In the icy heights of the central Himalaya, the Gangotri Glacier System (GGS) has for centuries fed the Bhagirathi, the headstream of the Ganga, India’s most iconic and culturally revered river. Now, new research shows how climate change is quietly but profoundly reshaping the flow of meltwater that sustains the river at its source — with consequences that could ripple far downstream.
A team of researchers from the Indian Institute of Technology Indore and partner institutions has reconstructed four decades of discharge from the GGS, offering the clearest picture yet of how snow, glacier ice, rainfall, and groundwater each contribute to the river’s flow, and how these proportions have shifted under a warming climate.
Published in the Journal of the Indian Society of Remote Sensing, the study deployed the high-resolution Spatial Processes in Hydrology (SPHY) model, calibrated with rare field discharge measurements (2000–2003), satellite-derived glacier mass balance data (2000–2019), and MODIS snow cover maps (2002–2020). The result is a 41-year hydrological reconstruction — the longest and most detailed yet for the Gangotri catchment.
The analysis found that snowmelt is the dominant contributor, supplying 64 per cent of annual flow, followed by glacier melt (21%), rainfall-runoff (11%), and baseflow (4%). On average, the glacier system discharges 28 cubic metres of water every second into the Bhagirathi.
“Over the last four decades, the composition of flow from the GGS is changing due to climate change, and this study offers the most detailed picture yet of how those changes have unfolded,” said lead author Parul Vinze, a PhD scholar at IIT Indore’s Glaci-Hydro-Climate Lab. While snowmelt remains the backbone of the system, its contribution has been declining over time, even as rainfall-runoff and baseflow have gradually increased. This subtle rebalancing of the basin’s water sources points to a changing climate regime.
“After 1990, the discharge peak in the Gangotri system shifted from August to July, linked to reduced winter precipitation and earlier summer melting,” Vinze explained. “The seasonal timing of flows is just as important as their volume, especially for downstream users.”
The study highlights how temperature rise and precipitation shifts are altering the dynamics of Himalayan hydrology. While mean annual temperatures across the catchment have increased, there was no significant long-term trend in precipitation. Instead, the form of precipitation is changing: more rain, less snow.
This has far-reaching implications. Statistical analysis revealed that annual discharge from the GGS is strongly controlled by summer precipitation (correlation coefficient r = 0.62) and winter temperature (r = 0.52). Simply put, wetter summers boost runoff, while warmer winters reduce snow accumulation — a double pressure on snow-fed systems.
“The Gangotri system is snowmelt-dominated, unlike the Indus basin which is more ice-dominated. That makes it especially vulnerable to shifts in snowfall and temperature patterns,” said Dr Mohd. Farooq Azam, senior author of the study, Associate Professor at IIT Indore and Senior Intervention Manager – Cryosphere at the International Centre for Integrated Mountain Development (ICIMOD).
He added: “Accurate modelling, backed by field data, is key for predicting future water availability in the Himalaya. This study provides a benchmark for how one of the most critical glacier systems of the Ganga basin is responding to climate drivers.”
The decadal breakdown tells a striking story. From 1980-1990, snowmelt contributed nearly three-quarters of annual discharge. By 2001-2010, that share had dropped to just over half, before rebounding somewhat in the last decade due to increased winter precipitation. Rainfall runoff jumped from just 2% in the 1980s to as high as 22% in the 2000s. “This increase in rainfall contribution, combined with declining snowmelt, is a signature of climate change,” said Vinze. “Even small changes in the balance matter when you are looking at water systems that millions depend on.”
Another notable finding is the shift in peak discharge. In the 1980s, August flows were highest. Since the 1990s, July has emerged as the new peak month — an advance of nearly a month. Such seasonal reshuffling can disrupt traditional irrigation schedules and hydropower operations designed around older flow patterns.
At the scale of the entire Ganga basin, meltwater contributions are smaller compared to rainfall. But at high elevations, such as the Gangotri catchment, the dominance of snow and ice melt makes communities, hydropower stations, and agriculture acutely sensitive to changes.
“Seasonality of flows from glacier-fed catchments is crucial,” Dr Azam said. “The observed changes in meltwater seasonality and runoff volume could severely affect hydropower generation and irrigation at higher elevations, even if impacts downstream are buffered by monsoonal rainfall.” The researchers emphasise that earlier studies on Gangotri, while valuable, were often limited by shorter time frames, coarser climate data, or fewer calibration datasets. This study, by spanning 1980–2020 with multiple data inputs, provides a far more robust and reliable estimate of discharge composition.
Both Vinze and Azam underline that the work is far from complete. “This study shows what’s possible when we combine field data with satellite observations and high-resolution modelling,” Vinze said. “But to refine predictions, we need sustained, long-term monitoring of discharge, snowfall, glacier mass balance, and meteorology.”
Azam agreed: “The Himalaya is a data-scarce region. Building a stronger observational network is critical if we are to anticipate and manage future water security challenges. Integrating high-resolution climate projections with field monitoring will allow us to model not just the past, but the future.”
For millions who depend on the Ganga downstream, the changes in the Gangotri catchment may seem distant. But experts caution that the headwaters are a harbinger of larger shifts to come.
“The Ganga may not be as meltwater-dependent as the Indus,” Azam said. “But at high elevations, where hydropower and irrigation systems rely on steady flows, these changes are already visible. If ignored, the risks will only intensify.”