Water and Climate Change
Water scarcity is expected to become an ever-increasing problem in the future, for various reasons. 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, the driest place on earth, receives imperceptible annual quantities of rainfall each year. On the other hand, 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 (Vorosmarty 2000).
Second, the rate of evaporation varies a great deal, depending on temperature and relative humidity, which impacts 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 (Konikow and Kendy 2005).
The Hydrological Cycle
The hydrological cycle begins with evaporation from the surface of the ocean or land, continues as the atmosphere redistributes the water vapor to locations where it forms clouds, and then returns to the surface as precipitation. The cycle ends when the precipitation is either absorbed into the ground or runs off to the ocean, beginning the process over again.
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 (Goudie 2006).