|The Regional Impacts of Climate Change|
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10.2.2. Hydrology and Water Resources
Global warming affects stream hydrology in many ways. In mid- and high-latitude regions-where warmer temperatures and a shift toward more rainfall and less snowfall in winter are projected-the seasonality of discharge regimes of snowmelt streams would shift toward higher winter flows and lower spring and summer flows. In semi-arid regions, streams and rivers may experience large increases in hydrological variability, with more frequent and larger floods with high sediment loads and longer droughts. In humid regions, streams and rivers are likely to experience reductions in total rainfall and/or increases in evapotranspiration that produce longer, more severe droughts during the warm season (IPCC 1996, WG II, Sections 10.3.5, 10.3.7, and 10.4.2).
In mountain regions, glaciers will provide extra runoff as the ice disappears. In general, the extra runoff would persist for a few decades; in areas with very large glaciers, it may last for a century or more. By 2050, the volume of runoff from glaciers in central Asia is projected to increase threefold. Tentative estimates have been made for central Asia based on mass balances from a small number of Tien Shan glaciers for the period 1959-1992. Extrapolation to the whole area of central Asia suggests that its glacier mass may have decreased by 804 km3 over that time, representing a 15% increase in glacial runoff. Eventually, however, glacial runoff will taper off or even cease. Projected glacier runoff in 2100 is about 68 km3/yr, compared with the present value of 98 km3/yr (IPCC 1996, WG II, Section 7.4.2).
Widespread loss of permafrost over extensive continental and mountain areas would trigger erosion or subsidence of ice-rich landscapes and change hydrological processes. Revegetation of terrain following deglaciation is slow in high-mountain areas. This lag leaves morainic deposits unprotected against erosion for extended periods (decades to centuries), which can result in increased sediment loads in alpine rivers and accelerated sedimentation in lakes and artificial reservoirs at high altitudes. On slopes steeper than about 25-30 degrees, stability problems such as debris flows will develop in freshly exposed or thawing nonconsolidated sediments (IPCC 1996, WG II, Section 7.4.3 and Box 7-2).
Overall, most 2xCO2 equilibrium scenario simulations show a decrease in water supplies throughout Temperate Asia, except in a few river basins. Warmer winters may affect water balances because water demands are higher in spring and summer. Glacial melt may lead to nonsustainable supplies of surface water (IPCC 1996, WG II, Chapter 14).
To balance water supply with water demand, more efficient water management is likely to be the approach taken by Japan. In other parts of Temperate Asia, water resource development will remain important. There, the central adaptation issue is how climate change might affect the design of new water resource infrastructure.
Because many river basins are international, climate change would exacerbate current international conflicts over water use and potentially cause new conflicts (IPCC 1996, WG II, Section 14.2.3). Clearly, multiple-stress impact studies on water resources in international river basins are needed.
Table 10-5 shows the impacts of climate change on
the annual runoff of seven river basins in China, located in different climate
zones from south to north. For all four GCMs, projected changes in runoff are
the result mainly of changes in precipitation in spring, summer, and autumn
because of the strong influence of the monsoon climate (Liu, 1997).
Adaptation strategies for water-resource management in China could include:
The most critical area of uncertainty is the lack of credible projections of the regional climate in Temperate Asia. In particular, the effect of climate change on the Asian monsoon or the ENSO phenomenon is unknown. Presently, there are two diametrically opposite projections: one showing a strengthening and the other a weakening of the Asian monsoon, which would result in completely opposite impacts on hydrology and water resources. Other uncertainties are introduced in the downscaling of precipitation from the GCM grid scale to the scale of hydrological models for river basins, through the stochastic or interpolation method. There also is considerable uncertainty in the translation of climate change into hydrological effects through hydrological models: The parameters in hydrological models, which are calibrated by historical data, are not transferable either geographically or in terms of a changing environment. In addition, the impact of climate change on evapotranspiration and groundwater-both of which are important for long-term projections of water resources-so far cannot be estimated appropriately.
Research and monitoring needs include projection of regional climate scenarios with high spatial and temporal resolution, improved hydrological models with appropriate land surface parameters under nonstationary climatic conditions, integrated impact studies that consider different sectors and their responses to adaptation strategies, and enhanced hydrological monitoring networks.
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