Abstract
ABSTRACTThe Koshi river basin is a sub‐basin of the Ganges shared among China, Nepal, and India. The river system has a high potential for investment in hydropower development and for irrigation in downstream areas. The upper part of the basin contains a substantial reserve of freshwater in the form of snow and glaciers. Climate variability, climate change, and climate extremes might impact on these reserves, and in turn impact on systems that support livelihoods, such as agriculture, biodiversity and related ecosystem services. Climatological variability and trends over the Koshi river basin were studied using RClimDex. Daily temperature data (20 stations) and precipitation data (50 stations) from 1975 to 2010 were used in the analysis. The results show that the frequency and intensity of weather extremes are increasing. The daily maximum temperature (TXx) increased by 0.1 °C decade−1 on average between 1975 and 2010 and the minimum (TNn) by 0.3 °C decade−1. The number of warm nights increased at all stations. Most of the extreme temperature indices showed a consistently different pattern in the mountains than in the Indo‐Gangetic plains, although not all results were statistically significant. The warm days (TX90p), warm nights (TN90p), warm spell duration (WSDI), and diurnal temperature range (DTR) increased at most of the mountain stations; whereas monthly maximum and minimum values of daily maximum temperature, TX90p, cool nights (TN10p), WSDI, cold spell duration indicator (CSDI), DTR decreased at the stations in the Indo‐Gangetic plains, while the number of cold days increased. There was an increase in total annual rainfall and rainfall intensity, although no clear long‐term linear trend, whereas the number of consecutive dry days increased at almost all stations. The results indicate that the risk of extreme climate events over the basin is increasing, which will increase people's vulnerability and has strong policy implications.
Highlights
Global climate change studies using observational temperature and precipitation data focused only on changes in mean values and there were few studies of climate extremes
Ji and Kang (2014) highlighted that changes in extreme temperature become increasingly pronounced from South to North China, with the most significant changes occurring on the Tibetan Plateau
The rate of increase in minimum seasonal temperature was somewhat higher in winter than in other seasons, and more often statistically significant, which is consistent with the results reported by Donat et al (2013)
Summary
Global climate change studies using observational temperature and precipitation data focused only on changes in mean values and there were few studies of climate extremes. A study done by Caesar et al (2011) considering seven stations from Nepal (among those three stations from Koshi river basin) showed that warm extremes are increasing and cold extremes are decreasing They found that trends in precipitation extremes are less spatially consistent across the region. Klein Tank et al (2006) investigated the changes in daily temperature and precipitation extremes in central and south Asia from 1961 to 2000 They found a significant increase in the percentage of warm nights and days and a decrease in the percentage of cold nights and days at 70% of stations; the daytime trends in maximum temperature extremes were smaller than the night-time trends in minimum temperature extremes. On a global scale, there was a tendency towards wetter conditions for most precipitation indices, i.e. the intensity, frequency, and duration of extreme precipitation is increasing on average
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