Understanding of glaciers’ health calls for precise estimations of ice losses into water equivalent

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Glaciers in the upper Indus supply more than half of the river water and are experiencing significant melting. There is much discussion on the recent melting rate, which involves considerable uncertainties. A recent study reported one of these uncertainties to be caused by density assumptions for volume-to-mass change conversion, hindering estimations of precise glacier mass change.

The majority of previous studies used constant densities for volume-to-mass change estimations. The average density assumption for volume-to-mass conversion represents a potential source of error and has substantial variability in geodetic mass balance measurements. The density assumption must be used with caution as it depends on several factors, including the magnitude of elevation changes, in addition to the terrain (as our study considered). A constant density of 850 ± 60 kg/m3 in a more extended period (>3 years as suggested by Huss (2013)) may not be useful for glaciers with significant positive thickness change. Our study presents the sensitivity of ice density assumptions for volume-to-mass change conversion in the Indus basin. We used four different criteria for converting ice loss into water equivalent with different combinations of slope (below and above 20° and 25°) with ice densities of 600, 800, 850, and 900 kg/m3.

The bias caused by the average density assumption of 850 kg/m3 varied between −0.20 and +0.09 m w.e.a-1 throughout the Indus basin. The bias comes mainly from the thickness change and glacier cover above 20° and 25° slopes where 600 kg/m3 density was assumed. The glacier cover area above slopes of 25° is approximately more than 40% of the whole glacierized region, but the bias is more concentrated in the Karakoram and adjacent region, where the magnitude of thickness change of glacier ice above 25° is higher. We also found a contrasting pattern between the east and west of the Karakoram glaciers with significant bias. The bias in the Hindu Kush and Himalaya is comparable. Overall, the bias due to the constant density assumption results in a reduction of the imbalance by 35%.

At the Indus basin scale, the mass balance is extremely negative in the northwest of the southern sub-basins. The glacier mass balance in the south of the Himalaya, including the Chenab, Jhelum, and Ravi sub-basins, is noticeably negative, compared with the other sub-basins. However, the losses in these sub-basins, excluding the Chenab, only exert a small effect on the river flows because of the small glacial coverage. The negative mass balance during the study period contributed approximately +0.014 ± 0.016 mm/a1 to global sea-level equivalent. This imbalance is equivalent to one-third of the contribution by all the glaciers in the High Mountain Asia (HMA) estimated during 2000–2016 by Brun et al. (2017), while covering one-fourth of the total glacierized area. These results suggest that declining mass in the Indus basin is as important as that in the rest of the HMA.

Surface elevation change in the whole period in most of the densely glacierized regions of the sub-basins (excluding Beas, Ravi, and Satluj for better visualization) of the Indus basin

Our study shows that the glacier mass losses are mostly from the Himalaya and the Hindu Kush regions in the Indus basin. Future changes in the climate will affect glaciers and downstream river flows, and the 21st-century projections of these changes are extremely uncertain. It is therefore crucial to collect more data, including accurate glacier ice density for volume to mass conversion, to understand the mass imbalance better in the future.

Additional information

Link to the full published article: https://doi.org/10.1016/j.jhydrol.2019.04.057

Citation for the full article: Muhammad, S., Tian, L., & Khan, A. (2019). Early twenty-first century glacier mass losses in the Indus Basin constrained by density assumptions. Journal of Hydrology, 574:467–475.