Impact of climate change and watershed development on river basin hydrology using SWAT - a case study

Abstract

Climate change is considered as a global phenomenon, but investigation at the regional level is essential to understand the changes induced, and to suggest suitable adaptation strategies. This study is mainly concerned with the analysis of possible changes in the hydrology of Bharathapuzha river basin in the state of Kerala, India. Initially the trend in historic climate data was analysed to get an idea about the changes happening in the area. The trend analysis of gridded data using Mann-Kendall and t-test showed that the mean, maximum and minimum temperatures during 1951-2013 showed a significant increasing trend and the increase in mean, maximum and minimum temperatures during the period was at the rate of 0.07°C/decade, 0.14°C/decade and 0.04°C/decade respectively. Trend analysis of gridded rainfall data for the period 1971-2005 showed statistically significant decreasing trend, at the rate of 15 mm/year. Trend analysis of seasonal rainfall indicated that there was no significant trend in seasonal rainfall except during the south-west monsoon period when there was an increasing trend. To find out the best suitable climate model for the region, the downscaled reanalysis data on precipitation and temperature from five regional climate models (RCM’s) derived from different Global Climate Models (GCM’s) were compared with observed data of area on the basis of the four statistical parameters (standard deviation, correlation coefficient, coefficient of variation and centred root mean square difference). The GFDL-CM3 RCM gave better comparison with the observed data and hence was used for further data analysis. Bias in precipitation was corrected using power transformation which corrects the mean and coefficient of variation (CV) of the observations. Since temperature is approximately normally distributed, it was corrected by fitting it to the mean and standard deviation of the observations. The model data for two emission scenarios RCP4.5 and RCP8.5 and two scenario periods 2041-70 and 2071-99 were selected for the study. Comparison of the post-processed climate data to observed climate data was carried out. Based on the results obtained, the annual maximum and minimum temperatures is expected to increase in future. It is also predicted that there will bea decrease of 4 to 7 per cent in average annual rainfall during 2041-70 compared to the present day average values, whereas the decrease will be up to 10 to 15 per cent during 2071-99. To evaluate the surface runoff generation and soil erosion rates from the area, the Soil and Water Assessment Tool (SWAT) model was used. The model was calibrated and validated on a monthly basis using the observed data and it could simulate surface runoff and soil erosion to a good level of accuracy. The model evaluation statistics used for the calibration and validation periods were Nash-Sutcliffe Efficiency (NSE), Coefficient of determination (R 2 ) and PBIAS. The study demonstrated that the SWAT model can be used to predict the monthly stream flow and sediment loss from the basin. So the calibrated and validated model was then used for studying the impact of changes in climate and watershed interventions on the hydrology of the river basin. The model predicts 15 to 20 per cent decrease in stream flow by the end of the century if the worst situation of climate change continues (RCP8.5). While analysing the water balance components, it is seen that ET ranges from 15 to 22 per cent of the annual rainfall in the current scenario, while it may increase to 29 to 32 per cent in the RCP4.5 scenario and 32 to 35 per cent in RCP8.5 scenario. Lateral flow component is the lowest, comprising only 8 to 10 per cent of the total rainfall and there is no much variation for this component within the scenarios. Monthly streamflow predicted for the two periods 2041-2070 and 2071-2099 when compared with the current scenario values shows that irrespective of the scenarios, the streamflow is found to be less than that of the current scenario in almost all months. During 2046-2070, the sediment loss in RCP4.5 scenario is predicted to be much less than the RCP8.5 scenario, whereas to the end of the century, the sediment loss in RCP8.5 scenario is greater than RCP4.5 scenario in almost all years, and the annual sediment loss goes up to 7 to 9 t/ha, from the present condition of 2.5 to 4 t/ha.The impact of watershed interventions on the river hydrology was studied based on 0.05, 0.1 and 0.2 per cent increase in Water Retention Structures (WRS) in the area. The monthly stream flow simulated for the period 2007 to 2011 after adding WRS showed that even though the annual river flow decreased, the flow during the summer months (base flow) increased after adding the WRS and the percent increase in flow was highest during the months of January to April when the river has a very lean flow. Rather than utilizing the stored water in the upper reaches for irrigation and domestic purpose, the increase in summer flow will be helpful for maintaining a better environmental flow regime. Though the decrease in annual streamflow due to the WRS is small (1 to 6 per cent), the redistribution of peak flow to the summer months is significant. The annual streamflow in the current scenario is found to be decreasing with increasing capacity of the water storage structures. Streamflow prediction for the period 2041-2069 under the two scenarios RCP4.5 and RCP8.5 with WRS showed that the monthly stream flow could be increased by 5 to10 per cent due to the addition of the WRS during December to March. The water stored on account of increased WRS can be utilized for irrigation and domestic purpose in the upper reaches and at the same time the increase in summer flow will be helpful for maintaining a better environmental flow regime.