to groundwater overdraft. Persistant glacial wastage and persistant overdraft are both self-terminating.
Glacial melt can affect total streamflow on a seasonal basis, and its significance is manifest in its timing, as water is stored as snow accumulation into the glacier system and the water equivalent runoff is delayed until icemelt in the late summer months of the otherwise low streamflow. Therefore, the importance of glacial melt in terms of percentage contribution to streamflow is primarily on a seasonal timescale.
An understanding of ice dynamics is required to understand the response of glaciers to climate change (Armstrong, 2010). If climate and ice dynamics result in a glacier extending farther downslope with time, the advance of the terminus2 will increase the total glacier area. A time lag on the order of decades or longer occurs between a change in climate and glacier advance or retreat, and year-to-year glacier terminus changes are likely a response to climatic events that occurred several decades or more in the past. The majority of glaciers in the HKH region have response times on the order of decades to a few centuries (Humphrey and Raymond, 1994; Johannesson et al., 1989). The response time is influenced by a glacier’s area and volume, precipitation regime, debris cover, and topographic shielding or shadowing (Kargel et al., 2011). All these factors vary widely over the HKH and High Mountain region of Asia.
The easiest glacial property to measure is the location of the terminus. Simply by walking uphill to the start of a glacier, one can locate the terminus of the glacier. The terminus position for that year can be marked in any number of ways, including a simple pile of rocks. Some glaciers have accurate records of their terminus position that go back a hundred years or more. However, this simple measurement may yield erroneous information about a glacier’s retreat and rate of retreat over short timescales of a decade or so. Prolonged retreat of the terminus of a glacier over time-scales of several decades does indicate that the glacier is retreating.
The “glaciological” method for determining glacier mass balance relies on a network of stakes and pits on the glacier surface and measuring the change in surface level between two fixed dates (an annual mass balance) or at the end of the ablation and accumulation seasons (a seasonal mass balance) (Racoviteanu et al., 2008). This method is considered the most accurate and provides the most information about spatial variation (Kaser et al., 2003). However, there are currently no long-term glaciological mass balance records for the HKH region, and few measurements of glacial mass balance at all (Kaser et al., 2006).
Mass balance can be estimated using the “geodetic method.” This indirect method consists of measuring elevation changes of the glacial surface over time from various digital elevation models constructed over the entire glacier surface (Racoviteanu et al., 2008). Because of large uncertainties, the geodetic method can only be used to estimate glacier changes at decadal or longer timescales (Kaser et al., 2003; Racoviteanu et al., 2008).
The logistical difficulties caused by the rugged topography and remote location of glaciers in the region make remote sensing techniques of particular interest. Remote sensing allows for regular monitoring of glacier area, length, surface elevation, surface flow fields, accumulation/ablation rates, albedo,3 ELA, accumulation area ratio, and mass balance gradient. A more detailed description of glaciological, hydrological, geodetic, and remote sensing glacier measurement methods is presented in Appendix C.
Glacier Extent and Retreat Rates
The HKH region is often referred to as the “third pole” because it contains the largest ice fields outside the polar regions. Some of the largest glaciers in the world are located here, including the Siachen glacier on the north slopes of the Karakoram Range, which stretches to a length of about 72 km and is the largest nonpolar glacier. Additionally, the mountains and glaciers of the Himalayas are culturally important to the region’s population (Box 2.1).
2 The glacier terminus, sometimes called the glacier snout, is the lower end of a glacier.
3 Albedo is the ratio of reflected solar radiation to incident solar radiation for a specific surface and has a value between 0 and 1. For example, fresh snow has an albedo of about 0.8 (AMS, 2000).