Spatio-Temporal Trends of Rainfall in the Ganga Basin Over the Last Century: Understanding Feedbacks and Hydrological impacts

Abstract:

The Indian Summer Monsoon (ISM) characterizes the hydrometeorological variability across the North Indian region. The ISM which originates from the Arabian Sea contributes more than 70% during monsoon (July – September) season. Here, we analyse the long-term rainfall data (1901-2019) from the Ganga river basin up to Allahabad (GRBA) to investigate its spatio-temporal variability. Since 1950, a statistically significantly decreasing trend in the ISM rain is observed in this region by past workers which has been attributed to reduction of recycled precipitation contribution driven by intensified land use and land cover (LULC) changes. Thus, a regional hydrological understanding for the GRBA is very crucial for water resource management. In this work, we have performed the Mann Kendall test on the rainfall data to understand the long-term trend in ISM rainfall in the GRBA. In addition, L-moment analysis is performed to determine the probability density function and we note that the Weibull is the best distribution to describe the rainfall frequencies of varying magnitudes and their trends. A statistically increasing (10-17 mm/y; p<0.05) trend is observed in ISM rainfall in the mountainous region since 1980, accompanied by an increase in temperature. The high, very high, and extreme rainfall events are also found to be increasing, which subsequently enhance the flash flood risk in the mountainous region. In contrast, the ISM rain in alluvium region was found to be statistically decreasing (-5 to -20 mm/y; p<0.05) with the combined influence of reduced vegetation and global oscillations, e.g. El Nino and Dipole Moment Index, in the Indian Ocean. Further, we compare rainfall signatures with river discharge and sediment load data and found significant roles of hydraulic structures e.g., dams and barrages on water and sediment flux movements. These findings provide important insights into the regional GRBA hydrology caused by natural and anthropogenic factors.