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The impact of climate change on the water resources of Tropical Asia may be significant. Whetton et al. (1994) suggest that increased evaporation (resulting from higher temperatures), combined with regional changes in precipitation characteristics (e.g., total amount, variability, and frequency of extremes), has the potential to affect mean runoff, frequency, and intensity of floods and droughts; soil moisture; and water supplies for irrigation and hydroelectric generation. Anglo et al. (1996) confirm that water resources in the region are very sensitive not only to changes in temperature and precipitation but also to changes in tropical cyclones. For example, runoff in Nepal is affected by snow cover, the southWest monsoon, and cyclones-all of which may be affected by climate change.
Water resources in the region also are vulnerable to increasing demand resulting
from population growth, urbanization, industrialization, and agriculture (Schreir
and Shah, 1996). At present, agriculture is the predominant water use throughout
the region (see Table 11-5).
The effects of climate change on hydrology in Tropical Asia would have many facets. In the Himalayas, the storage of precipitation in the form of snow and ice (in glaciers) over a long period provides a large water reservoir that regulates annual water distribution. The majority of rivers originating in the Himalayas have their upper catchments in snow-covered areas and flow through steep mountains. This factor and the perennial nature of the rivers provide excellent conditions for the development of hydropower resources, despite the temporal variability in the sources of runoff for Himalayan rivers. Studies of one large catchment in the western Himalayas (the Chenab, a tributary of the Indus) show that the average snowmelt and glacier-melt contribution to the annual flow is 49.1%; a significant proportion of runoff is derived from snow in the dry season, when water demand is highest (Singh et al., 1997). Climate change-related increases in temperature also could increase the rate of snowmelt and reduce the amount of snowfall, if the winter is shortened.
If climate change does alter the rainfall pattern in the Himalayas, the impacts could be felt in the downstream countries-that is, India and Bangladesh. By and large, dry-season flow in the major Himalayan rivers in a given year results from the monsoon rainfall of the previous year. Catchments in Nepal supply about 70% of the dry-season flow of the Ganges River, and tributaries of the Brahmaputra River originating in Bhutan supply about 15% of the total annual flow of that river. If climate change disrupts these resources and alters mountain hydrological regimes, the effects will be felt not only in the montane core of Tropical Asia but also downstream, in countries that depend on this water resource.
Any change in the length of the monsoon also would be significant. For instance, if the southWest monsoon arrives later or withdraws earlier, soil moisture deficits in some areas may get worse. On the other hand, prolonged monsoons may contribute to more frequent flooding and increase the depth of inundation in many parts of the large river basins. If precipitation increases, dry-season river flow may increase because of increased recharge in the monsoon season. For instance, flooding is experienced on the east coast of Malaysia and in the coastal areas of Sarawak and Sabah almost every year during the rainy northeast monsoon season; these floods become severe and catastrophic with heavy rainstorms around the same time of the year (Sooryanarayana, 1995). Increases in rainfall during the northeast monsoon, as well as increases in the magnitude of extreme rainfall events-both of which have been projected with climate change-are expected to increase the frequency and intensity of flooding in the region.
Divya and Mehrotra (1995) examined regional effects of climate change on various components of the hydrological cycle-such as surface runoff, soil moisture, and evapotranspiration-by applying a conceptual model on a monthly time scale. The experiment was conducted using hypothetical scenarios in three drainage basins located in different agroclimatic zones of central India. The authors found that basin characteristics-such as soil type, moisture-holding capacity, and runoff coefficients-significantly influence basin runoff. More recently, Mirza (1997) modeled nine subbasins of the Ganges River using the GISS transient scenario (a global coupled ocean-atmosphere climate model with 4x5-degree resolution). The GCM experiment is based on 74 years, during which CO2 increases by 1% (compounded) per year. Mirza (1997) found that changes in mean annual runoff in the range of 27-116% occurred in the subbasins at doubled CO2 and that runoff was more sensitive to climate change in the drier subbasins than in the wetter subbasins.
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