST was between 425-519 m. Land use type in the watersheds surrounding these reservoirs was generally similar (paddy field). Water depth was also similar (Table 1). So, the key difference among these three reservoirs was their climatic zones location. Therefore, the main driving factor of LWST in the South Asian subcontinent is climate zone, rather than lake morphometric characteristics.
Regression models were generated between in-situ air temperature and lakes LWST derived from MODIS to examine relationship between these variables (Figure 8). Positive relationships were found for daytime, nighttime and the daily mean air temperature.