IMPACT OF CLIMATE CHANGE ON HYDROMETEOROLOGICAL VARIABLES IN BHIMA BASIN (INDIA)

Abstract

The impacts of future climate change on water resources and agriculture have become a world-wide concern these days. India and many other developing countries are expected to be seriously affected by the consequences of climate change (IPCC, 2001). Consequently, assessment of climate change impacts on hydrology and water resources are becoming an integral part of the water resources management and planning studies. In this context, assessment of the climate change impacts at basin-scale in the Bhima River basin was undertaken. Data of two basin’s namely the Bhima River basin and the Nira River basin were analysed in this study. The Bhima River, tributary of the Krishna River (India), is a major river in the central India. It flows for about 861 km through Maharashtra, Karnataka, and Telangna before joining the Krishna River near Kudlu in Raichur districts in Karnataka state covering an area of about 70,614 km2. The river originates in the Sahyadri Range (popularly known as the “Western Ghats”) in Maharashtra. The Nira River basin is a nested sub-basin of the Bhima River basin which is located in the state of Maharashtra having an area of 6900 km2 and a total length of about 180 km. Following were the major objectives of the study: i. To study the spatio-temporal variability of meteorological variables (i.e. temperature and rainfall) in the Bhima River basin. ii. To analyse the association and impact of ENSO on monsoon rainfall in the Bhima River basin. iii. To evaluate changes in meteorological variables (i.e. temperature and rainfall) under various emission scenarios in the Bhima basin using statistical downscaling approach. iv. To assess the impact of projected climate change on streamflows of the Bhima River using Variable Infiltration Capacity model. v. To analyse the suitability of gridded rainfall datasets in studying the impact of climate change using a case study of the Nira River basin. vi Assessing the impacts of climate change on water resources is a multi-step evaluation process as described by Frederick and Gleick (1999). The present study also follows the proposed methodology and the steps are enumerated below: (i) analysis of spatio-temporal changes in hydrometeorological variables (temperature, rainfall and streamflow) using Mann-Kendall/Modified Mann-Kendall test, Theil and Sen’s Median Slope test and Van Belle and Hughes’ Homogeneity of Trend test in the Bhima basin; (ii) assessment of the association of monsoon rainfall with El Niño Southern Oscillations (ENSO) using correlation analysis; (iii) projection of meteorological variables using LARS-WG under three emission scenarios (A1B, A2 and B1) during three time spans (i.e 2011-2030, 2046-2065 and 2080-2099) and their analysis using change factor approach; and, (iv) assessment of climate change impacts on streamflows using a macro-scale hydrological model Variable Infiltration Capacity (VIC) . An attempt was also made to compare the results of the climate change analysis on rainfall in the Nira basin using two rainfall datasets; observed dataset obtained from IMD and gridded dataset obtained from IMD. Analysis of Spatio-temporal Variability of Meteorological Variables The analysis of past temperature records showed decreasing trend in minimum temperatures during summer and winter seasons whereas an increasing trend during monsoon and post-monsoon seasons. However, the seasonal maximum and seasonal mean temperatures showed increasing and decreasing trends respectively. On an annual scale, minimum, maximum, and mean temperatures showed an increasing trend. The rainfall data was analysed in two time periods i.e. 1901-2004 and 1961-2004 in order to assess the effects of increased anthropogenic activities in the latter period. Analysis of the seasonal rainfall data indicated a decrease in the summer and winter rainfall during the 1901-2004 timescale whereas an increase in the monsoon and post monsoon rainfall during the same period. For the second time scale (1961-2004), the summer and monsoon rainfall showed a decreasing trend while an increasing trend was found in the rainfall in the other two seasons. vii The analysis of the data on annual scale shows that the rainfall increased over the last century (15% increase), however there is a decrease in the annual rainfall during 1961-2004 (11% decrease). This disparity is indicative of the effects of anthropogenic climate change. There is considerable spatial variation in the rainfall data over the basin. While the central parts of the basin showed an increase in rainfall (1901-2004) the upper and lower parts showed decreasing trends of rainfall (1961-2004). Analysis of Relationship of Monsoon rainfall and ENSO The monsoon rainfall showed significant correlation with the monsoon ENSO indices at majority of grid points in the basin. A significant positive correlation with the monsoon SOI and a negative correlation with monsoon MEI and N3.4 was found. On an average 15% more rainfall was received during monsoon season in La Niña phase and 9% less during El Niño phase in the basin. Impact of Climate Change on Meteorological Variables The minimum and maximum temperatures of all seasons are projected to increase with a maximum increase in the A2 scenario and a minimum increase in the B1 scenario. The highest/lowest increase in minimum and maximum temperatures were observed in winter/monsoon season. Spatial pattern of change factors of annual minimum and maximum temperatures over the basin indicated a greater rise in the middle part than other parts. There is considerable variation in the rainfall projections of the GCMs. However the seasonal rainfall was projected to increase at most of the grids. The annual rainfall projections indicated 3.2 to 18.4% increase with respect to the baseline period (i.e 1961-90). Overall, an increase in annual rainfall is projected across the entire basin, except for a small portion of the upper part of the basin. Impact of Climate Change on Future Streamflow The analyses of the results show that the flows are likely to increase across all time spans and scenarios. This is in consonance with the overall trend of future rainfall. Apart from post-monsoon, the flows are likely to increase in the other seasons. viii Comparative Analysis of Observed and Gridded datasets in Nira basin The comparison of observed and gridded rainfall data suggested that the observed data give better representation of changes than the gridded data. Thus, it was concluded that the observed data should be preferred over the gridded data whenever both are available for a basin. However, in case of limited data availability, gridded data can also prove to be reliable in terms of identifying the general trends in the hydrometeorological series, and comparing the relative differences between various scenarios. Overall, it was concluded that results of the analysis carried out using observed rainfall data are in agreement with the findings of the gridded data.