Understanding the Flood Seasonality in the Himalayan River Basin under Changing Climate

Abstract This study addresses critical knowledge gaps about climate change impacts on hydrology in a highly vulnerable and strategically important region. A sub-catchment, namely Sarasgaon in the Bhilangana River basin in the Central Himalayas, was analyzed using observed and projected future climate data for rainfall and temperature. Examining the maximum monthly rainfall data provided by the India Meteorological Department (IMD) spanning from 1901 to 2020 reveals a noteworthy shift in the peak towards earlier months, particularly up to June. This observation suggests compelling evidence of seasonal changes in flood peaks potentially associated with a warming climate scenario. On comparison of different data sets with observed data sets, it is found that the Indian Meteorological Department (IMD) precipitation dataset best matched the observations. The projected climate ensemble of the chosen dataset (NEX-GDDP) required significant correction, and quantile-based mapping was used to counter underestimation. The hydrological modeling was done with the CemaNeigeGR4J hydrological model in the airGR package in R software to project the changes in floods for past and future scenarios. The model was calibrated with observed discharge data for Sarasgaon for 2016-2018 and validated with data for 2018-2021. Nash-Sutcliffe Efficiency (NSE) values of 0. 79 and 0. 73, respectively, were obtained. The simulation was done for Historical (1971-1999), RCP 4. 5 (2071-2099), and RCP 8. 5 (2071-2099) scenarios using calibrated parameters. The results indicated the possibility of vanishing snowpacks in the study area. Due to higher temperatures and precipitation in the future scenarios concerning the Historical (1971-1999) period, the simulated discharge series of RCP 8. 5 (2071-2099) is found to be magnitude-wise higher than that of RCP 4. 5 (2071-2099), which in turn is found to be higher than that of the Historical (1971-1999) during the monsoon season. The results of the time of occurrence of annual maximum flood peaks indicate a possible early shift of the future streamflows (combination of snowmelt and rainfall-induced floods) over the study region. This shows the possible occurrences of changes in floods, though we are well aware of the high uncertainties of projected future precipitation and temperature conditions and projected future streamflows. The study highlights the potential impact of climate change on floods in the Garhwal Himalayas region, emphasizing the importance of understanding different flood-generating regimes for better preparedness against natural hazards. The findings underscore the need for ongoing research and adaptation strategies in the Himalayan region to address the uncertainties associated with projected future precipitation and temperature conditions.