10.4.2.2 Water quality
Over-exploitation of groundwater in many countries of Asia has resulted in a drop in its level, leading to ingress of sea water in coastal areas making the sub-surface water saline. India, China and Bangladesh are especially susceptible to increasing salinity of their groundwater as well as surface water resources, especially along the coast, due to increases in sea level as a direct impact of global warming (Han et al., 1999). Rising sea level by 0.4 to 1.0 m can induce salt-water intrusion 1 to 3 km further inland in the Zhujiang estuary (Huang and Xie, 2000). Increasing frequency and intensity of droughts in the catchment area will lead to more serious and frequent salt-water intrusion in the estuary (Xu, 2003; Thanh et al., 2004; Huang et al., 2005) and thus deteriorate surface and groundwater quality.
10.4.2.3 Implications of droughts and floods
Global warming would cause an abrupt rise of water quantity as a result of snow or glacier melting that, in turn, would lead to floods. The floods quite often are caused by rise of river water level due to blockage of channels by drifting ice, as happened in Central Siberia, Lensk, or enormous precipitation from destructive shower cyclones, as it was in the North Asia Pacific coast, Vladivostok (Izrael et al., 2002a). A projected increase in surface air temperature in North-West China will result in a 27% decline in glacier area (equivalent to the ice volume of 16,184 km3), a 10 to 15% decline in frozen soil area, an increase in flood and debris flow, and more severe water shortages (Qin, 2002). The duration of seasonal snow cover in alpine areas, namely the Tibetan Plateau, Xinjiang and Inner Mongolia of China, will shorten and snow cover will thaw out in advance of the spring season, leading to a decline in volume and resulting in severe spring droughts. Between 20 to 40% reduction of runoff per capita in Ningxia, Xinjiang and Qinghai Province is likely by the end of 21st century (Tao et al., 2005). However, the pressure due to increasing population and socio-economic development on water resources is likely to grow. Higashi et al. (2006) project that future flood risk in Tokyo, Japan between 2050 to 2300 under SRES A1B is likely to be 1.1 to 1.2 times higher than the present condition.
The gross per capita water availability in India will decline from about 1,820 m3/yr in 2001 to as low as about 1,140 m3/yr in 2050 (Gupta and Deshpande, 2004). India will reach a state of water stress before 2025 when the availability falls below 1000 m3 per capita (CWC, 2001). The projected decrease in the winter precipitation over the Indian subcontinent would reduce the total seasonal precipitation during December, January and February implying lesser storage and greater water stress during the lean monsoon period. Intense rain occurring over fewer days, which implies increased frequency of floods during the monsoon, will also result in loss of the rainwater as direct runoff, resulting in reduced groundwater recharging potential.
Expansion of areas under severe water stress will be one of the most pressing environmental problems in South and South-East Asia in the foreseeable future as the number of people living under severe water stress is likely to increase substantially in absolute terms. It is estimated that under the full range of SRES scenarios, 120 million to 1.2 billion, and 185 to 981 million people will experience increased water stress by the 2020s, and the 2050s, respectively (Arnell, 2004). The decline in annual flow of the Red River by 13 to 19% and that of Mekong River by 16 to 24% by the end of 21st century will contribute in increasing water stress (ADB, 1994).