Deciphering the role of meteorological parameters controlling the sediment load and water discharge in the Sutlej basin, Western Himalaya

Highlights

• Summer monsoon River flow is mostly augmented by the precipitation & glacial melt.

• SSC increases exponentially with temperature.

• Top 1% of extreme summer monsoon events contribute up to 45% of SSL.

Abstract

The Sutlej River basin of the western Himalaya (study area), owing to its unique geographical disposition, receives precipitation from both the Indian summer monsoon (ISM) and the Westerlies. The characteristic timing and intensity of the ISM and Westerlies, leaves a distinct footprint on the sediment load of the River. Analysis with the last forty years data, shows an increasing trend for temperature. While for precipitation during the same period, the Spiti watershed on the west has highest monthly accumulated precipitation with long term declining trend, in contrast to the other areas where an increasing trend has been observed. Thus, to probe the hydrological variability and the seasonal attributes, governed by the Westerlies and ISM in the study area, we analyzed precipitation, temperature, snow cover area (in $\%$), discharge, suspended sediment concentration (SSC) and suspended sediment load (SSL) for the period $2004$ - $2008.$ To accomplish the task, we used the available data of five hydrological stations located in the study area. Inter-annual shift in peak discharge during the monsoon period is controlled by the variation in precipitation, snow melt, glacier melt and temperature. Besides seasonal variability has been observed in generation of the sediments and its delivery to the river. Our analysis indicates, dominance of the Westerlies footprints in the hydrological parameters of the Spiti region, towards western part of the study area. While, it is observed that the hydrology of the Khab towards eastern part of the study area shows dominance of ISM. Further downstream, the hydrology of Nathpa station also shows dominance of ISM. It also emerged out that the snowmelt contribution to the River flow is mostly during the initial part, at the onset of the monsoon, while for rest and major part of the summer monsoon season, the River flow is augmented by the precipitation, glacial melt and some snow melt. We observed, that the SSC increases exponentially in response to increase in temperature and correlates positively with River discharge. The average daily SSL in the summer monsoon is many times more than that in the winter monsoon. The downstream decrease in steepness of the sediment rating curve is attributed to either a change in the River-sediment dynamics or on account of the anthropogenic forcing. The top $1\%$ of the extreme summer monsoon events (only $4$ events) in our study area contribute up to $45\%$ of SSL to the total sediment load budget. It has also been observed that the River-sediment dynamics in the upstream catchments are more vulnerable and sensitive to the extreme events in comparison to the downstream catchments. The present study for the first time gives a holistic insight in to the complex dynamics of the hydrological processes operational in the study area. The research findings would be crucial for managing the water resources of the region and the linked water and food security.

Introduction

Climate change has impacted the hydrological cycle with considerable implications for the water resources in major river basins (Shrestha et al., 2015). Research efforts have been made by various researchers to examine the effects of meteorological parameters on hydrological resources including Himalayan river (Akhtar et al., 2008; Beldring et al., 2008; Bloschl and Montanari, 2010; Goswami et al., 2006; Li et al., 2016; Dahal et al., 2020). Important to be noted that the Glaciated cover in the Himalayas has decreased from $6332$ km² to $5329$ km² between $1962$ and $2004$ (Kulkarni et al, 2010) and the mountain ranges of the Himalayas are vulnerable to retreat of the glacier (Barnett et al., 2005).

The Hindukush-Karakoram-Himalaya (HKH) in particular, is extremely important for the nourishment of several Indus-Ganga basin rivers, fed by Himalayan glaciers and snowmelt runoff. It is important for the livelihood of several billion people dependent on river water for their food security, irrigation purposes, hydropower generation, and for several other practical reasons. To understand the role of Indian summer monsoon and westerlies on the Himalayan glacier and their interlinkage with climate change several research have been carried out (Mölg et al., 2014; Kumar et al., 2020, 2021a,b, 2021a,b; Dimri and Niyogi, 2013; Kotlia et al., 2015). In the central Himalaya, ISM plays an important role in precipitation by contributing more than $80\%$ of the total annual rainfall whereas eastern and Western syntaxes record only $50\%$ of the same during summer season (Bookhagen and Burbank, 2010; Azam et al., 2014). The Westerlies in the Western parts of the Himalayas (Kumar et al., 2020; Inoue, 1978) plays a key role in defining the behavior of the glaciers (Shekhar et al., 2017; Azam et al., 2014). Precise role of water resources in Himalayan river catchments in response to meteorological factors is not well documented due to lack of data (Haritashya et al., 2006; Raina, 2009; Srivastava et al., 2014). According to a study in the western Himalayan part of Chenab basin, temperature and precipitation shows a significant relationship with discharge $(R^2=0.67)$ and sediment load $(R^2=0.86)$ (Singh et al., 2020). An important need is felt to conduct a study concerning the effects of meteorological parameters on the hydrological process in different catchments of the Himalayas. Some researchers had though made attempts to address the crucial problems in relation with different rivers of the Himalayas such as Sutlej, Beas, Chenab & Ravi (Bhutiyani et al., 2008; Singh and Jain, 2002; Wulf et al., 2012, 2016). However, an integrated research with holistic understanding of the hydrological processes of the Sutlej basin, western Himalaya has not been attempted.

The Snow cover area (SCA) in the western and Karakoram Himalaya shows an increasing trend (Kour et al., 2016; Negi et al., 2017; Shafiq, 2019; Tahir et al., 2015) whereas in rest of the HKH region it shows a decreasing trend (Immerzeel et al., 2009; Shrestha and Joshi, 2009; Gurung et al., 2011; Maskey et al., 2011). With the embark of summer, the increase in temperature leads to an escalated runoff on account of elevated snow and glacier melt whereas, in late summer, the decreased snowmelt is compensated by the increased glacier melt and the stream flow also surges to climb up due to the augmented precipitation in some parts of the Himalayas (Engelhardt et al., 2017; Mandal et al., 2020; Pratap et al., 2019; Singh et al., 2016). The Sutlej River has $16\%$ of its total discharge during Spring March, April and May (MAM) and $66\%$ during the months of June, July, August and September (JJAS) (Singh and Jain, 2002). Studies also document the interrelationship between the discharge and sediment flux for the Indian Summer Monsoon and the Westerlies-dominated catchments (Dubey et al., 2013; Rasul et al., 2008; Bhutiyani et al., 2007; Singh and Jain, 2002; Bookhagen and Burbank, 2006; Chakrapani, 2005; Bookhagen et al., 2005). Under the climatic stress and associated hydrological impacts, HKH catchment significantly affects the sediment load in its streams. Sediment budget in different HKH river catchments indicate an increasing or decreasing trend in sediment production based on its flow type, climatic setup and geological control (Lenard et al., 2020; Jain et al., 2020). Since the past two-decades, researchers have shown their interest in understanding the river sediment transport and environment-management related problems occurring across the Himalayas and in other parts of the world (Wulf et al., 2010; Akhtar et al., 2008; Miller et al., 2012).

However, data scarcity in these terrain limits our understanding on the climatic impact on water resources. Hydro-meteorological data obtained from European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation Description (ERA5), Tropical Rainfall Measuring Mission (TRMM), and Asian Precipitation - Highly-Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) are the reliable data sources in absence of hydro-meteorological observational dataset in the high-altitude areas of the HKH region. (Ren et al., 2017; Sun et al., 2017; Zhan et al., 2017). The research here integrates the derived data from the sources like ERA5, TRMM, APHRODITE with the field observed data from five hydrological stations for holistic understanding of the catchment-based hydrological processes. We adopted an integrated approach using SSC, SCA, precipitation, temperature, river discharge and other parameters. The study also exerts significant emphasis on climate driven control on the hydrological processes in the Sutlej river catchment. The research aims to mainly address the objectives: 1. To estimate the spatial and temporal variations in the meteorological (temperature, precipitation, SCA) and hydrological parameters (SSC, SSL and River discharge). 2. An assessment of the interlinkage between these variations, in perspective of the overall hydrological processes. 3. To assess the importance of extreme events in contribution of sediment load to the River.