7. Analysis of Climate Variability in a Part of Brahmaputra River Basin in India

Regional specific study of the most important climatic variables is essential to reduce the adverse effects of climate change in developing countries. In the present study, an attempt has been made to detect the trends in rainfall and temperature (past and future) time series and the possibility of any rational relationship between the trends and elevation over study region. The study was mainly focused on a part of Brahmaputra river basin in Northeast India, i.e., the Dikhow catchment (area = 3100 km²). The Dikhow catchment is a set of unique composite catchment topography comprising fairly higher altitude hills as well as alluvial plains at much lower elevation. Historically (from the year 1901 to 2002), annual precipitation and monsoon precipitation are decreasing significantly (5 % level of significance), whereas significant rise was observed on annual and seasonal basis for both maximum and minimum temperature time series. Further, during the entire time series (1901–2002), significant negative correlation was obtained between precipitation (annual and monsoon) trend magnitude and elevation of the study region. Significant rise for future precipitation was identified over the region for both A2 and B2 SRES scenarios, and winter precipitation is likely to increase more (59.18–69.44 % under A2 and 47.71–54.90 % under B2 over the catchment) than summer and monsoon precipitation. The maximum temperature has higher rate of rise than the minimum temperature for annual, summer, and monsoon temperature time series, while winter season shows higher rate of rise rather than the maximum temperature (1.02 °C over 102 years). For diurnal temperature range (DTR), monsoon season shows the significant rising which varies from 0.0016 to 0.0021 °C/year. Extreme temperature indices (TXx, TXn, TNx, and TNn) also show the significant warming picture over the catchment. Trend magnitude of extreme temperature indices and elevation shows significant relationship, while trend magnitude of the average maximum and minimum temperature and DTR does not show the noticeable connection between warming rate and elevation of the study area. Future Tmax and Tmin are increasing sharply for both A2 and B2 scenarios. A2 scenario shows higher rate than the B2 scenario.