Water resource system degradation

In addition to the above, existing patterns of water pollution and quality degradation are likely to both exacerbate and be exacerbated by the impacts of climate change. These water quality impacts, while important, are beyond our focus here.

Over-exploitation of Groundwater Resources

The over-exploitation of groundwater resources in South Asia, particularly India, is now well documented (World Bank and Ministry of Water Resources - Government of India 1998; Shah, Roy et al. 2003; Centre for Water Policy 2005). In many regions water levels are falling and groundwater quality is declining as the concentrations of naturally occurring contaminants (such as salinity and arsenic) and pollutants (such as agricultural and industrial chemicals) increase. The combination of depletion and quality degradation is already undermining the agricultural, economic and other benefits groundwater development has delivered in recent decades.

Since the late 1960s, groundwater has played a major role in stabilizing agricultural production and as a lead input underlying yield growth across much of South Asia. It has, in essence, served as the primary source of water buffering the inherent and natural variability of weather and regional climatic patterns on weekly to seasonal time scales. This has generated major additional benefits with respect to poverty alleviation and economic development (Moench 2003). In addition, groundwater serves as a major source of water for most large cities and towns as well as for rural populations.

Climate change processes are likely to increase both the variability of precipitation and the duration and intensity of extreme drought as well as flood events (IPCC 2001). In areas where groundwater resources are either depleted or their quality has degraded, the inability of groundwater resources to serve as a “buffer” source of supply will greatly increase the impact of such variability. Groundwater depletion could, in fact, serve as a major lead indicator of likely climate impacts on water supply for multiple uses for regions where variability increases.

Urban Water Supply Problems

Urban water supply systems across South Asia are already stretched beyond capacity. In most cities, a large proportion of the population is not served by the existing municipal utility and depends on informal sources of supply. Furthermore, even for populations who do receive water from the formal system, supplies are highly irregular (one or two hours per day) and of questionable quality.

Climate change processes are likely to substantially increase pressure on such already over-stretched urban supply systems. Prior research indicates that income diversification through migration to urban and peri-urban areas is a core strategy rural populations utilize to buffer the impacts of climatic variability (Moench and Dixit 2004). As a result, urbanization rates are likely to further increase as climate change proceeds.

Alteration of Drainage Basins

Flow blockages in large river basins such as the Ganges are a major factor contributing to the extent of flood problems in many areas. Such blockages are caused by the construction of roads, railways, buildings and, perhaps most importantly, water control infrastructure. Disputes between India and Nepal have, for example, been occurring for more than a decade due to the flooding of land in Nepal by the Kosi Barrage. Within India, although they were intended to control floods, the construction of embankments has actually resulted in the flooding of large areas (Mishra 1997). This happens in part because much of the precipitation occurs within the basin, and it is drainage, rather than the control of flows coming down the river, that is the primary physical requirement to avoid flooding. It also happens because sedimentation and other fundamental problems within embankment systems cause frequent breeching resulting in much more severe inundation than would otherwise have occurred in areas where levees fail.

Problems related to the alteration of drainage patterns are not confined to rural areas. The Bombay floods of 2005 and the flooding of the Lei River basin in Rawlpindi, Pakistan, both resulted in substantial loss of life and property. Both of these floods were caused when extreme rainfall events occurred in urban areas where buildings and impermeable pavements combined to block drainage and increase runoff.  

If the intensity of extreme storm events increases as a consequence of global warming, changes in drainage basins such as those identified above are likely to substantially increase the severity of flood related impacts. 

Sediment Transport

Global scientific results indicate that “a more active hydrological cycle with more heavy precipitation events” (IPCC 2001 p.6) is one of the most likely impacts of climate change. If storm intensities increase and a greater proportion of precipitation occurs as rainfall instead of snow, average erosion rates would logically increase as would the massive pulses of sediment movement that occur as base load.

The implications of changes in sediment load for water management are important to recognize. The Yellow River, for example, is known to carry the heaviest sediment load of any river on the planet. This high sediment load has plagued attempts to manage floods and water in the river throughout China’s history. Silt deposition between flood control embankments has caused the riverbed to rise by up to one meter per decade (Gray, Osterkamp et al. 2002). Riverbed levels are, in some locations, now as much as 10 meters above the surrounding plain. Sediment load is closely correlated with precipitation intensity. Studies of the Yellow River show that “most of the sediment load is produced by a few major storms during the flood season, when the daily precipitation reaches 100-200 mm. In some areas, one storm event can contribute to more than 50 percent of the total annual sediment load. Very heavy storms can increase the annual sediment yield of small watersheds by a factor of two or more" (Mou 1991). The Yellow River passes through the Loess Plateau, a region where soils are highly vulnerable to erosion. The pulsed nature of sediment fluxes isn’t, however, limited to this type of situation. Studies on 20 of the largest streams entering the Pacific Ocean along the California coast between Monterey Bay and the Mexico border documented tremendous changes in sediment flux (Inman and Jenkins 1999). According to Inman, between 1948 and 1968 dry climatic conditions prevailed in the region. This abruptly transited in 1969 to a much wetter period that extended for an equal period of time. During three major flood years in the wet period, the sediment flux averaged 27 times the average annual flux in the dry period. In some rivers the amount of sediment moved in 1969, when the transition from wet to dry occurred, exceeded the entire amount of sediment moved during the preceding 25-year dry period.

In the Yellow River, high sediment loads have a major impact on dam storage capacity, food production in the lowlands (due to excessive sedimentation) as well as increasing the potential for major floods and the breaching of levees. This is also common in other regions. Furthermore, it is not a recent phenomenon associated with the large-scale water resources development activities of the last century. Studies of ancient irrigation systems in desert areas show similar problems with larger systems having failed in large part due to siltation (Evenari, Shanan et al. 1971). The core problem facing these historical irrigation systems was similar to those currently present in many large systems:

Such problems are proportional to the scale of structural intervention because larger structures tend to have a larger and more concentrated impact on river hydraulic properties. Problems are also influenced by overall basin sediment loads (the more sediment being moved, the greater the impact of hydraulic changes) and by flow variability. Where flows are highly variable, sediment movement occurs in pulses that are particularly difficult to manage and water control structures are more likely to be subject to sudden failure.

What is the key lesson from experiences with sediment transport for climate change? The implications are fairly straightforward: Anticipated increases in the variability and intensity of climatic events could logically exacerbate the types of sedimentation problems already present in many systems. If dry periods are followed by intense storms, the large pulses of sediment documented in the California watersheds are likely to be increasingly common. This may pose particular challenges for water management approaches that attempt to address water supply variability by increasing storage in reservoirs. It is also likely to pose particular problems for flood control strategies that rely on levees and other conventional flood control mechanisms.  

The extent of changes such as the sediment pulses discussed above, their regional distribution and the timing of change is uncertain. Some climate related changes, such as those related to average sea level, can be specified well in advance and are likely to occur in a gradual manner.  Other changes, such as those associated with floods, droughts and extreme storm events, will occur in abrupt pulses.

Bibliography

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