Glaciers and Hydrology in the South Asia Region l 33
Indus River Basin A substantial body of research exists on the importance of glaciers in the HKHK to the volume and timing of water in the Indus River and its tributaries as well as the potential impact of climate change to these glaciers. Analyses describing the role of glaciers in the hydrologic regime of these mountains are more limited, however, due in part to the relative inaccessibility of the Himalayan glaciers, which occupy altitudes between roughly 4,000 to 7,000 meters. For this reason, estimates of the potential impacts of glacial retreat are derived, rather than directly observed, using a variety of data sources, including disaggregated low-altitude databases, topography derived from satellite imagery, and simple process models of water and energy exchange in mountain regions. Principal sources of runoff from the upper Indus basin include winter precipitation and glacier melt. Winter precipitation is a key determinant of the volume of seasonal snow runoff, while summer temperature is a key determinant of the volume of glacier melt. The primary zone of melt water from both winter precipitation and glacier melt is located at between 4,000 to 5,000 meters, due to the combination of maximum-terrain surface area, maximum glacier surface area, and maximum deposition of snow water equivalent occurring there. This altitudinal zone is generally reached by the upward migration of the freezing level during the months of July and August, which are also the time of maximum runoff. More than 60,000 square kilometers of the upper Indus basin are above 5,000 meters—the estimated mean altitude of the summer-season freezing level—and it is generally assumed that significant glacial melt does not occur over most of this upper zone. The glaciers of the region, flowing outward from this zone, have an estimated surface area of approximately 20,000 square kilometers, of which between 7,000 to 8,000 square kilometers are below the summer season freezing level. This 7,000–8,000 square kilometers, in the ablation zone, is the source of the bulk of the annual glacier melt water flowing into the Indus River tributaries. The principal sources of water in the upper Indus basin are snow and ice melt. According to estimates from a recent World Bank report (World Bank 2013), glacier runoff contributes approximately 16 mean annual flow (MAF) to the total annual flow of the upper Indus basin: 12 MAF from the Karakoram Himalayas, 0.8 MAF from the western Himalayas, and 3 MAF from the Hindu Kush. This flow from glacial runoff represents an estimated 15 percent of the total flow (104 MAF) from the mountain headwaters of the Indus River, with the remaining 85 percent of the total flow representing melt water from the winter snowpack. Stream flow in the upper Indus basin is highly seasonal: approximately 80 percent of the volume of annual stream flow in the tributaries of the upper Indus basin occurs during the summer months of mid-June to mid-September (World Bank 2013).
