Comparative analysis of hydroclimatic changes in glacier-fed rivers in the Tibet- and Bhutan-Himalayas

Both glacial and climatic changes have increased the amount of water supplying Himalayan rivers. Kurichu River in the Bhutan-Himalayas (BH) and Karuxung River in the Tibetan Himalayas (TH) were selected as representative glacier-fed rivers with high glacier coverage (20.8% and 14.6%) in their respective watersheds and 20 years of hydrometeorological data. The Mann-Kendall trend test and correlation analysis were used to analyze data from the gauge stations (1600 m and 4550 m asl, respectively) and related meteorological stations (2600 m and 4450 m asl). The results indicate that there are close correlations between monthly air temperature and discharge in the studied watersheds. In contrast, annual runoff has a significant increasing trend with increasing air temperature and decreasing precipitation in TH, but annual runoff decreases in BH with increasing air temperature and decreasing precipitation. Monthly runoff was the most sensitive parameter to climatic warming, especially during autumn and winter. It was concluded that far more trends were observed than were expected to occur by chance. In the past 20 years, the mean annual air temperature has risen dramatically by 0.38 °C per decade in TH and 0.68 °C per decade in BH, and the mean annual air temperature in BH is 0.26 °C warmer than in TH. The increasing trend in runoff rate in TH varied with different times of the year, increasing by 44% for the winter flow (from October to February), 24% for the spring flow, and 27% for the summer flow, while the streamflow decreased in BH. The change in runoff in TH is mainly affected by climatic warming from April to June, while in BH, the runoff is affected by both air temperature and precipitation when the Indian Monsoon prevails, and summer rainfall has unpredictable influences on the runoff. During the non-monsoon period, the change in runoff is significantly influenced by air temperature and subsurface water, and the reduction in runoff in BH is balanced by the increasing runoff in TH.