The Regional Impacts of Climate Change

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11.3.3.3. Runoff and River Discharge

Water resources in Tropical Asia could be significantly affected by climate change. Results from early greenhouse warming simulations in MPI-ECHAm3 model experiments reported in Lal (1994) suggest that mean annual surface runoff throughout the region could increase by about 15% by the end of the century, with the greatest increases in northeast India and Indonesia. However, a more recent study (Lal et al., 1995b) that takes into account the combined effects of GHGs and aerosols suggests a future decline in surface runoff throughout the region.

In many parts of the region, the ratio of monsoon-to-dry-season runoff is very high (e.g., nearly 6:1 for the Ganges River). Because water requirements for agriculture and other water-use sectors are significantly higher during the dry season than during the monsoon, water supply in the dry season often cannot meet demand. Any change in the availability of water resources as a consequence of climate change may have a substantial effect on agriculture, navigation, fisheries, industrial and domestic water supply, reservoir storage and operation, and salinity control (Divya and Mehrotra, 1995; Mirza and Dixit, 1997).

Mirza (1997) applied empirical models using standardized precipitation scenarios from the CSIRO9, UKMO, GFDL, and LLNL GCMs to estimate changes in mean annual discharge of the Ganges and Brahmaputra rivers in Bangladesh. For the CSIRO9-E1 (Whetton et al., 1993), GFDL-A2 (Whetherald and Manabe, 1986), and LLNL GCM experiments, the global warming value is equivalent to the climate sensitivity of the climate model (i.e., the equilibrium global mean temperature change for a doubling of CO2-equivalent concentration). For the UKMO-X5 GCM experiment (Murphy and Mitchell, 1995), the changes are defined as the difference between the mean climate state of one decade in the perturbed integration minus the climate state of the equivalent decade in the control integration. In the study, a 2-6C temperature increase was considered; under a 4C global mean temperature change scenario, mean annual discharge of the Ganges River could increase by 27% (CSIRO9), 42% (UKMO), 15% (GFDL), or 2% (LLNL). Under the same temperature scenario, mean annual discharge of the Brahamaputra River may change by -0.1%, +13%, +9%, and +2% for the four GCMs, respectively. For higher temperature increases, the changes vary linearly.

The sediment load transported by these rivers is a nonlinear function of discharge. The implications of these increases in discharge on the sediment load of the Ganges and Brahamaputra-which already carry an extraordinarily heavy sediment load and have a high, though irregular, rate of downstream deposition-could be severe (Subramanian and Ramanathan, 1996).

Riebsame et al. (1995) conducted a study on the potential effect of climate change on water resources of the Mekong River basin. Equilibrium precipitation scenarios for doubled CO2 from the GISS (Hansen et al., 1983), GFDL (Mitchell et al., 1990), and UKMO (Wilson and Mitchell, 1987) GCMs, as well as a transient scenario (for the year 2030) from the GISS GCM (Hansen et al., 1988), were used in the assessment. Riebsame et al. (1995) found slight or no changes in annual river flows, but they did detect changes in seasonality. Reduced hydropower production and low-flow augmentation from a planned cascade of 13 dams also were projected.



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