Authors:
Walter W. Immerzeel1,2, Ludovicus P. H. van Beek2, Marc F. P. Bierkens2,3
1 FutureWater, Costerweg 1G, 6702 AA Wageningen, The Netherlands.
2 Department of Physical Geography, Utrecht University, Post Office Box 80115, Utrecht, The Netherlands.
3 Deltares, Post Office Box 80015, 3508 TC Utrecht, The Netherlands.
More than 1.4 billion people depend on water from the Indus, Ganges, Brahmaputra, Yangtze, and Yellow rivers. Upstream snow and ice reserves of these basins – important in sustaining seasonal water availability – are likely to be affected substantially by climate change, but to what extent is yet unclear. Here, we show that meltwater is extremely important in the Indus basin and important for the Brahmaputra basin, but plays only a modest role for the Ganges, Yangtze, and Yellow rivers. A huge difference also exists between basins in the extent to which climate change is predicted to affect water availability and food security. The Brahmaputra and Indus basins are most susceptible to reductions of flow, threatening the food security of an estimated 60 million people.
Mountains are the water towers of the world (1), including for Asia, whose rivers all are fed from the Tibetan plateau and adjacent mountain ranges. Snow and glacial melt are important hydrologic processes in these areas (2, 3), and changes in temperature and precipitation are expected to seriously affect the melt characteristics (4). Earlier studies have addressed the importance of glacial and snow melt and the potential effects of climate change on downstream hydrology, but these are mostly qualitative (4–6) or local in nature (7, 8). The relevance of snow and glacial melt for Asian river basin hydrology, therefore, remains largely unknown, as does how climate change could affect the downstream water supply and food security.
We examined the role of hydrological processes in the upstream areas, which we defined as all areas higher than 2000 meters above sea level (masl), on the water supply of the five major Southeast Asian basins (Figure 1). These basins, which provide water to more than 1.4 billion people (over 20% of the global population), vary considerably in their characteristics (Table 1). The Yangtze has the largest population of the five basins, whereas the Ganges is the most densely populated. The Indus and Brahmaputra basins have extensive upstream areas (i.e., above 2000 m) and larger glaciated areas than the Yangtze and Yellow river basins (9). The Ganges, Brahmaputra and Yangtze basins are wetter than the Yellow and Indus basins (10). The Indus, Ganges, and Yangtze basins support large-scale irrigation systems (11) with high net irrigation water demand, but in the Indus the difference between basin precipitation and net irrigation demand is highest. We investigated three related components of these river basins:
(i) the current importance of meltwater in overall river basin hydrology;
(ii) observed cryospheric changes; and
(iii) the effects of climate change on the water supply from the upstream basins and on food security.
Figure 1: Basin boundaries and river courses of the Indus, Ganges, Brahmaputra, Yangtze, and Yellow rivers. Blue areas denote areas with elevation exceeding 2000 masl. The digital elevation model in the background shows the topography ranging from low elevations (dark green) to high elevations (brown).
We used the Normalized Melt Index (NMI) over the period 2001 to 2007 to quantify the importance of meltwater from the upstream areas on overall basin hydrology. NMI is defined as the volumetric snow and glacier upstream discharge divided by the downstream natural discharge. Upstream discharge is calculated with a calibrated Snow-melt Runoff Model (SRM) (12, 13). Downstream natural discharge is calculated by subtracting the Natural Evaporation (En) of the basins, calculated with a hydrological model (14), from Precipitation (P) (15). En excludes additional evaporation from irrigated areas, because irrigation water is derived from upstream sources. The difference P – En is, therefore, a measure of natural downstream discharge. The NMI is amore reliable measure than the commonly used meltwater fractions of total river discharge, which are affected by reservoirs, and water extractions (13). The great size of the basins that we analyze allows us to use melt parameters calculated for whole basins, rather than a different set of melt parameters for each different glacier, because each basin contains many glaciers of all types. Results from the NMI analysis (Figure 2) indicate that for the present-day climate, meltwater plays an important role in the Indus and Brahmaputra river basins. This is most evident in the Indus: Discharge generated by snow and glacial melt is 151% of the total discharge naturally generated in the downstream areas. In the Brahmaputra basin, this amounts to 27%. The contribution of snow and glacier water to the Ganges (10%), Yangtze (8%), and Yellow (8%) rivers is limited owing to comparatively large downstream areas, limited upstream precipitation, smaller glaciers, and/or wet monsoon-dominated downstream climates (Table 1). In the Indus and Ganges basins, about 40% of the meltwater originates from glaciers, whereas in the other basins the glacial melt contribution is much less.
Since the end of the last ice age, an almost worldwide recession in glaciers has been observed (16), a trend that also applies to most of the glaciers in the Himalayas. Annual net imbalance rates of 0.5 to 0.9 m year-1 have been observed from time series of digital elevation models in the Everest region in Nepal (17) and SPOT satellite imagery in the western Himalayas (18), whereas radioactivity analysis in ice cores revealed no net accumulation of ice in a high-elevation glacier in Tibet (19). However, there are some regional anomalies (13). We used the DMT-1 GRACE gravity model (20) in combination with derived precipitation trends (10) to identify large-scale trends in snow and ice storage in each of the five basins. Results were inconclusive. We identified a negative trend of -0.22 T 0.05 m year-1 only in the Ganges basin. A positive trend of 0.19 T 0.02 m year-1 was observed in the Indus basin, while in the other basins no discernable trends were identified (13). On the basis of this review, we conclude that there is a general decrease in the ice volumes of Asian basins, although regional anomalies exist and, as regional quantification of these trends is lacking, the uncertainty about these trends is substantial.
