126.96.36.199 Freshwater resources
The direct influences of sea-level rise on freshwater resources come principally from seawater intrusion into surface waters and coastal aquifers, further encroachment of saltwater into estuaries and coastal river systems, more extensive coastal inundation and higher levels of sea flooding, increases in the landward reach of sea waves and storm surges, and new or accelerated coastal erosion (Hay and Mimura, 2005). Although the coast contains a substantial proportion of the world’s population, it has a much smaller proportion of the global renewable water supply, and the coastal population is growing faster than elsewhere, exacerbating this imbalance (see Section 6.2.2 and Chapter 3, Section 3.2).
Many coastal aquifers, especially shallow ones, experience saltwater intrusion caused by natural and human-induced factors, and this is exacerbated by sea-level rise (Essink, 2001). The scale of saltwater intrusion is dependent on aquifer dimensions, geological factors, groundwater withdrawals, surface water recharge, submarine groundwater discharges and precipitation. Therefore, coastal areas experiencing increases in precipitation and run-off due to climate change (Section 6.3.2), including floods, may benefit from groundwater recharge, especially on some arid coasts (Khiyami et al., 2005). Salinisation of surface waters in estuaries is also promoted by a rising sea level, e.g., Bay of Bengal (Allison et al., 2003).
Globally, freshwater supply problems due to climate change are most likely in developing countries with a high proportion of coastal lowland, arid and semi-arid coasts, coastal megacities particularly in the Asia-Pacific region, and small island states, reflecting both natural and socio-economic factors that enhance the levels of risks (Alcamo and Henrichs, 2002; Ragab and Prudhomme, 2002). Identifying future coastal areas with stressed freshwater resources is difficult, particularly where there are strong seasonal demands, poor or no metering, and theft of water (Hall, 2003). Overall efficiency of water use is an important consideration, particularly where agriculture is a large consumer, e.g., the Nile delta (see Chapter 9, Box 9.2) and Asian megadeltas.
Based on the SRES emissions scenarios, it is estimated that the increase in water stress would have a significant impact by the 2050s, when the different SRES population scenarios have a clear effect (Arnell, 2004). But, regardless of the scenarios applied, critical regions with a higher sensitivity to water stresses, arising from either increases in water withdrawal or decreases in water available, have been identified in coastal regions that include parts of the western coasts of Latin America and the Algerian coast (Alcamo and Henrichs, 2002).