No abstract

ABSTRACT:The 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.

The association between the mid–latitude circulation and rainfall over the Indian region on an intraseasonal time–scale is investigated by considering 11 years (1974–1984) of Northern Hemisphere 500 hPa geopotential heights and rainfall data for the Indian summer monsoon months June through to September. On the basis of extensive correlation analysis between the geopotential heights and rainfall, it is seen that three regions over the mid–latitudes, the Manchurian region, the Algerian region and the Caspian sea region show positive correlation with rainfall over India, with higher values north of 20°N latitude. Lead and lag correlations between the heights at the locations identified above and rainfall over India reveals that some common element of low–frequency variability is influencing the mid–latitude circulation and Indian rainfall. On the interannual scale the connections between the winter–time low–frequency patterns (the Pacific/North Atlantic, the West Pacific Oscillation, the North Atlantic Oscillation and the Eurasian) and Indian summer monsoon rainfall (ISMR) are investigated. Only the West Pacific Oscillation pattern shows a significant relationship with the ISMR. Further, the interannual and the decadal variability is examined by using the Northern Hemisphere zonal index data for the period 1900–1993. Results reveal that the decadal–scale variability of the ISMR and the circulation features of the Northern Hemisphere are connected. © 1997 The Royal Meteorological Society.

No abstract

No abstract