Motomu Ibaraki Abstract
Issues of unsustainability related to water are discussed to enrich our understanding of water resources and potential approaches for sustainability management are delineated. Concepts of the hydrologic cycle and water budget are used to organize and identify characteristics of the various issues. Sustainable characteristics of the groundwater system, which is the largest natural freshwater reservoir, are examined. Conjunctive management for both groundwater and surface water is required, and sustainable water management may not be feasible for certain types of aquifers. The impacts of various factors, such as quality of water, and human-induced input and output flows to a watershed are also examined. These factors influence each other greatly and affect ecological systems and human health on a large scale. Comprehensive analyses and management involving physical necessities of sustainabilities including energy, land and nonenergy resources are key factors for water sustainability.
Water is a priceless commodity to life on Earth. Unlike other resources, there are no suitable substitutes for it in most cases. If we are to allocate Earth's limited water supply optimally in the future, we must have a fundamental understanding of water.
What We Have: Water on the Earth
Like any conversation, discussion about sustainability and resources of water must be set in context. Therefore, let us begin with the necessary background of hydrologic systems and water resources.
Water is one of the most important commodities. However, unlike other commodities, there are no suitable substitutes for freshwater and most of its uses. Water resources are fundamental components for both economic development and the maintenance of natural environments. Technically, water changes only in form. It does not disappear, but rather moves from one place to another.
The movement of water from one place to another can be described at many different scales. The hydrologic cycle is the global-scale, endless recircula-tory process that links water in the atmosphere, oceans, and on the continents (Figure 15.1). The hydrologic cycle constitutes reservoirs that store water, such as oceans, and the movement of water between them. Water can be stored within the reservoirs and transported between them in three phases: gas, liquid, or solid. Oceans are the largest reservoir and store 97.5% of total global water (Table 15.1). Only 2.5% of all water is available as freshwater, and this is stored primarily on the continents. This endless recirculatory process is driven by solar energy, gravity, and other forces.
As illustrated in Figure 15.1, more water moves from the oceans to the atmosphere through evaporation (502,800 km3/yr) than from the atmosphere to the oceans through precipitation (458,000 km3/yr). This indicates that there is a continuous movement of freshwater from the ocean to the continents through the atmosphere.
On the continents, water precipitated from the atmosphere may be stored in groundwater, streams, lakes, and glaciers. However, it eventually moves back to the oceans through runoff (groundwater and surface water) or to the
Evaporation Lakes f 1
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