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2 Physical Geography G laciers play an important role in the global snow and through metamorphic processes, the density hydrological cycle, through the storage of of snowpack gradually increases. Snow that does not water for thousands of years (Figure 2.1). melt is carried over to the next season, where it can be Water is stored in a series of reservoirs, including the buried by subsequent snowfall. Snow that is older than ocean, lakes, groundwater, atmosphere, snowpack, and a year but not yet glacial ice is called "firn" or "névé." glaciers. Water movement is driven by energy: warmer The density of firn gradually increases over time, and air temperatures speed up the water cycle; colder air eventually the air trapped in pockets or bubbles is no temperatures slow the water cycle down. Water move- longer in contact with the atmosphere. The firn has ment from the atmosphere to the oceans and continents become glacial ice. Local climate determines the rate at occurs as precipitation, including snow, sleet, and other which seasonal snow changes to glacial ice (cf. Cuffey forms of solid precipitation. Snow that accumulates for and Paterson, 2010). many years may turn into a glacier. This chapter reviews Glaciers move by gravitational processes, including the current understanding of Hindu-Kush Himalayan internal deformation caused by shear stress imposed by (HKH) glaciers in the context of the modern climate overlying ice and snow, and potentially by basal sliding setting, impacts of aerosols and black carbon1 on the on a layer of liquid or quasi-liquid water. Ice masses energy budget affecting the glaciers, what paleoclimate can flow down slopes or across flat terrain because of records can tell us about current regional climate condi- the pressure produced by overlying snow and ice. Once tions, regional hydrology, and physical hazards in the a mass of ice flows as a solid, it is considered to be a Himalayas. glacier. Patches of ice and snow that do not flow are not glaciers. GLACIAL MASS BALANCE The fundamentals of glacial behavior can be read- ily understood by recognizing that glaciers have both Glacial ice is characterized by (a) a density between a zone of accumulation in which the volume of the about 830 and 920 kg m-3 (83 to 92 percent water glacier grows and a zone of ablation in which volume content) and (b) air that is trapped in bubbles within is lost. During the accumulation season (summer in the ice and no longer in contact with the atmosphere. the eastern HKH and winter in the western HKH), When snow falls on a surface, it initially has a density of a glacier gains mass. During the melt season (summer 50 to 70 kg m-3 and within a few days has a density on in both the eastern and western HKH), some or all of the order of 100 to 300 kg m-3 (10 to 30 percent water that accumulation is lost. Thus, over the course of a year content). Over time, through compaction of overlying the size of a glacier may increase, decrease, or remain static. This is determined by whether accumulation or 1 Black carbon refers to particulate matter derived from the ablation predominates or whether they are equal. The incomplete combustion of a hydrocarbon. accumulation area is the upper elevation zone where 15
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16 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY FIGURE 2.1 The global hydrological cycle, or water cycle, is the process by which water moves through a series of reservoirs, including the ocean, lakes, groundwater, atmosphere, snowpack, and glaciers. Water can be in any phase (solid, liquid, gas) in these reservoirs. Water moves from the terrestrial and oceanic reservoirs to the atmosphere through transpiration, evaporation, or sublimation. Water moves from the atmosphere to the terrestrial and oceanic reservoirs through precipitation. Precipitation can occur in liquid form (rain) or solid form (snow, sleet, other types). SOURCE: U.S. Geological Survey. there is an annual net gain in mass, and the ablation water removal from a lake, which includes processes area is the lower elevation zone where there is an annual such as evaporation, water carried out of the lake by net loss in mass. The equilibrium-line altitude (ELA) streams, rivers, and groundwater channels, and extrac- is the elevation where the accumulation and ablation tion by humans. When water input sources equal water zones meet and where the annual net mass balance output sources, the lake is in steady state and the lake is zero (Figure 2.2). The annual mass balance is the level does not change. With glaciers, when accumula- net difference between accumulation and ablation (cf. tion equals ablation, the volume of water stored in the Cuffey and Paterson, 2010). glacier does not change and the ELA does not move. Accumulation includes all processes by which gla- Glacial volumes decrease when ablation persistently ciers increase in snow and ice mass, such as snowfall, exceeds accumulation, the ELA moves up, and the gla- condensation, refreezing of rainfall, avalanche transport cier in question ultimately disappears. This is analogous onto the glacier, and blowing snow transport onto the to a lake where persistent overdraft, in which extrac- glacier. Ablation includes all of those processes by tions exceed water input, is always self-terminating. which glaciers lose snow and ice mass, such as snow- Several important principles follow from this melt, icemelt, sublimation, blowing snow transport off discussion. First, it is the change in the volume of the glacier, calving and avalanche removal (cf. Cuffey the glacier, not the change in its downhill extent or and Paterson, 2010). areal extent that determines whether the net change is When viewed as water supply systems, glaciers positive or negative. However, it is difficult to directly are analogous to lakes. Water storage in glaciers is measure the volume of a glacier; thus measurements analogous to the total quantity of water stored in a of glacial volumes are scarce throughout the world. lake. Glacial accumulation is analogous to water input Second, where the entirety of the glacier is below the to a lake, which includes processes such as precipitation equilibrium line, there will be no accumulation and and water carried into the lake by streams, rivers, and with time the glacier will disappear. Third, glacial groundwater channels. Glacial ablation is analogous to mass balance information will provide an important
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PHYSICAL GEOGRAPHY 17 Glacier contribution to streamflow can be dis- cussed in terms of the hydrological cycle (Figure 2.1) following the approach of Comeau et al. (2009). For their investigation of glacier hydrology in the Canadian Rockies, they defined "glacial melt" and "glacial wast- age" in terms of the water equivalent. They simplified the annual glacial mass balance by treating sublimation as negligible, and assuming no snow inputs or outputs from avalanching, snowdrifting, or blowing snow, and no ice losses from calving. At the high elevations of the HKH, as well as in central and northern Tibet, where it is very cold and dry, sublimation is an important glaciological term in considering the mass balance of the glacier, but not in hydrological considerations. FIGURE 2.2 Schematic of glacial mass balance indicating the accumulation area at higher elevation and the ablation area at Therefore, the Committee has followed the approach lower elevation. Accumulation includes all processes by which of Comeau et al. (2009) in using the terms "glacial melt" solid ice (including snow) is added to a glacier, and ablation in- and "glacial wastage" when discussing the relationship cludes all processes by which ice and snow are lost from the gla- cier. The equilibrium-line altitude occurs at the elevation contour between glacial meltwater and streamflow. Because where the accumulation and ablation areas meet and the annual there are differences in meaning implied between gla- net mass balance is zero. SOURCE: Armstrong (2010). cial melt and glacial wastage for different disciplines, when reporting results from other sources, the Com- mittee has been consistent with the language used in link between variations in glacial volume and climate the original reference. changes (Meier, 1962). A general understanding of Comeau et al. (2009) defined the annual glacial glacial mass balance is essential to understanding what mass balance as being equivalent to the annual snowfall happens to glaciers over time. minus the annual snowmelt from the glacier and minus Glaciers respond to climate to reach steady state, a the annual glacial icemelt. If a glacier is in equilibrium state with no change in the mass balance or ELA over or has a positive mass balance, then the glacial icemelt time. A glacier advances due to cooling temperatures or term is defined as the icemelt volume that is equal to, snowfall increase, resulting in a positive mass balance. or less than, the water equivalent of snow that accumu- Warming temperatures or a decrease in snowfall results lates into the glacier system in a hydrological year. If in a negative mass balance and glacial retreat. A glacier a glacial mass balance is negative, then glacial wastage that is in disequilibrium with a warming climate will is defined as the volume of icemelt that exceeds the retreat until equilibrium is reestablished or the glacier water equivalent of the annual volume of snow accu- disappears. mulation into the glacier system, causing an annual Glaciers in mid-latitude mountain regions of the net loss of glacier volume. In short, glacial melt does world, including those in the HKH region, experience not by itself imply a negative mass balance or wastage. melt in their ablation zones at some time in most years. By these definitions, on an annual basis, the presence Melting of glacial ice is a normal phenomenon. Most, of a glacier in a basin affects total streamflow volume if not all, mid-latitude glaciers contribute meltwater through wastage contributions only. Glacial melt is a to streams and rivers. This contribution of glacial storage term and does not contribute to increased total meltwater to the discharge of mountain streams and annual streamflow. Within the hydrological cycle, both rivers occurs even in years with a positive glacial mass glaciers and groundwater are storage reservoirs. Follow- balance. A steady-state situation occurs when climate ing the convention of Comeau et al. (2009), snowfall on conditions are such that glacial melt equals accumula- glaciers is analogous to groundwater recharge, glacial tion and there is no change in the mass balance over melt is analogous to groundwater extraction (or outflow some time period. from artesian aquifers), and glacial wastage is analogous
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18 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY to groundwater overdraft. Persistant glacial wastage and The "glaciological" method for determining glacier persistant overdraft are both self-terminating. mass balance relies on a network of stakes and pits on Glacial melt can affect total streamflow on a sea- the glacier surface and measuring the change in surface sonal basis, and its significance is manifest in its timing, level between two fixed dates (an annual mass balance) as water is stored as snow accumulation into the glacier or at the end of the ablation and accumulation seasons system and the water equivalent runoff is delayed until (a seasonal mass balance) (Racoviteanu et al., 2008). icemelt in the late summer months of the otherwise low This method is considered the most accurate and streamflow. Therefore, the importance of glacial melt provides the most information about spatial variation in terms of percentage contribution to streamflow is (Kaser et al., 2003). However, there are currently no primarily on a seasonal timescale. long-term glaciological mass balance records for the An understanding of ice dynamics is required to HKH region, and few measurements of glacial mass understand the response of glaciers to climate change balance at all (Kaser et al., 2006). (Armstrong, 2010). If climate and ice dynamics result Mass balance can be estimated using the "geodetic in a glacier extending farther downslope with time, the method." This indirect method consists of measur- advance of the terminus2 will increase the total glacier ing elevation changes of the glacial surface over time area. A time lag on the order of decades or longer occurs from various digital elevation models constructed over between a change in climate and glacier advance or the entire glacier surface (Racoviteanu et al., 2008). retreat, and year-to-year glacier terminus changes are Because of large uncertainties, the geodetic method likely a response to climatic events that occurred several can only be used to estimate glacier changes at decadal decades or more in the past. The majority of glaciers in or longer timescales (Kaser et al., 2003; Racoviteanu the HKH region have response times on the order of et al., 2008). decades to a few centuries (Humphrey and Raymond, The logistical difficulties caused by the rugged 1994; Johannesson et al., 1989). The response time is topography and remote location of glaciers in the influenced by a glacier's area and volume, precipita- region make remote sensing techniques of particular tion regime, debris cover, and topographic shielding or interest. Remote sensing allows for regular monitor- shadowing (Kargel et al., 2011). All these factors vary ing of glacier area, length, surface elevation, surface widely over the HKH and High Mountain region of flow fields, accumulation/ablation rates, albedo,3 ELA, Asia. accumulation area ratio, and mass balance gradient. A more detailed description of glaciological, hydrologi- Measuring Glaciers cal, geodetic, and remote sensing glacier measurement methods is presented in Appendix C. The easiest glacial property to measure is the location of the terminus. Simply by walking uphill to Glacier Extent and Retreat Rates the start of a glacier, one can locate the terminus of the glacier. The terminus position for that year can The HKH region is often referred to as the "third be marked in any number of ways, including a simple pole" because it contains the largest ice fields outside pile of rocks. Some glaciers have accurate records of the polar regions. Some of the largest glaciers in the their terminus position that go back a hundred years world are located here, including the Siachen glacier or more. However, this simple measurement may yield on the north slopes of the Karakoram Range, which erroneous information about a glacier's retreat and stretches to a length of about 72 km and is the largest rate of retreat over short timescales of a decade or so. nonpolar glacier. Additionally, the mountains and gla- Prolonged retreat of the terminus of a glacier over time ciers of the Himalayas are culturally important to the scales of several decades does indicate that the glacier region's population (Box 2.1). is retreating. 3 Albedo is the ratio of reflected solar radiation to incident solar 2 The glacier terminus, sometimes called the glacier snout, is the radiation for a specific surface and has a value between 0 and 1. For lower end of a glacier. example, fresh snow has an albedo of about 0.8 (AMS, 2000).
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PHYSICAL GEOGRAPHY 19 BOX 2.1 Cultural Importance of the Himalayas People have traditionally revered mountains as places of sacred est mountain in the world. This mountain is venerated by Hindus, power and spiritual attainment, and the Hindu Kush-Himalayan (HKH) Buddhists, Jains, Sikhs, and believers of Bonri, the ancient Tibetan mountains play a central role in the spiritual, as well as practical, lives religion (Peatty, 2011). For Hindus, Mt. Kailash is the heavenly abode of millions of people (Bernbaum, 1998). It is from the Himalayas that the of Lord Shiva and his consort Parvati. Tibetan Buddhists view Mt. Ganges River, considered by Hindus to be the holiest of all rivers in India, Kailash as the pagoda palace of Demchog, the One of Supreme Bliss rises and cuts its path through the valleys and gorges before it enters (Bernbaum, 2006). Mt. Kailash is considered sacred in these religions the plains. The Ganges River draining the southern area of the Himalaya in part because it is the headwaters of four major rivers aligned in is considered by Hindus to be both a goddess and a river, Ganga Mata the cardinal directions: the Indus, the Brahmaputra, the Karnali (a (Mother Ganges; Eck, 1998, 2012), and is seen as sacred along its entire major tributary of the Ganges), and the Sutlej (a major tributary of length. Many believe that bathing in the Ganges frees one from past sins the Indus). and liberates the soul from the cycle of birth and death. One of Nepal's most famous places of religious pilgrimage is The glaciers have particular cultural importance as the perceived Gosainkunda Lake (4,400 m in elevation). Every year during the Janai source of water for the Ganges and other rivers in the HKH. This is Purnima festival in August, thousands of Hindu and Buddhist pilgrims demonstrated by anecdotal evidence from pilgrims who bathe in rivers travel there by foot to bathe in the holy lake. Glacial meltwater is strongly and lakes near the outlet of glaciers. For example, the Gangotri glacier associated with the major lakes and rivers in the HKH region. Rivers, is a traditional Hindu pilgrimage site. Devout Hindus consider bathing glaciers, and mountains in the HKH are intertwined with the daily activi- in the waters near Gangotri town a holy ritual. ties, spiritual lives, and the cultures of the local populations. Uncertainty The HKH region is also home to Mt. Kailash, in western Tibet surrounding the health of the glaciers and the rivers and lakes resonates (6,600 m in elevation), considered by many religions to be the holi- deeply throughout these cultures. The major concentrations of glaciers in the high (i.e., below the ELA). Glacial hypsometry plots the mountain area of Asia cross more than 12 mountain distribution of glacial area with elevation. Bajracharya ranges (Dyurgerov and Meier, 2005). There are cur- et al. (2011) have developed a glacial hypsometry for rently no complete glacier inventories, but there is Nepal (Figure 2.3). The hypsometry shows that glacial general agreement on the area of the glaciers in the ice ranged in elevation from about 3,200 m to 8,500 region (Armstrong, 2010; Bolch et al., 2012). The m. The highest amount of glaciated area was in the total glacier coverage of the HKH and the Tibetan 100-m-elevation band centered around 5,400 m. Gla- Plateau north to the Tien Shan4 is thought to exceed cial area decreases with both increasing and decreasing 110,000 km2, with about 50,000 identifiable glaciers elevation. (Dyurgerov and Meier, 2005). Table 2.1 summarizes Detailed glacier area measurements are not avail- glacial area estimates from different sources. However, able for the full study area. However, the Committee comparisons of glacial area among different studies are calculated the proportion of glacier area in different difficult because spatial extents are often different or elevation bands for the Indus and Ganges/Brahmapu- not well categorized. tra. In both basins, the majority of glacier area is in the Recent work by Jacob et al. (2012) shows that 5,000- to 6,000-m band (Figure 2.4), with a significant although previous estimates of mass loss in the region amount in the 4,000- to 5,000-m band. The Indus ranged from 47 to 55 Gt yr-1, the rate may be closer Basin has a slightly greater proportion of its glacier area to 4 ± 20 Gt yr-1. The gaps and discrepancies in these below 4,000 m than the Ganges/Brahmaputra Basin, various reports emphasize the need for a comprehen- whereas the Ganges/Brahmaputra has a slightly greater sive glacial inventory of the region. In addition, more proportion of its glacier area above 6,000 m. Although information about how glacier area is distributed with these values should be considered qualitative, they are elevation would lead to a better understanding of how consistent with the more rigorous hypsometry data much glacial area is in vulnerable low-elevation areas from Nepal (Figure 2.3). The differences are small, but they suggest that glacial retreat would be more sensi- 4 A large mountain system located in Central Asia and to the tive to changes in climate in the Indus Basin than in north of this report's study area. the Ganges/Brahmaputra Basin; however, this qualifies
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20 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY TABLE 2.1 Glacial Area Estimates from Different Studiesa Region Glacier Area (km2) Data Source HKH 114,800 WGMS (2008) 116,180 Xu, J., et al. (2009) 60,000 ICIMOD (2011b) 99,261 Yao et al. (2012) Central HKH 33,050 WGMS (2008) 32,182 ICIMOD: Eriksson et al. (2009) 71,182 Indian Space Agency: ISRO (2011) Himalayas 33,050 Dyurgerov and Meier (1997, 2005) Karakoram 15,400 Dyurgerov and Meier (1997) 16,000 Dyurgerov and Meier (2005) 16,600 Yao et al. (2012) Indus Basin 32,246 ISRO (2011) 36,431 Raina (2009) 21,192 ICIMOD (2011b) Ganges Basin 18,392 ISRO (2011) 9,012 ICIMOD (2011b) Brahmaputra Basin 20,542 ISRO (2011) 14,020 ICIMOD (2011b) China 59,406 Chinese Academy of Sciences: Liu, et al. (2000) India 37,959 Geological Survey of India in ICIMOD (2011b) Nepal 4,212 ISRO (2011) aComparisons of glacial area among different studies are difficult because spatial extents are often different or not well categorized. evidence that glaciers are more stable in the western of Nepal glaciers is at altitudes above approximately Himalayas. This is because glacial retreat is sensitive to 5,400 m (Alford et al., 2010; Bajracharya et al., 2011). more factors than simply elevation, including precipita- Therefore, glacier AX010 is not a good indicator of tion regime, local temperatures, and debris cover. general trends in the HKH region. In a study of gla- Rates of glacial retreat in the HKH are not well ciers in northern India, Kulkarni et al. (2011) found understood because of a lack of field data (Kargel et al., that glaciers smaller than 1 km2 lost an average of 28 2011; Thompson, 2010), making it difficult to under- percent of their area between 1962 and 2001, while gla- stand regional climate change impacts (Scherler et al., ciers greater than 10 km2 lost an average of 12 percent 2011b). One of the most studied glaciers in the region, of their area in the same time period, further indicat- AX010 in Nepal, has consistently been shown to have a ing that smaller glaciers cannot be used to determine negative mass balance. If the climate conditions remain regional trends. consistent with the period 1992 to 1996, AX010 has Extrapolation of these few mass balance studies been predicted to disappear by the year 2060 (Kadota, over the greater High Asian region has been used to 1997). However, this glacier is relatively small, with estimate a rate of water loss from glacial retreat between an area of only 0.38 km2, and exists at a low altitude, 2002 and 2006 of -55 Gt yr-1 for this entire region, extending to only 5,300 m, and thus only represents with -29 Gt yr-1 over the eastern Himalayas alone small, low-elevation glaciers (Fujita and Nuimura, (Dyurgerov, 2010). In contrast, Jacob et al. (2012) 2011). However, approximately 50 percent of the area used new information from the Gravity Recovery and
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PHYSICAL GEOGRAPHY 21 FIGURE 2.3 Glacial area in Nepal is shown as a function of elevation. Glacial ice ranges from about 3,200 m to 8,500 m in eleva- tion. Total glacial area decreased between 2001 (red line) and 2010 (black line) although the highest amount of glacial ice remained at about 5,400 m in elevation. Glacial area decreases as the elevation increases or decreases from 5,400 m. SOURCE: Bajracharya et al. (2011). Climate Experiment (GRACE)5 satellite mission to have estimated the retreat of individual glaciers in the estimate a mass loss of only -4 Gt yr-1 for the region region: the Bhagirath Kharak glacier in Uttarakhand, of High Asian mountains for the period 2003 to 2010. India, retreated 7 m per year between 1962 and 2005 The much lower estimate of glacier loss from analysis (Nainwal et al., 2008); the Dokriani glacier in Uttara- of the GRACE data is at least in part because the khand, India, retreated 550 m between 1962 and 1995 GRACE satellite information integrates over the entire (Dobhal et al., 2004); the Parbati glacier in Himachal region, in contrast to the study by Dyurgerov (2010), Pradesh, India, retreated 578 m between 1990 and which by necessity extrapolated the few glacial mass 2001 (Kulkarni et al., 2005); the Satopanth glacier in balance measurements collected at low elevations over the entire region. Similarly, a recent time series using the geodetic approach based on recently released stereo Corona satellite imagery (years 1962 and 1970), aerial images, and recent high-resolution satellite data (Cartosat-1) to determine mass changes for 10 glaciers south and west of Mt. Everest, Nepal, show a specific mass loss between 1970 and 2007 of 0.32 ± 0.08 m of water equivalent per year. These results are consistent with the global average (Bolch et al., 2011). Terminus mea- surements of 466 glaciers in the Chenab, Parbati, and Baspa basins in the Indus catchment showed significant deglaciation (Kulkarni et al., 2007). Various studies 5 The GRACE signal is heavily influenced by groundwater ex- FIGURE 2.4 Glacial area is shown as a function of elevation traction and subcrustal mass and plate movement. The HKH region for the Indus (black bars) and Ganges/Brahmaputra (gray bars) is a large, complex, and tectonically active area with substantial basins. In both basins, the majority of glacier area is in the groundwater depletion. Therefore, use of GRACE satellite data 5,000- to 6,000-m-elevation band. Comparing the two basins, for mass balance measurements in the HKH region leads to sub- the Indus Basin has a greater proportion of its glacier area below stantial uncertainties (e.g., Bolch et al., 2012). Moreover, the use of 4,000 m in elevation than the Ganges/Brahmaputra, and the GRACE satellite data to understand groundwater is complicated by Ganges/Brahmaputra has a greater proportion than the Indus the fact that the coarse resolution of GRACE disallows understand- of its glacier area above 6,000 m in elevation. SOURCE: Based ing of groundwater overdraft patterns at the scale of individual or on data from the Natural Earth dataset of the Digital Chart of local community consumptive use. the World product and the Hydrosheds database.
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22 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY Uttarakhand, India, retreated 23 m per year between several decades in this part of the HKH region. In the 1962 and 2005 (Nainwal et al., 2008). Although there eastern HKH, glaciers are retreating, at rates similar to is remaining uncertainty about the retreat rates of those in the rest of the world. Recent evidence shows specific glaciers (e.g., the Gangotri glacier in northern that glaciers may be receding at smaller rates than pre- India; Ahmad and Hasnain, 2004; Bhambri et al., 2011; viously estimated ( Jacob et al., 2012), although there Kumar, K., et al., 2008; Kumar, R., et al., 2009; Naithani is still uncertainty in estimates of glacial retreat. The et al., 2001) and more mass balance measurements are evidence to date suggests little change in rates of glacial needed, glaciers of the eastern HKH region, in general, retreat and glacial extent in the eastern HKH over the have a negative mass balance and are retreating, but not next two to three decades. Even if this is the case, the at higher rates than other mid-latitude glaciers (Bolch rate of glacial retreat could increase in the future with et al., 2012; Racoviteanu, 2011). appropriate changes in climate forcing. In contrast to the eastern HKH, Hewitt (2005) Currently, retreat rates in the eastern HKH are report that in the western HKH, there has been expan- highest at elevations below 5,000 m. This is par- sion of the larger glaciers in the Karakoram region6 ticularly serious for glaciers with maximum elevation since about 1990, particularly those at higher altitude. below 6,000 m. These small, low-elevation glaciers are Similarly, Scherler et al. (2011b) report that for the expected to sustain high rates of retreat. High-elevation Karakoram region, 58 percent of the studied glacier communities and activities that depend on glacial melt- fronts were stable or slowly advancing with a mean water generated by these small glaciers are most likely rate of about +8 ± 12 m yr-1. Surging of glaciers has to experience the impact of these retreating glaciers. been observed in Karakoram glaciers, but more field The Committee cannot state with certainty whether observations are needed to confirm whether this indi- major changes in either rates of glacial retreat or glacial cates a positive mass balance. Data from the late 1980s extent in the HKH region will occur for the next several indicated a possible trend of negative mass balance for decades. However, below is a worst-case scenario over the Siachen Glacier (Bhutiyani, 1999), but more recent a timescale of several decades that could result in very evidence from remote sensing data shows that glaciers high rates of glacial retreat. in the central Karakoram had a slightly positive mass A worst-case scenario of extensive glacial retreat balance between 1999 and 2008 (Gardelle et al., 2012). is within the bounds of possibility. One scenario The western end of the HKH appears to show slower involves albedo feedback processes. Because of the rates of retreat, less formation of pro-glacier lakes asso- large energy-albedo feedbacks associated with snow ciated with flood hazard, and frequent observations of and ice, small changes in the amount and timing of advancing glaciers, compared with the eastern region snow, and in the overall energy balance can have large (Armstrong, 2010; Bolch et al., 2012; Hewitt, 2005). effects on a glacier's mass balance. Fresh snow has an For the region as a whole, the loss of glacial ice over the albedo range of about 0.75 to 0.95. In contrast, glacial last decade is much less than previously thought (e.g., ice has an albedo range of about 0.3 to 0.4. Remov- Dyurgerov, 2010). ing snow from a glacier, holding other factors (air temperature, cloud cover, etc.) constant, results in a Possible Changes in Glacier Extent and Volume 200 to 300 percent increase in the delivery of energy to the surface of the glacier. As the exposed glacier ice There are few studies of the response of HKH gla- heats up and then melts, more nearby snow also melts, ciers to changes in climate (Cogley, 2011). Glacial mass resulting in more energy delivery to the glacier, and balances in the Karakoram area of the HKH appear to more glacial wastage. This process results in a runaway be stable, with some of the larger glaciers experienc- positive feedback signal that can accelerate the wastage ing positive mass balances. These results suggest that of glacial ice. This albedo feedback process is currently there will be little change in glacier extent over the next occurring in Arctic sea ice. Another scenario involves a regional change in 6 The Karakoram region is a large mountain range spanning the monsoonal activity that reduces snowfall in the Hima- border between Pakistan, India, and China. layas, coupled with high amounts of black carbon depo-
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PHYSICAL GEOGRAPHY 23 sition, resulting in very high wastage rates. Imposition sea level to the great height of the Tibetan Plateau of increased air temperatures caused by black carbon (~ 8,000 m) in a distance of 100 to 400 km across the heating of the atmosphere would accelerate the rates width of the arc. Together with the Tibetan Plateau, the of glacial wastage. A change in monsoonal activity Himalayas exert great influence on the powerful Asian could result in less snowfall and more exposed glacial monsoon system (Figure 2.5). As such, there is a very ice, which could itself lead to high wastage rates. As high climatic gradient across the region. discussed later in this chapter, high amounts of black The region's climate ranges from tropical at the carbon are being entrained in the atmosphere and base of the foothills to permanent ice and snow at the deposited in the HKH region, decreasing the albedo of highest elevations. During the late spring and early glacier ice and snowpack. This decrease, even with no summer, the Plateau surface heats up quickly and serves increase in air temperature, could lead to increased sur- as an elevated heat source, which draws warm and moist face wastage. In the monsoonal region of the Himalayas, air from the Indian Ocean toward the Himalayas and decreased albedo from deposition of black carbon is Tibetan Plateau region. As the monsoon flow trans- mitigated by repeated, almost daily, snowfall during the ports moisture from the Arabian Sea to the Indian monsoon. Black carbon deposition is also mitigated by subcontinent, it spurs heavy monsoon rain over the snow turnover processes. However, if new snow does not Indo-Gangetic Plain and the Bay of Bengal. During fall because of changes in monsoonal activity, then black the winter, the low-level monsoon flow reverses to carbon transported from the Indo-Gangetic Plain could northeasterly, with prevailing large-scale subsidence accumulate on the snow surface, causing an acceleration and relative dry conditions over India. of wastage rates. Furthermore, in this scenario with Over the Tibetan Plateau, rainfall is scarce all year less monsoon precipitation and more glacial wastage, round with annual totals of 100 to 300 mm. Most of the the contribution of glacial wastage to summer stream- precipitation falls in the form of snow in winter, with flow would become more important. Such accelerated more than 50 percent of the land at elevation above wastage rates could occur even without a change in air 5,000 m covered by snow. In the summer, the snow temperature. However, high atmospheric loads of black carbon heat the atmosphere, which would further accel- erate melt rates. Another important effect of black car- bon could be to change the phase of precipitation (e.g., from snowfall to rainfall). Changes in climate could also result in a shift in the precipitation phase and number of snow days in the region. For example, Shekhar et al. (2010) found significant variations in snowfall trends in the western Himalayas. More precipitation phase data are needed to fully understand whether snowfall events in the region are changing and how such changes will affect glacial mass balance. With the right conditions, accelerated rates of glacial retreat beyond present rates are a possibility. If such a situation does arise, most likely it would be local in origin and not global or even consistent throughout the entire HKH region. REGIONAL CLIMATE AND METEOROLOGY FIGURE 2.5 The moist air currents that drive the South Asian The HKH region features one of the world's Monsoon are indicated by white arrows. Monsoon flow trans- ports moisture from the Arabian Sea to the Indian subcontinent, steepest slopes of an extended mountain range, rising resulting in heavy monsoon rain over the Indo-Gangetic Plain from its base in the alluvial Indo-Gangetic Plain near and the Bay of Bengal. SOURCE: Hodges (2006).
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24 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY cover fraction drops to below 30 percent at the same Further east along the arc is the Greater Himalayan elevation. The melted snow reveals an arid stepped Range. This region includes snow-capped high moun- landscape interspersed with scattered glaciers and large tains and foothills in northwestern India, Nepal, and brackish lakes. Bhutan, forming the northern boundary of the fertile The climatic gradient is strong not only across, but and populous Indo-Gangetic Plain, where the Ganges also along the arc of the Himalayas (Figure 2.6). The River flows. Rainfall is higher in the east, mostly from Sutlej valley serves as a rough dividing line between the summer monsoon rain. The Bay of Bengal, in which climate regimes of the western and eastern Himalayas the Ganges/Brahmaputra rivers flow out to sea is the (Bookhagen and Burbank, 2010). In the Karakoram wettest part of the Indian monsoon region. Bookhagen in the west, about two-thirds of high-altitude snowfall and Burbank (2010) reviewed precipitation data for the is due to the mid-latitude westerlies. In the east, more 10-year period from 1998 to 2007. They showed that than 80 percent of annual precipitation is from the mean annual rainfall ranges from ~1 to more than 4 m summer monsoon. (Bolch et al., 2012). in the monsoon-precipitation-dominated portions of The westernmost portion of the region includes the the region. high mountains and glaciers of the Hindu Kush and the Karakoram, with a large number of rivers flowing Role of Aerosols in Regional Climate into the upper Indus River Basin in Pakistan, eventually draining into the northern Arabian Sea. This region Aerosols are suspended fine particles in the atmo- adjoins the arid, rugged regions of Afghanistan in the sphere that have both natural and manmade sources. west, and the Thar Desert of northwestern India to the Aerosols from natural sources such as desert dusts have south. It has a relatively dry climate, with annual pre- been known to coexist with the Indian monsoon in the cipitation of 400 to 600 mm, primarily from wintertime eastern HKH region for a long time. During April and storms associated with the mid-latitude westerlies. In May, dusts are transported by the mid-latitude wester- the cold arid regions of Ladakh, India, the precipitation lies from the deserts in the Middle East and Afghani- is somewhat higher in summer, but the mean annual stan and the Thar Desert in northwestern India to the precipitation is as low as 115 mm per year (Thayyen Indo-Gangetic Plain and Himalayas. and Gergan, 2010). Since the Industrial Revolution, atmospheric load- ing of aerosols from manmade sources such as factories, power plants, cooking and heating, and slash-and-burn agricultural practices has greatly increased, making the Indo-Gangetic Plain one of the most polluted regions in the world. These aerosols often appear in the form of a brownish haze known as atmospheric brown clouds (Ramanathan et al., 2005). A key component of these brown clouds is black carbon, commonly known as soot. Black carbon sources include internal combustion engines, power plants, heat boilers, waste incinerators, slash-and-burn agricultural activities, and forest fires. Although some aerosol species have a cooling effect, airborne black carbon strongly absorbs FIGURE 2.6 The climate varies across the HKH region. In the solar radiation and heats up the atmosphere. Recent west, indicated by purple, the climate is alpine and dominated by the mid-latitude westerlies. Most precipitation takes the form studies have shown that aerosols, in particular, black of winter snow. This area adjoins a cold arid climate regime, carbon because of its ability to heat the atmosphere, can indicated by blue. In the east, indicated by yellow, the climate is affect the regional and global water cycles, including dominated by the summer monsoon, with most of the precipita- the Himalayan snowpack and glaciers, by altering the tion coming during the summer months. The Indus, Ganges, and Brahmaputra watersheds are also shown. SOURCE: Thayyen radiation balance of the Earth's atmosphere and surface and Gergan (2010). and modulating cloud and rain formation processes
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PHYSICAL GEOGRAPHY 25 (Lau et al., 2010; Ramanathan et al., 2005; Rosenfeld Indo-Gangetic Plain and the Himalayan foothills. The et al., 2008). Because aerosols have the capability to mixture of dust and black carbon in the deep aerosol regulate atmospheric heat sources and sinks, modulate layer provides efficient heating of the atmosphere. It monsoon rainfall, surface evaporation, and river runoff, interacts with the warm moist monsoon air and maxi- and possibly affect melting of high mountain snowpack mizes the atmospheric water-cycle feedback, and may and glaciers, they are an integral component of the significantly modulate the summer monsoon rainfall monsoon climate system. (Lau et al., 2006, 2008). The atmospheric loading of aerosols is measured In contrast, during winter, the prevailing mon- in terms of the aerosol optical thickness (AOT), which soon flow is cold, dry northeasterly with large-scale is quantitatively determined by the amount of solar subsidence. Local emissions of aerosols from the radiation attenuation at Earth's surface by the aerosol. Indo-Gangetic Plain are transported by the north- During the late spring and early summer season (April easterly flow in the form of atmospheric brown cloud to June), the AOT builds up dramatically over the plumes emanating from the source region over the Indo-Gangetic Plain and northwestern India (Figure Indo-Gangetic Plain to the adjacent ocean (Figure 2.8, 2.7, upper panels). The monsoon flow is blocked by upper panels), and are trapped within the stable and the Tibetan Plateau and forced to rise over the Hima- low boundary layer (<1 km; Figure 2.8, lower panel). layas foothills and northern India. As a result, aerosols In winter, the atmospheric brown clouds have a higher transported from remote deserts and from local emis- contribution from local black carbon emissions, but sions accumulate against the Himalayan foothills to a little contribution from dust, because of lack of deserts great height (>5 km) and spread over the entire Indo- upwind. The black carbon aerosol heats the boundary- Gangetic Plain and regions to the south (Figure 2.7, layer air, but cools the land surface, thus further increas- lower panel). Additionally, the southwest monsoon flow ing atmospheric stability, suppressing convection and brings increasingly warm, moist, and unstable oceanic the already-scarce wintertime rainfall. The wintertime air from the Indian Ocean and the Arabian Sea to the high aerosol loading within the boundary layer in the FIGURE 2.7 Climatological monthly distribution of aerosol optical thickness (AOT) during April, May, June from MODIS (upper panels) and vertical distributions across the Tibetan Plateau from CALIPSO (lower panel, horizontal scale shows latitude/longitude coordinates) show the deep and extended layer of aerosols over vast regions of the Indo-Gangetic Plain and Himalayan foothills. Some aerosols can be detected over the top of the Tibetan Plateau. SOURCE: upper panel, Gautam et al. (2010); lower panel, Gautam et al. (2009b).
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38 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY FIGURE 2.15 One thousand year records of 18O and mineral dust concentrations, shown as decadal averages, from five Tibetan Plateau ice-core study sites along with a three-decade moving average. of the Himalayas. The larger concentration of dust in the drought of greatest intensity occurring from 1790 cores in the north is due in part to the proximity to to 1796. Recent increases in anthropogenic activity in Takla Makan and Gobi deserts and the fact that they India and Nepal are shown by a doubling of chloride are located in regions of the plateau that are domi- concentrations and a fourfold increase in dust in the nated by the westerlies. Puruogangri, Naimona'nyi and upper sections of these cores. The Dasuopu ice core also Dasuopu located in the central Tibetan Plateau and the suggests a 20th-century warming trend that appears Himalayas to the south are under greater influence of to be amplified at higher elevations (Thompson et al., the monsoons and contain far lower concentrations of 2000). mineral dust. The lowest mineral dust concentrations The ice fields located in more arid regions of the are found in the 7,200-m a.s.l Dasuopu ice-core study Tibetan Plateau (i.e., Guliya, Puruogangri, Dunde) site. The ice-core data files along with the metadata have rather similar annual net mass balances, averag- files for Dunde, Guliya, Dasuopu, and Puruogangri ing 220 mm water equivalent (liquid water obtained ice cores are archived at the National Climatic Data from melting snow or ice) per year for Guliya in the Center (NCDC).8 Naimona'nyi ice core data files and far northwestern Tibetan Plateau, 350 mm water metadata will be archived pending publication. equivalent per year for Puruogangri in central Tibetan A high-resolution ice-core record from Dasuopu Plateau, and 390 mm water equivalent for Dunde in at 7,200 m in the central Himalayas (located just northeastern Tibetan Plateau. Ice-core records from north of the China-Nepal border) shows that this these three ice fields have rather similar histories with location responds to fluctuations in the intensity of net balance higher in the 17th and 18th centuries, con- the South Asian monsoon. Measurements of dust sistently lower values in the 19th century, and a general and chloride concentrations yield information about increase in the 20th century. In contrast, the net balance changes in monsoonal intensity. Older sections of the history on Dasuopu in southern Tibetan Plateau in the ice cores reveal periods of drought in the region, with Himalayas shows a different pattern, with current net balances averaging 1,000 mm of water equivalent per 8 http://www.ncdc.noaa.gov/paleo/icecore. year. The net balance history shows consistently high
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PHYSICAL GEOGRAPHY 39 values over the 19th century. Although the 600-year region. Ice-core datasets from exposed mountain sum- net balance history for the Himalayas (e.g., Dasuopu) mits away from the effects of urbanization and topo- is quite different from that in the Tibetan Plateau graphic sheltering provide relatively unbiased records to the north, their oxygen isotope histories, or proxy of the planet's climate. temperature records, are remarkably similar at lower frequencies. During the 17th century, temperatures REGIONAL HYDROLOGY were warmer over Guliya and Puruogangri than over Dunde and Dasuopu, but there has been persistent, Mountains are the water towers of the world, gradual warming from the 18th through the early 20th characterized by high precipitation and little evapora- century and accelerated warming over the second half tion because of lower air temperatures and longer snow of the 20th century. The recent isotopic enrichment coverage, resulting in large contributions of snowmelt is consistent among the Tibetan Plateau sites and and icemelt to the runoff of lowland areas (Viviroli et independent of the net balance (Duan et al., 2006; al., 2007). This is especially true for the HKH region, Thompson et al., 2006). where the snow and ice stored in high-altitude glaciers A sulfate record, which indicates deposition of in the Greater Himalayas are a source of water for sulfate aerosol, for 1000 to 1997 from the Dasuopu ice almost every major river system in the region. However, core shows that this site is sensitive to anthropogenic a complete understanding of the regional hydrology-- activity originating in southern Asia. Before 1870, including the actual contribution of snow and glacial sulfate concentrations in the atmosphere were rela- meltwater to surface waters and groundwater of the tively low and constant, but after 1870, concentrations region--is lacking because of the same incomplete increased and the rate of increase has accelerated since science and unresolved uncertainties discussed earlier 1930. This trend in sulfate deposition is accompanied in the report. by growing SO2 emissions in South Asia. This is in The lack of understanding of the regional hydrol- contrast to sulfate concentrations derived from Green- ogy is intimately tied to water security concerns in the land ice cores, which have declined since the 1970s. region. Some reports and peer-reviewed publications This is a result of regional differences between Europe suggest that glacial meltwater provides a large share and Asia in emission and transport of sulfate, as well of the water feeding discharge into major rivers such as different levels of environmental regulation (Duan et as the Ganges. This apparent contribution of glacial al., 2007). As discussed earlier in this chapter, a number meltwater to these large rivers, combined with the of recent studies have concentrated on the impact of misconception that the region's glaciers are experienc- black carbon and aerosols on atmospheric heating and ing the highest rates of glacial retreat in the world, glacier melting (Lau et al., 2010; Menon et al., 2010; have combined to create a sense of water scarcity in the Ramanathan and Carmichael, 2008). region (e.g., Kehrwald et al., 2008). Regional composites for the Tibetan Plateau have been constructed using decadal averages of oxygen iso- Uncertainty About the Contribution of Glacial topes over the last 2,000 years to reveal larger temporal- Melt to the Hydrology of the Region scale changes. The 2,000-year perspective from these Tibetan Plateau ice cores shows large and unusual Barnett et al. (2005) report that "there is little doubt warming at high elevations. The oxygen isotope record that melting glaciers provide a key source of water for clearly shows that large-scale dynamics have changed the region in the summer months: as much as 70 per- over the past century, regardless of whether the record cent of the summer flow in the Ganges and 50 to 60 is used as a proxy for temperature, precipitation, or percent of the flow in other major rivers." This state- atmospheric circulation (Thompson and Davis, 2005; ment is based on estimates of glacial melt contributions Thompson et al., 2006; Vuille et al., 2005; Yao et al., to river flow derived from models that relied on many 1996). Similar to tree-ring chronologies, ice-core assumptions because of the lack of field data (Singh records collected across the Tibetan Plateau dem- and Bengtsson, 2004; Singh and Jain, 2002; Singh et onstrate that it is a climatically diverse and complex al., 1997). Rees and Collins (2006) used a theoretical
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40 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY modeling approach to conclude that for large distances However, it is generally accepted that the percent con- downstream, the contribution of discharge from glacier tribution increases from east to west across the region icemelt often dominates flow, particularly when other (Immerzeel et al., 2010). sources of runoff are limited. Finally, contributing to the confusion about the Attempts to improve the understanding of the con- relative importance of glacial melt to the discharge of tribution of glacial wastage to the regional hydrology rivers is the often-overlooked differentiation between confound these interpretations by identifying scientific relative contributions of snowmelt versus glacial melt. gaps, important nuances in geography, and contrasting For example, any water source from high elevation is results when using different scientific methods. For sometimes assumed to be glacial melt, when in fact example, modeling9 showed that in Nepal the glacial snowmelt may be a major contributor. Snowmelt is meltwater contribution to tributaries to the Ganges a renewable resource that is replenished every year, near the base of the Himalayan sub-basin streamflow in contrast to the fossil water11 contributed by gla- varies from approximately 20 percent in the Budhi cial wastage (Barnett et al., 2005). In addition, some Gandaki Basin to approximately 2 percent in the Likhu reported values include runoff from rain and other Khola Basin. The average across nine basins in Nepal forms of precipitation that would occur with or without was 10 percent (Alford et al., 2010), a far lower per- the presence of glacial ice. In most basins, the contribu- cent than that discussed above. Using remote sensing tion of rain far outweighs the combined contributions approaches,10 Racoviteanu (2011) corroborated these of snowmelt and glacial wastage to discharge. For lower values by showing that for the Langtang basin in example, Andermann et al. (2012) found that snowmelt Nepal, glacial meltwater contributes about 10 percent and glacial melt contributed roughly 10 percent to the of discharge at an elevation of 900 m. This work also discharge of the three main Nepal rivers. Therefore, in showed that the contribution of glacial meltwater to these catchments, rainfall could contribute as much as discharge increases with increasing elevation, reach- 90 percent to the total discharge. ing about 50 to 70 percent at an elevation of 3,800 m (Racoviteanu, 2011). Using ice-core records and mea- A Complex Hydroclimate System suring radioactivity, Kehrwald et al. (2008) suggested that reports of the relationship between glacial retreat Glaciers are only one part of the complex HKH and downstream water resources have not accounted hydroclimate system,12 where the relative importance for mass loss through thinning of high-elevation, low- of the contribution of glacial meltwater to runoff latitude glaciers. For example, this is apparently occur- depends on the magnitude of other components of the ring in the Naimona'nyi glacier in Tibet. Thinning of hydrological cycle. The different climate regimes of the high-elevation glaciers could result in a decrease in region are characterized by differences in the spatial water availability in regions where the water supply is and temporal distribution of precipitation (type and dominated by high-elevation glacial melt. Armstrong amount) and runoff. As mentioned previously, summer (2010) reported that previous assessments of the rela- monsoon rains dominate the annual precipitation cycle tionship between glacial meltwater and surface water of the eastern Himalayas while the west is dominated supply have been highly qualitative or local in scale. by winter snowfall with low amounts of summer pre- Direct evidence is lacking to support the higher-end cipitation. A similar variation in the relative contribu- values reported for the contribution of glacial icemelt tions of rain, snowmelt, and melt of glacier ice to the to total river flow volume (i.e., 50, 60, 70 percent). discharge of different rivers throughout the HKH is expected. 9 The model was based on limited mass balance measurements from glaciers in the region, remote sensing measurements of glacier area, and a variety of assumptions. 11 Fossil water is water that has been stored in a glacier or an 10 Datasets were derived using remote sensing resources discussed aquifer for a long (years or more) period of time. further in Appendix A, such as the Advanced Spaceborne Thermal 12 The hydroclimatic system includes the processes by which the Emission and Reflection Radiometer (ASTER) sensor, declassified climate system causes global and local variations in the hydrologi- Corona imagery, and Landsat ETM+. cal cycle.
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PHYSICAL GEOGRAPHY 41 Methods comparing measurements or models of for the Ganges, Yangtze, and Yellow Rivers." Prelimi- glacial meltwater production with measurements of nary results show that in the Indus, snow and glacial downstream discharge volume are problematic. Glacial melt contribute one and a half times as much discharge meltwater can be interpreted as raw volume input into as that generated naturally downstream below 2,000 m. the system, but downstream of the glaciers discharge In the Brahmaputra, snow and ice discharge is about has been modified by a number of factors, including a quarter of that generated downstream. The model precipitation, evaporation, irrigation, damming, and found snow and glacial melt to be less important in groundwater exchange. With increasing distance from the Ganges, with discharge being about one-tenth of the glaciers, these modifications increase in relative that generated downstream. For the Indus, this indi- importance, while the relative contribution of glacial cates that the source of much of the streamflow at low meltwater decreases. In a direct comparison between elevations is snowmelt and/or icemelt from elevations glacial meltwater and runoff downriver, the volume greater than 2,000 m. The values for the Ganges are contribution from glaciers can be overestimated with remarkably similar to those of Alford et al. (2010) and increasing distance from the glaciers (Kaser et al., suggest that glacial melt is not a major contributor for 2010). river systems to the east but is much more important Furthermore, limited data on the water cycle of the for river systems to the west. region, due to a variety of reasons ranging from difficult Recently, Wulf et al. (2011) quantified the water terrain to political instability, is a chronic problem. resources and discharge components for the Sutlej Scientists compensate by using many assumptions in River from 2004 to 2009, which is a major tributary models or remote sensing imagery. The overall result, of the Indus River flowing through northern India however, is more overall uncertainty in the body of and Pakistan. As discussed earlier in this chapter, the scientific literature than in other parts of the world Sutlej Basin is located at the interface between the that are not limited by these challenges. Despite this monsoon-dominated precipitation regime to the east uncertainty, many studies have contributed to reducing and the winter-snow-dominated regime to the west. this uncertainty associated with the complexity of the Results indicate that the discharge of the Sutlej River at HKH hydroclimate system. Bhakra located at low elevation and situated at the base There is strong seasonality in annual precipita- of the mountains is sourced predominately by snowmelt tion. Annual hydrographs for the Ganges and Indus (48 percent) followed by effective rainfall (rainfall- rivers clearly show strong seasonality in the amount evapotranspiration,13 39 percent) and glacial melt (13 of discharge, leading to seasonal differences in water percent). Average runoff per square meter is less than availability. The Ganges River exhibits a significant 0.2 m yr-1 in the high-elevated, low-relief Transhima- discharge during the summer months, resulting in a layan part of the Sutlej Valley, peaks at about 1.5 m water surplus that can maintain in streamflows and in yr-1 in the snowmelt-dominated High Himalaya, and some areas recharge groundwater storage. However, is about 0.9 m yr-1 at the rainfall dominated- mountain water consumption exceeds natural runoff in the winter front. Snowmelt is thus a much more important com- months of February and March when there is some ponent of discharge than glacial melt for the Sutlej reliance on groundwater and/or storage (Hoekstra and Basin, where monsoon rains are less important than Mekonnen, 2011). The Indus River has a lower peak farther to the east in Nepal and Bhutan. discharge and lower annual discharge than the Ganges The hydrograph of the upper Indus Basin is highly River. The Indus River discharge also varies seasonally seasonal, with about 85 percent of annual discharge and interannually. (The relationship between natural occurring between May and September. Figure 2.16 runoff and water use in the major river basins in the shows the time series of monthly and yearly variability study region is discussed in more detail in Chapter 3.) in May to September discharge at the historic Partab Using a modeling approach, Immerzeel et al. (2010) concluded that glacial melt and snowmelt are 13 Evapotranspiration is the combined processes through which "extremely important in the Indus Basin and important water is transferred to the atmosphere from open water and ice for the Brahmaputra basin, but plays only a modest role surfaces, bare soil, and vegetation (AMS, 2000).
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42 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY record at Partab Bridge is too short to determine whether these are indicative of long-term trends or shorter random changes at the decadal scale, hence the value of producing a much longer May to September river flow reconstruction from tree rings (Ahmed and Cook, 2011). However, the results do indicate that recent measurements of increased snowfall in the area and positive glacial mass balances are correlated with recent increases in discharge. Furthermore, the sub- basins and tributaries of the Upper Indus Basin are not always in phase with respect to their relative con- tributions to the overall discharge. Fowler and Archer (2006) report conflicting signals of temperature and discharge in tributary inflows that ultimately discharge into the Tarbela Reservoir, farther downstream on the Indus Basin. In summary, if the observed +11 percent change is a short-term phenomenon, it falls within the interannual variability of flows and is of little import for water supply; if it indicates a long-term step increase in discharge, +11 percent over 5 months represents approximately 4 million acre-feet, which would be significant for downstream mountain communities and also for downstream reservoir operation, storage, and deliveries. But again, the period of record is too short to speculate about the trend, driving forces, or water resources significant of this record at present. FIGURE 2.16 The monthly hydrograph of upper Indus River The Role of Groundwater discharge at Partab Bridge, Pakistan, for the period 1962 to 2008 (top) and the cumulative discharge series for the May to September months (bottom) that collectively account for Groundwater is an important part of the hydro- about 85 percent of the annual discharge on average. In both logical system in any part of the world. It is the primary the top and bottom, the discharge on the y-axis is in units of source of freshwater in many areas around the world cubic meters per second (m3 s-1), or CMS. The red curve is a and responds more slowly to meteorological condi- 50 percent L OWESS robust smoothing of the yearly discharge data. The annual discharge has the appearance of two flow tions when compared with the surface components of regimes: about 3,500 m3 s-1 from 1962 to 1987 and about the water cycle. The full role of groundwater in the 3,900 m3 s-1 from 1988 to 2008, an approximate 11 percent HKH region is not clearly understood. For example, increase. This is reasonably consistent with periods of observed glacial recession from 1985 to 1995 and expansion from 1997 the amount of groundwater storage in the HKH to 2002 in the Karakoram.Plateau ice-core study sites along with region is largely unknown because storage is notori- a three-decade moving average. ously difficult to measure, especially in mountainous terrain. Furthermore, the relationship between glacial Bridge, Pakistan, for the period 1962 to 2008. Median melt and snowmelt and groundwater recharge is not discharge is 3,658 m3 s-1, but has the appearance of fully understood. Understanding the average spatial two flow regimes: 3,519 m3 s-1 from 1962 to 1987 and temporal characteristics of groundwater fluxes, and 3,902 m3 s-1 from 1988 to 2008, an approximate recharge, and discharge is a research frontier facing 11 percent increase. These two periods are reasonably the hydrological science community (NRC, 2012a) consistent with periods of observed glacial retreat in and the HKH region is no exception; the mechanisms 1985 to 1995 and expansion in 1997 to 2002 in the and pathways of groundwater recharge and discharge Karakoram (Hewitt, 2005). The upper Indus Basin remain unclear.
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PHYSICAL GEOGRAPHY 43 There is no evidence that glacial melt or wast- Although there is still a great deal of uncertainty, age contributes to groundwater recharge outside local non-glacial mountain runoff (recharge) from the regions in any significant way. Research from other Himalayas toward low-elevation areas, in other words, mountainous regions has shown that groundwater the mountain water tower effect, could be a significant recharge outside local impacts is negligible and there source of recharge to northern India. Preliminary are no circumstances that hint that this would not be results from a simple but effective hydrological model so in the HKH. The effect of glacial melt on ground- combined with daily rainfall and discharge measure- water recharge on the plains is likely small because the ments found that groundwater flow through bedrock contribution of glacial meltwater to flows downstream is approximately six times the annual contribution from is generally small, and therefore any groundwater glacial icemelt and snowmelt to central Himalayan riv- recharge from major rivers downstream would be little ers (Andermann et al., 2012). However, this hydrologi- affected by changes in flows of glacial meltwater. This cal model used by Andermann and colleagues accounts is important because there is substantial evidence that only for snowmelt waters and not for the melting of groundwater withdrawals are increasing and it appears glacial ice, which is likely to be an important discharge unlikely that increases in glacial wastage would have contribution during the late summer. Furthermore, any significant impact on the supplies of groundwater with a model based on mean monthly values, it is dif- available to meet the increased demands. ficult to establish the infiltration behavior of relatively Groundwater extraction across northern India short, but strong, monsoon storms (Bookhagen, 2012). and the surrounding area in response to the growing Groundwater storage in the middle and upper demand for water14 is exceeding groundwater recharge, Indus River plains of Pakistan is also significant, and causing a lowering of the water table (Qureshi, 2003; it has been increasingly pumped to fulfill agricultural Shah, 2009; Sikka and Gichuki, 2006). The GRACE15 and urban water demands. This is consistent with satellite mission revealed a steady mass loss that has the decrease in discharge (discussed above) below the been proposed to be due to this excessive groundwater Partab Bridge over the last 10 years. However, ground- extraction (Rodell et al., 2009; Tiwari et al., 2009). water in this region is recharged by the distribution of Tiwari et al. (2009) estimate the region lost groundwa- upper Indus basin runoff through Pakistan's extensive ter at a rate of 54 ± 9 km3 yr-1 between 2002 and 2008, canal irrigation system across the Indus plains, mitigat- which is likely the largest rate of groundwater loss for ing groundwater depletion. But in some areas the canal comparably sized regions. A more recent analysis by system contributes to waterlogging, a high water table Moiwo (2011) of the loss of groundwater storage in this resulting in the saturation of soil to the degree of hin- area using GRACE data shows a similar depletion. If dering or preventing agriculture (Briscoe and Qamar, this trend is sustained, there could be water shortages 2006). Salinization of waterlogged lands soon follows. in the region when the aquifers become economically These groundwater balance processes are difficult exhausted (Tiwari et al., 2009). More specifically, it is to estimate, however, because they involve water-level possible that groundwater withdrawals during the low- fluctuations over the year, as well as high levels of flow period of these river basins may cause these rivers spatial variation in depths to groundwater and salinity to become seasonally dry. Finally, advances from satel- levels (Van Steenbergen and Gohar, 2005). Waterlog- lite gravimetry in the GRACE mission have helped to ging and salinity issues increase with the successive quantify groundwater depletion in the plains of north- reuses of water downstream in the canals of the lower west India, but the spatiotemporal resolution of the Indus Basin (Bhutta and Smedema, 2005). Again, the current satellite system is too coarse to fully distinguish signal of the contribution of glacial meltwater in the between all mass changing processes (groundwater, ice- downstream hydrological cycle can be lost in the noise melt, sediment, and tectonic forces (Bookhagen, 2012). of other processes, such as irrigation and the resulting hydrological impacts. In contrast to northern India and Pakistan, ground- 14 water resources in the Kabul Basin of Afghanistan Groundwater accounts for 45 to 50 percent of irrigation and 50 to 80 percent of domestic water use (Rodell et al., 2009). appear to be adequate for current needs and for at 15 See http://www.csr.utexas.edu/grace/. least the next several decades. A quantitative study of
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44 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY groundwater resources in the Kabul Basin (a very arid Immerzeel et al. (2012) evaluated the hydrological region), Afghanistan by the U.S. Geological Survey and response to future changes in climate for the Langtang Afghanistan authorities (Mack et al., 2010),16 has shed Basin in Nepal using a high-resolution combined light on the role of groundwater in this country. This cryospheric-hydrological model that explicitly simu- study integrated a variety of hydrological datasets--for lates glacier evolution and all major hydrological pro- example, streamflow data, water quality data, and satel- cesses. The analysis showed that both temperature and lite imagery--into a groundwater flow model to assess precipitation are projected to increase over the next current and future water availability in the region. century. These increases will lead to greater evapotrans- The study gleaned that groundwater from the piration and greater snowmelt and icemelt. This, com- upper aquifers has been the primary source of water for bined with more snow falling as rain, results in a steady agriculture and municipalities (Mack et al., 2010). The decline of the glacier area in the model. Furthermore, availability of groundwater in the Kabul Basin primarily the analysis shows that increased precipitation and depends on (a) surface-water infiltration from rivers icemelt will lead to increased streamflow. The seasonal and streams, (b) water leakage from irrigated areas, peak in meltwater coincides with the monsoon peak; (c) subsurface groundwater inflows from mountain therefore no shifts in the hydrograph are expected. fronts and, (d) groundwater storage in thick sediments. If these results are representative of the region, However, most recharge is derived from leakage of the Committee expects little change in the hydro- streamflow. Snowmelt in the mountains surrounding graph of large rivers in the HKH region in response Kabul Basin, particularly the Paghman Mountains, to changes in climate and potential glacial retreat over contributes an unknown but important amount to the the next several decades. If anything, there may be an water resources of the basin (Mack et al., 2010). increase in discharge. Potential changes in climate that Groundwater resources in the upper aquifer dur- result in drier and/or warmer conditions will result in ing years of normal precipitation and in the northern negative glacial mass balances, providing an additional Kabul Basin are considerable. Existing community water source for these large rivers over the next several water-supply wells that are shallow, or screened near decades. Decreases in available water from changes in the water table, likely would be affected by increased climate (less precipitation, more evapotranspiration, groundwater withdrawals, however, and could be etc.) will be compensated by the release of water from rendered inoperable or dry during summer months storage in glaciers. However, higher elevation areas with groundwater-level declines as small as about 1 m. can receive more than 50 percent of their annual water Simulations of the effects of increasing water use on flow from glacial meltwater. Populations that live in groundwater levels indicate that a large percentage of high-elevation areas--or use them for activities such as existing shallow water-supply wells in urban areas may seasonal grazing--may face water shortages in the near contain little or no water by 2057 (Mack et al., 2010). future (years to decades) if their basins have glaciers where the upper end of the glacier is at or near the Possible Changes in Regional Hydrology elevation of the local ELA. There is the possibility of reduced availability of The analysis of future climate change impacts on groundwater below the front of the HKH, for example, the hydrology of the HKH region is complex because the Gangetic plain in northern India. Andermann et of climate variability, sparse data, and uncertainties in al. (2012) have recently shown that groundwater stor- climate projections and the response of glaciers to cur- age in the fractured bedrock significantly influences rent and future changes in climate, as discussed earlier the Himalayan river discharge cycle for the Ganges in this chapter. River Basin. They show that water from rainfall and snowmelt is stored temporarily in a groundwater res- ervoir with a characteristic response time of about 45 16 The study was conducted by the U.S. Geological Survey days. Further, they suggest that water traveling through (USGS), under an agreement funded by the U.S. Agency for groundwater reservoirs in the eastern HKH represents International Development (USAID). It was conducted with co- operation from the Afghanistan Geological Survey (AGS) and the about two-thirds of annual discharge. Groundwater Afghanistan Ministry of Energy and Water (MEW). is the primary source of water during low-flow times
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PHYSICAL GEOGRAPHY 45 such as winter months, where consumptive use of water rates. One such misunderstanding is that retreating in the Ganges Basin is already greater than flow. The glaciers will lead to widespread flooding. While this is short response time and large amount of water flow- not likely, the region does face other physical hazards, ing through the groundwater systems, in combination including flash flooding due to extreme precipitation, with the GRACE measurements that suggest a rapid flooding due to monsoon rainfall, lake outbursts, land- reduction in the amount of groundwater in northern slides, and avalanches. Monsoon flooding and lake India, are consistent with a system that is experiencing outbursts are covered in detail here. overdrafting of groundwater. Current recharge rates do not appear to be able to replenish present rates of Monsoon Flooding groundwater removal. Continued or accelerated rates of groundwater removal can easily lead to water short- Monsoon flooding can cause loss of life and prop- ages on the scale of years. erty, and potentially economic and social calamities. The largest changes to the hydrological system over During the historic flooding in Bangladesh in 1998, the next decade or two will most likely be because of two-thirds of the country was submerged; about 1,000 changes in the timing, location, and intensity of mon- people perished from flooding or succumbed to water- soonal activity. Interannual variability of the monsoon borne diseases such as typhoid and cholera. Nearly strongly affects spatial patterns. At some locations in 16,000 km of roads and more than 700,000 hectares the central Himalayas, one monsoon depression alone of cropland were damaged or destroyed, and over one can account for 10 to 20 percent of all monsoon rainfall. million people were displaced (BBC News, 1998; del A small shift in storm path from one year to another Ninno et al., 2001). can cause large differences in water availability. An Monsoon flooding can have important political example is the flooding in Pakistan in July and August impacts. Because of the enormous cost involved in 2010 that resulted from an unusual combination of rebuilding businesses, agriculture, and infrastructure, severe weather events (Lau and Kim, 2011a). The 2010 a major monsoon flood can have profound and long- Pakistan flood caused historic social and economic term impacts on the policy and politics of the local losses for the country. The flooding was primarily due and national governments in the Himalayan region. to heavy rain that fell during late July and early August The way governments respond, and how they interact 2010, from a shift of monsoon activities from the Bay with international relief groups in managing the relief of Bengal to northern Pakistan. Glacial wastage from efforts, may also contribute to public perception of inef- the Himalayas likely did not play a role in this case. ficiency, favoritism, political discord, and unrest, as in While flow in the Indus and the Ganges/ the case of the 2010 Pakistan flood. Another example Brahmaputra basins will be highly affected by changes is the 2008 flooding of the Kosi River, which flows in precipitation, climate change will also have other from the Himalayan foothills of Nepal to a conflu- impacts on the hydrological cycle. For instance, evapo- ence with the Ganges in the Bihar region of northern transpiration rates may increase over large parts of the India. The flood event led to a dispute between Nepal irrigated area of both basins. To compensate, farmers and India regarding mismanagement of the Kosi River may apply more irrigation water, further drawing down (Malhotra, 2010). surface water and groundwater. This trend may be most The main cause of monsoon floods is heavy rain. important during the dry months of the year, and may Monsoon floods occur most frequently in the foothill be most significant on the Indus River, which already regions along the arc of the Himalayas, and low-lying has dry periods when available flow does not meet areas in the head of the Bay of Bengal during the sum- demand for irrigation water. mer monsoon season from June through September. The heavy monsoon rains range from Bangladesh, PHYSICAL EXTREME EVENTS Bhutan, Nepal, and northeastern India, to north and northwestern India and Pakistan. With few exceptions, As discussed in Chapter 1, lack of observational every year somewhere in the region, some degree of data in the HKH region has led to misunderstandings monsoon flooding will occur, due to the uneven distri- about the effects of climate change on glacial retreat bution of monsoon rain.
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46 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY A number of additional factors have the potential retreating glacier (moraine-dammed; Figure 2.17). The to contribute to the severity of monsoon floods by com- phenomenon of ice-dammed lakes is more prevalent in pounding the impact of heavy rains (NRC, 2012a). For the Karakoram Mountains in northern Pakistan and the example, if the heavy rains are coupled with unusually Pamirs. In the eastern Himalayas (e.g., Bhutan, Nepal) high volumes of runoff from melting snowpack in the GLOFs are generally caused by water draining rapidly Himalayas (especially rain-on-snow events), the result from supraglacial lakes or the collapse of moraines. Fail- might be devastating. Land use changes and defores- ure of the confining dam can have a variety of causes, tation could also affect the severity of monsoon floods including earthquakes, catastrophic failure of slopes through an increase in surface-water flow leading to into the lake (avalanches, rock slides, ice fall from a more severe flooding. Changes in the distribution of glacier into the lake), a buildup of water pressure, or monsoon rainfall in response to climate and other fac- even simple erosion of the confining dam over time. tors may bring heavy rain to relatively dry regions (e.g., An example of a potential threat from ice-dammed the 2010 Pakistan flood), where the local population lakes is the Medvezhi Glacier in the Pamir Mountains. may be less prepared or equipped to carry out preven- In 1963 and 1973, the surge of the glacier was large tion, evacuation, and mitigation measures. Although enough that the ice dam exceeded 100 m in height, cre- these scenarios are speculative, it is clear that how ating a lake of over 20 million m3 of water and debris. hydrological extremes (in this case, monsoon flooding) A series of large floods resulted from the outburst of are intertwined with anthropogenic effects is poorly that lake, but there were no victims because of monitor- understood (NRC, 2012a). ing and early warning systems. Infrastructural damage, The monsoon varies with many factors, includ- however, was significant (UNEP, 2007). ing air-sea interactions and land processes. Increased New glacial lake formation and the enlargement of aerosol concentrations may also influence the mon- existing lakes have resulted from thinning and retreat soon through local heating. The severity of monsoon of glaciers in the HKH region. Many glacial lakes in flooding also depends on the local topography and Nepal are growing at a considerable rate, increasing the infrastructure. Although the magnitude of the heavy risk to local populations. Twenty-four GLOF events monsoon rain during the Pakistan flood of 2010 was have occurred in Nepal in the recent past, causing small compared with that of the 1988 Bangladesh considerable loss of life and property. For example, the flood, the impacts of the flooding were equally devas- 1981 Sun Koshi GLOF damaged the only road link to tating. One-fifth of the country was underwater; nearly China and disrupted transportation for several months, 2,000 people perished; 20 million people were affected. and the 1985 Dig Tsho GLOF destroyed the nearly The direct damage caused by the floods was estimated completed Namche Small Hydroelectric Project, in to be US$6.5 billion, with an additional US$3.6 billion addition to causing other damage farther downstream in indirect losses (Asian Development Bank and World (Bajracharya et al., 2007). Bank, 2010). Outburst Floods A glacial lake outburst flood (GLOF) is a type of flood that occurs when water dammed by a glacier or a moraine is rapidly released by failure of the dam (e.g., Bajracharya et al., 2007; Hewitt, 1982; Xin et al., 2008). There are two distinctly different forms of glacial lake outbursts: those that result from the collapse or over- topping of ice dams formed by the glacier itself, and those that occur when water drains rapidly from lakes FIGURE 2.17 Schematic diagram of a moraine-dammed gla- formed either on the lower surface of glaciers (supragla- cial lake formed by glacial meltwater. Failure of the confining cial) or between the end moraine and the terminus of a moraine dam leads to an outburst flood.
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PHYSICAL GEOGRAPHY 47 There is no doubt that people and property for Sah, 2008), LLOFs will be a greater future risk. That considerable distances downstream from the unstable being said, the development of landslide lakes is almost lakes are facing a serious threat; the problem, however, certainly less predictable than meltwater lakes because is how to determine the degree of probability of such landslides are more random and where they will occur an event. Analysis of the rapidly growing worldwide is harder to predict than glacial retreats. Thus, LLOFs literature, including field and theoretical knowledge, are even less predictable than GLOFs. on the outburst of glacial lakes, led a recent ICIMOD commission on GLOFs in Nepal to conclude that it CONCLUSIONS is not feasible to make a reliable prediction of a spe- cific occurrence on the basis of existing knowledge Key features of the physical geography of the HKH (ICIMOD, 2011a). Because direct predictions cannot region were identified at the workshop by the breakout be made, a careful selection of prioritized lakes needs groups on Climate and Meteorology and on Hydrol- to be monitored on a regular basis. ogy, Water Supply, Use, and Management. Starting GLOFs are not the only outburst lake hazards in from those concepts, the Committee used its expert the HKH region. Another is the landslide lake out- judgment, reviews of the literature, and deliberation to burst flood (LLOF), which is a catastrophic release of develop the following conclusions: impounded water from behind a natural dam formed by a landslide. In the steep mountainous Himalayas, · The climate of the Himalayas is not uniform landslides are a common event, whether they are trig- and is strongly influenced by the South Asian monsoon gered by normal weathering and erosion processes, and the mid-latitude westerlies. Projecting impacts of extreme rainfall events, or earthquakes. The release climate change in the Himalayas is challenging because potential of water by LLOFs can exceed that for of complex interactions between global, regional, and GLOFs (Hewitt, 1982). This is because landslide local forcing and responses. dams can be very large. Dunning et al. (2006) describe · Evidence suggests that the eastern Himalayas a landslide dam that formed in Bhutan in 2003 and and the Tibetan Plateau are warming, and this trend subsequently impounded 4 × 106 to 7 × 106 m3 of water is more pronounced at higher elevations. However, a before its failure in 2004. Landslide lakes can also occur lack of sufficient paleoclimate data makes assessing at much lower elevations than glacial meltwater lakes, the long-term significance of this warming trend a where they can impound runoff from larger upstream challenge. catchment areas compared with the areas contributing · There are sparse historical climate data in the to glacial meltwater lakes. Hewitt (1982) provides a region, but scientists are fairly confident about projec- detailed history of outburst floods in the Karakoram, tions of future temperature increases. There is more including some massive LLOFs, and Gupta and Sah uncertainty in projections of amounts and timing of (2008) present an example of a LLOF in the Satluj precipitation. catchment in Himachal Pradesh, India, well below the · Aerosols from the combustion of fossil fuels, termini of any glaciers above it. wood, and other sources are increasing in the Indo- Like GLOFs, LLOFs pose a serious hazard to Gangetic Plain and the foothills of the Himalayas. people, property, and infrastructure downstream from Absorbing aerosols such as desert dust and black carbon the landslide dams. However, compared with the may contribute to the rapid warming of the atmosphere, large number of glacial meltwater lakes forming and model results indicate that this may in turn contrib- today because of climate warming and glacial retreat, ute to accelerated melting of snowpack and retreat of landslide lakes are less commonly formed in the high glaciers. Black carbon deposited directly on non-debris- Himalayas (e.g., Hewitt, 1982). Therefore, the risk covered glaciers and snowpack could increase the rate posed by future LLOFs is likely to be significantly less of retreat by reduction of surface albedo. than that posed by GLOFs in the HKH region. In the · Over the next few decades, atmospheric concen- lower trans-Himalayan regions where landslide lakes trations of greenhouse gases are projected to continue replace meltwater lakes as hazards (e.g., Gupta and to increase globally, while black carbon aerosols are
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48 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY likely to continue to increase in the Indo-Gangetic becomes more problematic for flows in the upper Plain and Himalayas. Unless these trends are stabi- reaches of the eastern HKH over the long term. lized or reversed, the impacts of greenhouse gases and · In the western HKH where more of the surface- black carbon on the rate of Himalayan glacial retreat water flow is from higher elevations, the contribution will increase. That is, both the rate and volume of the of glacial wastage could be particularly important glacial retreat will be relatively greater than it would in affecting the timing and volume of surface-water be otherwise. discharge. · The rate of retreat and growth of individual · Overall, retreating glaciers over the next several glaciers is highly dependent on glacier characteristics decades are unlikely to cause significant change in and location. The most vulnerable glaciers are small flows at lower elevations, which depend primarily on glaciers at low elevation and with little debris cover. monsoon rain. However, for high-elevation areas, cur- These characteristics also make glaciers more suscep- rent glacial retreat rates, if they continue, appear to be tible to black carbon deposition, and model results sufficient to alter the seasonal and temporal streamflow indicate black carbon deposition may make them more in some basins. Removing water stored as glacial ice vulnerable to retreat. does not imply any a priori effect on average annual · In the eastern and central Himalayas there is discharge in the long term, assuming annual precipita- evidence of glacial retreat with rates accelerating over tion remains the same. In the short term with constant the past century. Retreat rates are comparable to other annual precipitation, glacial wastage will augment the areas of the world. Glaciers in the western Himalayas quantity of streamflow. appear to be more stable overall, with evidence that · Limited streamflow data in upper basin regions, some may even be advancing. along with government constraints on scientific access · In the short term, climate change is likely to to international streamflow data, increase the uncer- increase glacial wastage. In the longer term the impact tainties surrounding hydrological trends, variability, of continued retreat of glaciers is not clear. The rate of and extreme events in the region. Limited streamflow glacial retreat depends not only on temperature, but data also limit the understanding of the relative contri- also on precipitation changes associated with the sum- butions of rain, snowmelt, and glacial meltwater, as well mer monsoon in the central and eastern HKH and the as groundwater recharge mechanisms in the region. winter westerlies in the western HKH. Black carbon · Uncertainties in the role of groundwater in aerosols, via atmospheric heating and deposition on the overall hydrology of the region are even greater snowpack and glaciers, may increase the rate of glacial than those of surface water. Current understanding of wastage. groundwater in the region is confounded by a variety · Surface-water flow is highly seasonal and varies of limitations including knowledge gaps about the across the region, as does the relative importance of interaction between surface water and groundwater; glacial meltwater. In most instances, the annual contri- difficult terrain; the fractured and variable nature of bution of snowmelt and rainfall to streamflow exceeds the underlying geological substrate; and the inability to that of glacier wastage. Recent literature indicates that easily distinguish the contributions of snowmelt, glacial the importance of the glacial contribution to runoff has meltwater, monsoonal precipitation, and human actions previously been overestimated. such as groundwater overdraft to flows. Evidence sug- · The contribution of glacial wastage can be more gests that sizable and extensive overdraft in the central important when the glacial wastage acts as a buffer Ganges Basin is likely to have an earlier and larger against hydrological impacts brought about by a chang- impact on water supplies than foreseeable changes in ing climate. For example, in the late summer when all glacial wastage. snow has melted and the monsoon-rainfall contribu- · For upstream populations, GLOFs and LLOFs tion is declining or in the eastern HKH during times are the dominant physical hazard risk. For downstream of drought. populations in the central and eastern Himalayas, · Although retreating glaciers will subsidize sur- floods from changes in monsoon rainfall and cyclones face flow and mitigate immediate losses to discharge are more likely to be important, along with changes in by retreating glaciers, the loss of glacier "insurance" the timing of extreme events.