Although seasonal snow and ice represent only 1% of the world's fresh water, the annual melt cycles can play a significant role in water resources management at the catchment scale (Chapter 25). For example, snowmelt can be a welcome source of replenishment for lakes, rivers, reservoirs, and groundwater; however, rapid melting of large volumes of snow can cause flooding and subsequent contamination (e.g., sewer backups) of water resources.
Land-based surface water includes rivers, lakes (both fresh and saline), surface soil moisture, and wetlands. On the global scale, the volume of land-based surface water is a small part (0.0153%) of the hydrologic cycle, but the rate of flux through these components, and hence the availability of fresh water, is critically important to human activities within individual watersheds. For thousands of years, humans have interacted with land-based surface water by building aqueducts, digging irrigation canals, and more recently, by diverting or damming rivers, and by pumping, which captures groundwater without allowing it to discharge naturally to a surface water body. With the occurrence of global climate change and the possibility of increased frequency of floods or droughts, responsible hydrologic management will be key to achieving sustainable water resources within catchments. This is possible only through scientists' continuous progress in understanding the various components of the hydrologic cycle in each catchment, improved modeling of all components of the hydrologic system, and narrowing the uncertainty bounds on hydrologic predictions.
Biological water is the primary constituent of living tissue in all plants and animals. It is another form of land-based surface water; yet it is only a minuscule percentage (0.0001%) of the total water on Earth. Regardless of its small percentage, the critical role of plants in the vertical transfer of water from soil and subsurface reservoirs to the atmosphere—particularly in semiarid regions—cannot be ignored (see Chapter 26).
Groundwater generally refers to the water that exists in saturated layers of porous geologic materials, called aquifers. On the global scale, groundwater is a slow-moving reservoir that comprises approximately 0.5% of the world's total water, yet it accounts for 30% of Earth's freshwater reserves. Accordingly, the existence and replenishment of these reserves is critical for maintaining a water supply for many human communities.
The global hydrological cycle generally depicts groundwater as slowly discharging to oceans, lakes, and rivers, but groundwater discharge at the catchment scale is rapidly accelerated by the electric turbine pump. A major change in groundwater discharge rates in the United States came about with the widespread agricultural use of electric turbine pumps with the electrification of rural America following World War II. The effect of widespread use of the electric turbine pump for irrigated agriculture has been a decrease in water tables (up to 400 ft within 50 years in fast-growing metropolitan areas such as Tucson and Phoenix, Arizona) and, in some cases, groundwater capture of surface water resources. Only within the past century have scientists and water managers begun to understand the depleting effects of groundwater pumping on surface water flows (e.g., Bouwer and Maddock, 1997; Maddock and Vionnet, 1998; Glennon and Maddock, 1997).
Regional precipitation patterns often determine whether groundwater supplies are a sustainable resource for a catchment's population. In semiarid and arid regions where precipitation is light and water demands increase with growing metropolitan populations, natural recharge is insufficient to maintain a long-term, dependable water supply, and can often result in land subsidence leading to considerable property damage. For some arid regions, the mining of groundwater as a nonrenewable resource is the only viable alternative (e.g., El Geriani et al., 1998; Gijsbers and Loucks, 1999), but in other areas, water conservation and artificial recharge efforts can prolong a basin's water supplies.
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