Temporal Trends in Water Discharge Characteristics of the Large Peninsular Rivers: Assessing the Role of Climatic and Anthropogenic Factors

Abstract

In recent years, the growing water scarcity at the regional and global level has become a topic of increased discussion, particularly concerning its response to changing societal use and climate vagaries. For the Indian peninsula, the southwest monsoon accounts for >80% of annual precipitation, and rain-fed peninsular rivers serve as an important source for domestic and agricultural uses. Therefore, this study examines the trends of peninsular rivers’ annual water discharge (WQ). We computed location-specific trends of more than 300 locations covering ten large and several coastal river basins using daily WQ data provided by the Central Water Commission, India. The results are based on 50 years’ annual discharge data for 7 locations, more than 30 years of data for around 150 locations, and more than 20 years of data for additional 100 locations spread across the Indian peninsula. The discharge trends for the peninsular rivers were statically calculated. On-parametric Mann-Kendall test and linear regression analysis were applied to identify peninsular rivers with remarkable discharge variations. In the case of large rivers, the change varies between −79.1% and 29.5%. Large peninsular rivers except Mahi and Sabarmati experience a regular reduction in their annual water flux. Annual discharge chart of the rivers such as the Brahmani (−14%), Mahanadi (−12%), Godavari (−11%), and Sabarmati (−20%) illustrated a relatively small reduction, whereas a significant reduction of the annual water discharge of the Krishna river (−79%; p < 0.001), Pennar river (−78%; p < 0.05), Cauvery river (−54%; p < 0.01), Tapti river (−28%; p < 0.1) and the Narmada river (−62%; p < 0.01) observed. However, Mahi exhibited a minor increase in the water discharge. Overall, large peninsular rivers showed a 30% reduction in annual water discharge. Studies suggest that despite a significant increase in the frequency and the intensity of extreme monsoon rain events, there is no appreciable upward trend in average annual rainfall in most peninsular basins. The peninsular region hosts 45 mega and more than 3800 large dams and several thousand minor structures. Therefore, diverting water from dams for various purposes could be one of the major reasons fora decline in annual WQ in these basins. We anticipate that local or sub-basin-scale changes in rainfall patterns might be another important factor for observed water discharge trends based on location-specific discharge trends. Factor analysis, a data reduction technique, revealed that in the case of large rivers, storage capacity, rainfall, and evapotranspiration rates (3 factors) have eigenvalue>1 and explain about 87% of the variance. The statistical analysis endorses that (i) the construction of dams has significantly impacted the water annual and seasonal discharge patterns across large basins; (ii) although the precipitation is the major water source to the peninsular basins, it becomes a subordinate factor in the case of large peninsular rivers, following evapotranspiration losses.