Increasing Flood Frequencies Under Warming in the West‐Central Himalayas

Abstract

Northern India is home to large river basins, particularly the Ganga and its Himalayan tributaries, transporting a vast amount of water to the Bay of Bengal annually (Sinha & Tandon, 2014). These river basins are also characterized as some of the world's most disastrous flood-prone regions (Sinha et al., 2022;Wasson et al., 2019), producing catastrophic floods annually. Over 18 million people are directly affected by increasing flood frequencies caused by extreme rainfall events each year (Kumar & Mishra, 2020;Rupa & Mujumdar, 2019;Swarnkar et al., 2021). The Himalayas have an extensive glacier cover (Zhang et al., 2015) that contributes meltwater to the Himalayan Rivers (Ganjoo, 2021) during pre-monsoon months (January-May). Lutz et al. (2014) observed that the rainfall contributes to streamflow significantly higher than the snowmelt, glacier melt, and baseflow in the entire Himalayan region. Their estimates also suggest that around 70% of discharge comes from the rainfall in the west-central Himalayan region. The projected increase in snow-melt contribution, however, Anand et al. (2021) will likely intensify the flood risk further.The increasing precipitation intensity in the Himalayan basins is a result of the observable hydrometeorological changes in a warming climate. Consequently, these basins frequently witness increased "flashiness" of storm events (Bookhagen, 2010;Nandargi & Dhar, 2011). For example, the western and west-central Himalayas have experienced a doubling in extreme flows during 1980 -2003(Chug et al., 2020. Several high-magnitude flooding events have been reported in the last decade, such as the 2013 and 2021 floods in Uttarakhand (Allen Abstract The west-central Himalayan River basins face significant damage caused by extreme floods annually. We selected the upper Ganga basin (UGB), the west-central Himalayan basin, to demonstrate how maximum cumulative storm and total rainfall received during monsoon control the extreme flow variability. In addition, the downscaled CMIP6 data sets were used for projections of extremes. The results suggest that historically Alaknanda basin receives a relatively higher magnitude of monsoon and storm rainfall than the other regions in the UGB, which manifested into significant extreme flow variability at the Alaknanda outlet. The extreme flows at the UGB outlet are projected to be more frequent under warming scenarios. The nonstationary flood frequency magnitudes are higher than those obtained from the stationary frequency method, implying a need for a review of return periods used in the designs of flood control structures in the region.Plain Language Summary Floods are frequent natural disasters in India that affect millions of people and disrupt socioeconomic conditions. Climate change and human interventions further exaggerate flood hazards in many regions of the country. The west and west-central Indian Himalayas are the worst flood-affected regions in recent decades. A detailed scientific analysis is, therefore, crucial for sustainable flood management. ...

Hydrological impacts of altered monsoon rain spells in the Indian Ganga basin: a century-long perspective

“…Thus, the likelihood of saturation excess runoff generation processes (Bronstert et al 2002) has been amplified in the upper Yamuna, upper Ganga, and upper Ghaghra after 1960. Consequently, we have witnessed several high magnitudes frequent floodings in the upper Ganga (Chawla et al 2018, Swarnkar et al 2021a, 2021b, Swarnkar and Mujumdar 2023, upper Yamuna (Kumar et al 2019, Tomar et al 2021, and upper Ghaghra (Houze et al 2017, Sudeepkumar et al 2023. Recently, high intensity rainfall in the upper Yamuna area caused the 2023 Yamuna flood.…”

“…Hence, the quick basin response due to the steep terrain of these upper sub-basins might have further exaggerated the flood hazards to the downstream populated regions. For example, Swarnkar and Mujumdar (2023) showed how topographic steepness has influenced the basin response of the Alaknanda and the Bhagirathi sub-basin of the upper Ganga basin. Many populated cities, including the capital of India (New Delhi; population over 11 million), are situated close to these flood-prone upper mountainous regions.…”

“…Typically, droughts in Indian regions can be linked with extended duration abnormally low rainfall over large spatial extent (Sikka 1999). However, floods are characterized as extreme hydrological disasters that occur in a relatively smaller region for a shorter duration due to excessive rainfall and bank full conditions of rivers (Dhar and Nandargi 2003, Sharma et al 2018, Swarnkar and Mujumdar 2023. Estimates from previous studies suggest that nearly 40 million hectares of Indian regions are flood-prone, and around 8 million hectares of Indian land are affected by floods every year (Ray et al 2019).…”

“…Estimates also indicate that approximately 18 million Indians are directly impacted by increased flood magnitude and frequency yearly (Rupa and Mujumdar 2019). In particular, Uttarakhand (2013), Jammu & Kashmir (2014), Tamil Nadu (2015, Gujarat (2017), andKerala (2018) were some of the catastrophic flood events that caused severe socioeconomic disbalance, loss of biodiversity, and human life (Mishra and Shah 2018, Ray et al 2019, Upadhyay et al 2020, Swarnkar et al 2021a, 2021b, Swarnkar and Mujumdar 2023. Similarly, drought events have significantly impacted socioeconomic and agricultural activities in the Indian regions.…”