Future Hydrological Regimes in the Upper Indus Basin: A Case Study from a High-Altitude Glacierized Catchment

Abstract

The mountain regions of the Hindu Kush, Karakoram, and Himalayas (HKH) are considered Earth's ''third pole,'' and water from there plays an essential role for downstream populations. The dynamics of glaciers in Karakoram are complex, and in recent decades the area has experienced unchanged ice cover, despite rapid decline elsewhere in the world (the Karakoram anomaly). Assessment of future water resources and hydrological variability under climate change in this area is greatly needed, but the hydrology of these high-altitude catchments is still poorly studied and little understood. This study focuses on a particular watershed, the Shigar River with the control section at Shigar (about 7000 km 2 ), nested within the upper Indus basin and fed by seasonal melt from two major glaciers (Baltoro and Biafo). Hydrological, meteorological, and glaciological data gathered during 3 years of field campaigns (2011-13) are used to set up a hydrological model, providing a depiction of instream flows, snowmelt, and ice cover thickness. The model is used to assess changes of the hydrological cycle until 2100, via climate projections provided by three state-of-the-art global climate models used in the recent IPCC Fifth Assessment Report under the representative concentration pathway (RCP) emission scenarios RCP2.6, RCP4.5, and RCP8.5. Under all RCPs, future flows are predicted to increase until midcentury and then to decrease, but remaining mostly higher than control run values. Snowmelt is projected to occur earlier, while the ice melt component is expected to increase, with ice thinning considerably and even disappearing below 4000 m MSL until 2100.

Future Water Availability from Hindukush-Karakoram-Himalaya upper Indus Basin under Conflicting Climate Change Scenarios

“…For instance, a consistent increase in the mid-21st century water availability from the Shigar sub-basin has been reported under SRES A2 scenario due to enhanced glacier melt until the glacial extent reduces to 50% [11]. Similar results of an increased future water availability due to enhanced glacier melt are reported for the Shigar sub-basin throughout the 21st century under various RCP scenarios as simulated by CMIP5 GCMs [14]. The assessment of future water availability from the Hunza, Astore and Gilgit sub-basins of the UIB under SRES A2 scenario likewise suggests an increased water availability for the intact-glacier but drastic decrease for the no-glacier realizations by the end of 21st century.…”

“…For instance, a month early snowmelt runoff peak has been reported for the UIB till the end of 21st century under SRES A2 emissions scenarios [5]. Similar is the case with the Shigar sub-basin of the UIB, for which an earlier occurrence of the snowmelt outflow has been projected under various RCP scenarios throughout the 21st century [14]. As for changes in the overall future water availability, a qualitative agreement is achieved amid the near-future scenario with intact-glacier and the no-glacier realization of the far-future scenario (reduction up to 10% and 50%, respectively), which emphasizes more on the importance of unique cooling phenomenon and its relevance than on changes in the cryosphere, while assessing the future water availability from the UIB.…”

“…Further, despite 50% reduction in the glacial extent by the mid-century, studies project consistent increase in the water availability due to enhanced glacier melt [5,6,13]. In fact, glacier melt contribution and subsequent water availability will consistently rise under warmer climates until unless the glacial cover declines to a tipping point, as clearly shown by [14]. Identifying timings of such tipping point glacial extent, however, is non-trivial, first in view of the huge multi-model spread of the projected warming [14,16], and second, the timings of occurrence of such projected warming is less likely to be realistic, and rather unknown, in view of conflicting cooling observed at present [4,17,20,21].…”

“…In contrast, employing a glacio-hydrological model over highly glacierzied (Shigar) sub-basin of the UIB, recent study has reported a negligible ice cover changes by the mid-century under warmer climates as projected under various RCP scenarios [14]. Hence, in view of somewhat stagnant UIB glacial extent at present and conducive hydroclimatic conditions under ongoing regional and global warming, reduction in the future glacial extent under more wetter and warmer climates by the mid of 21st century needs to be rationalized for the UIB.…”

“…In fact, glacier melt contribution and subsequent water availability will consistently rise under warmer climates until unless the glacial cover declines to a tipping point, as clearly shown by [14]. Identifying timings of such tipping point glacial extent, however, is non-trivial, first in view of the huge multi-model spread of the projected warming [14,16], and second, the timings of occurrence of such projected warming is less likely to be realistic, and rather unknown, in view of conflicting cooling observed at present [4,17,20,21]. Hence, for assessing the far-future water availability under the extreme-end forcing of RCP8.5 by the end of 21st century, three glacial extents are assumed regardless of their time of realization: (1) intact-glacier cover; (2) 50%-glacier cover, and; (3) no-glacier, at all.…”