WATER

The vast majority of the Earth's water resources are salt water, with only 2.5% being fresh water. Approximately 70% of the fresh water available on the planet is frozen in the icecaps of Antarctica and Greenland leaving the remaining 0.7% of total water resources worldwide available for consumption. However from this 0.7%, roughly 87% is allocated to agricultural purposes.

These statistics are particularly illustrative of the drastic problem of water scarcity facing humanity. Water scarcity is defined as per capita supplies less than 1700 m3/year.

Source: World Resources 2000-2001: People and Ecosystems: The Fraying Web of Life. World Resources Institute, Washington DC (2000).
Figure 1: Freshwater availability: groundwater and river flow (2000). In UNEP/GRID-Arendal Maps and Graphics Library. Retrieved 15:50, March 26, 2008.

The Comprehensive Assessment of Water Management in Agriculture revealed that one in three people are already facing water shortages (2007). Around 1.2 billion people, or almost one-fifth of the world's population, live in areas of physical scarcity, while another 1.6 billion people, or almost one quarter of the world's population, face economic water shortage (where countries lack the necessary infrastructure to take water from rivers and aquifers); nearly all of which are in the developing countries.

Figure 2: Water use in the world (2005)

There are four main factors aggravating water scarcity:

• Population growth: in the last century, world population has tripled. It is expected to rise from the present 6.5 billion to 8.9 billion by 2050. Water use has been growing at more than twice the rate of population increase in the last century, and, although there is no global water scarcity as such, an increasing number of regions are chronically short of water.
• Increased urbanization will focus on the demand for water among an ever more concentrated population. Asian cities alone are expected to grow by 1 billion people in the next 20 years.
• High level of consumption: as the world becomes more developed, the amount of domestic water that each person uses is expected to rise significantly.
• Climate change will shrink the resources of freshwater.

Water and Climate Change

Water scarcity is expected to become an even more important problem than it is today. There are several reasons for this.

First, the distribution of precipitation in space and time is very uneven, leading to tremendous temporal variability in water resources worldwide (Oki et al., 2006). For example, the Atacama Desert in Chile receives imperceptible annual quantities of rainfall whereas Mawsynram, Assam, India receives over 450 inches annually. If all the freshwater on the planet were divided equally among the global population, there would be 5 000 to 6 000 m3 of water available for everyone, every year.

Second, the rate of evaporation varies a great deal, depending on temperature and relative humidity, which impact the amount of water available to replenish groundwater supplies.

The combination of shorter duration but more intense rainfall (meaning more runoff and less infiltration) combined with increased evapotranspiration (the sum of evaporation and plant transpiration from the earth's land surface to atmosphere.) and increased irrigation is expected to lead to groundwater depletion.

The Hydrological Cycle

The hydrologic cycle begins with evaporation from the surface of the ocean or land, continues as air carries the water vapor to locations where it forms clouds and eventually precipitates out. It then continues when the precipitation is either absorbed into the ground or runs off to the ocean, ready to begin the cycle over again in an endless loop. The amount of time needed for groundwater to recharge can vary with the amount or intensity of precipitation.

According to Pr. Andrew Goudie of Oxford University, key changes to the hydrological cycle associated with an increased concentration of greenhouse gases in the atmosphere and the resulting changes in climate include:

• Changes in the seasonal distribution and amount of precipitation
• An increase in precipitation intensity under most situations
• Changes in the balance between snow and rain
• Increased evapotranspiration and a reduction in soil moisture
• Changes in vegetation cover resulting from changes in temperature and precipitation
• Consequent changes in management of land resources
• Accelerated melting glacial ice
• Increases in fire risk in many areas
• Increased coastal inundation and wetland loss from sea level rise
• Effects of CO2 on plant physiology, leading to reduced transpiration and increased water use efficiency

Changes in Precipitation and Drought Patterns

Projections of changes in total annual precipitation indicate that increases are likely in the tropics and at high latitudes, while decreases are likely in the sub-tropics, especially along its polar edge. Thus, latitudinal variation is likely to affect the distribution of water resources. In general, there has been a decrease in precipitation between 10°S and 30°N since the 1980s. With the population of low latitude regions increasing, water resources are likely to become more stressed in many regions, especially as global warming intensifies.
Increasing intensity of precipitation is likely to increase a region's susceptibility to a multitude of factors including:

• Flooding
• Rate of soil erosion
• Mass movement of land
• Soil moisture availability

These factors are likely to affect key economic components of the GDP such as agricultural productivity, land values and an area's habitability. In addition, warming accelerates the rate of land surface drying, leaving less water moving in near-surface layers of soil. Less soil moisture leads to reduced downward movement of water and so less replenishment of groundwater supplies (IPCC). In locations where both precipitation and soil moisture decrease, drying of the land surface is magnified, and areas are left increasingly susceptible to reduced water supplies.

Although projecting how changed precipitation patterns will affect runoff is not yet a precise science, by interpreting historical discharge records, it is likely that for each 1°C rise in temperature, global runoff will increase by 4% (Labat, 2004). Applying this projection to changes in evapotranspiration and precipitation allows us to conclude that global runoff is likely to increase 7.8% globally by the end of the century. This places a region experiencing a higher annual precipitation amount and a larger volume of runoff at an increased likelihood for flooding.

Furthermore, in areas that are already vulnerable due to their limited groundwater storage availability, this cycle intensifies with increased warming and diminishing water supplies. In water stressed regions, variability of precipitation patterns is likely to further reduce groundwater recharge ability. Water availability is likely to be further exacerbated by: poor management, elevated water tables, overuse from increasing populations, and an increase in water demand primarily from increased agricultural production (IPCC).

In a recent global analysis by Dai et al, variations in PDSI indicated that the area of land characterized as very dry has more than doubled since the 1970s while the area of land characterized as very wet has slightly declined () during the same time period. In certain susceptible regions, increased temperatures have already resulted in diminished water availability. Precipitations in both western Africa and southern Asia have decreased by 7.5% between 1900 and 2005.

Most of the major deserts in the world including the Namib, Kalahari, Australian, Thar, Arabian, Patagonian and North Saharan, are likely to experience decreased amounts of precipitation and runoff with increased warming. In addition, both semiarid and arid areas are expected to have a decrease and seasonal shift in flow patterns. If increased temperatures cause an intensification of the water cycle there will be more extreme variations in weather events, as droughts will become prolonged and floods will increase in force.

Melting Glacial Ice

Decreasing water supplies can also be affected by warmer winter temperatures that decrease snowpack and result in diminished water resources during the summer months. This water supply is particularly important at the midlatitudes and in mountainous regions that depend upon glacial runoff to replenish river systems and groundwater supplies. Consequently, these areas will become increasingly susceptible to water shortages with time, because increased temperatures will initially result in a rapid rise in glacial meltwater during the summer months that will be followed by a decrease in melt as the size of the glacier continues to shrink. This reduction in glacial runoff water is projected to affect approximately 1/6 of the world's population (IPCC).

A reduction of glacial runoff has already been observed in the Andes, whereby the usual trend of glacial replenishment during winter months has been insufficient. This is due to increased temperatures which have caused the glaciers to retreat. It is likely that Andean communities such as El Alto in Bolivia have already observed a reduction in glacial runoff due to the scattered distribution of smaller sized glaciers, which further reduces the potential for runoff. In these areas, approximately one third of the drinking water is dependent upon these supplies, and the recurrent trend of increased melt with diminished replenishment provides a dismal projection for water reserves if this continues unabated.

Water Quality

Freshwater bodies have a limited capacity to process the pollutant charges of the effluents from expanding urban, industrial and agricultural uses. Water quality degradation can be a major cause of water scarcity.

Although the IPCC projects that global warming of several degrees will lead to an increase in average global precipitation over the course of the 21st century, this amount does not necessarily relate to an increase in the amount of potable water available.

One reason is a decline in water quality from an increase in runoff and precipitation that carries with it higher levels of nutrients, pathogens and pollutants. These contaminants were originally stored in the groundwater reserves but the increase in precipitation flushes them out in the discharged water.

Similarly, when drought conditions persist, and easily recoverable groundwater reserves are depleted, the residual water that remains is often of inferior quality due in part to the leakage of saline or contaminated water from the land surface, confining layers, or adjacent waters that have highly concentrated quantities of the depleting element(s). This occurs because decreased precipitation and runoff results in a concentration of effluent in the water, which leads to an increased microbial load in waterways and drinking-water reservoirs.

One of the most significant sources of water degradation results from an increase in water temperature. The increase in water temperatures can lead to a bloom in microbial populations, which among other things can have a negative impact on human health. Additionally, the rise in water temperature can adversely affect different inhabitants of the ecosystem due to a species' sensitivity to temperature. The health of a body of water, such as a river, is dependent upon its ability to effectively self purify through biodegradation, which is hindered when there is a reduced amount of dissolved oxygen. This occurs when water warms and its ability to hold oxygen decreases (IPCC).
Consequently, when precipitation events do occur, the contaminants are flushed into waterways and drinking reservoirs which has significant health implications.

Effects on Coastal Populations

For coastal populations, water quality is likely to be affected by salinization, or increased quantities of salt in water supplies. This will result from a rise in sea levels (projected between 14 cm and 44 cm by the end of this century), which will increase salt concentrations in groundwaters and estuaries. Sea-level rise will not only extend areas of salinity, but will also decrease freshwater availability in coastal areas. Saline intrusion is also a result of increased demand due in part to growing coastal populations that leave groundwater reserves increasingly vulnerable to contamination and diminishing water reserves.

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