Glaciers as water resources

Glaciers are perennial ice features that temporarily store freshwater at the higher altitudes and latitudes of Earth. Although comprising only a small fraction of Earth's cryosphere, the net mass loss of mountain glaciers worldwide has become one of the most widely recognized indicators of the reality of human-induced climate change. And since freshwater is essential for human society, there is a critical link between the fate of glaciers and sustainability of water resources downstream. However, it is not accurate to assume retreating glaciers comprise a generalizable water shortage to society. Many variables relating to location, watershed scale, and timing (of both seasonal availability and demand) impact glacier water resources, and present challenges for models used to project future scenarios. Assessing the relative magnitude of glacier influence on water resources requires not only careful assessments of climate control and hydrologic response, but also a critical review of the social factors that ultimately hold sway over water allocation and access [1]. The key conditions necessary for glacier formation are possible at nearly all latitudes (Figure 11.1), while regional differences in climate and proximity to human society impact the supply and exploitation of glacier water. In general, mountain topography constrains the nature of glacier water resources, from modifying climate required to sustain glaciers, to influencing all surface and subsurface flow and reservoirs. Glaciers are effective “water towers” [2], accumulating seasonal snow and ice above the snowline, while also releasing freshwater as the ice flows downward and melts. The gravitational potential energy yields secondary resource gains as water flowing from mountain regions is often utilized for hydroelectrical power generation. Nevertheless, human demands for water vary regionally and impact risks to water resources. With ongoing global climate change, glacier water resources are vulnerable to a high degree of climate sensitivity. Studies of warming climate scenarios mostly feature glacier recession that will alter the volume and variability of stream flow. The hydrological response to glacier retreat follows a characteristic sequence for regions undergoing net glacier mass loss. Under conditions of continuous retreat, glaciers generate a temporary increase in glacial-melt-derived stream runoff as they lose mass. As the remaining glacier volume diminishes, the annual runoff hydrograph reaches a maximum called “peak water” and is then followed by a persistent annual decrease [3].