atmosphere through evapotranspiration. Freshwater resources, upon which we are totally dependent, consist of this temporarily stored water on the continents within the hydrologic cycle.

Finite Nature of Water

Within the freshwater body (which comprises 2.5% of global water), 69.55% is stored in glaciers and permanent snow cover in polar and mountainous regions (mainly in Antarctica and Greenland); thus, it is not readily accessible for use. Most of the usable freshwater exists only in continent reservoirs, in the form of groundwater, lakes, streams, marshes, and wetlands, and comprises only 0.76% of the global water resource (Table 15.1).

We could increase limited freshwater resources by moving water from oceans to continents through saltwater desalination, which is identical to the process that occurs in the hydrologic cycle. However, a large-scale increase in freshwater resources through desalination is limited by energy resources as well as by capability and efficiency constraints of canals and aqueducts needed to transport water from coastal areas to inland areas. Although Earth is sometimes depicted as "the water planet," only 0.76% of total global water is available for use for both human activities and the maintenance of natural environments. To describe this finite water resource quantitatively, let us apply the concept of water budget.

The Water Budget

Water budget calculation is a very useful tool to evaluate the availability and sustainability of a water supply. An understanding of water budgets can provide a foundation for effective water resource management and environmental planning. We can quantitatively evaluate the effects of human activities and climatic variations through changes in observed water budgets over time. We can also assess the effects of controlling factors (e.g., geology, geography, and land uses) on the hydrologic cycle by comparing different regions.

The water budget for freshwater resources, which is a part of the hydrologic cycle, can be quantitatively described by applying the principal of conservation of mass. The mass conservation law can be described as the difference between the rate of inflow and outflow is equal to the time rate of change of mass inside of the control volume. By assuming that the density of water is approximately constant, we can express the conservation of mass as:

where V is the volume of water within the control volume (L3), I and ° are volumetric inflow and outflow rates (L3T-1), respectively.

The global water budget can be described by applying the principal of mass conservation (Equation 15.1) and setting the continents as the control volume. The volume of water within the control volume (V) represents the volume of water stored on or within the continents (e.g., as glaciers, streams, lakes, and groundwater). Volumetric inflow (I) is precipitation, and outflow (O) is evapotranspiration, and surface water and groundwater runoff. The global water budget can be re-written as:

where p is the precipitation rate (L3T-1), rs and rg are the surface water and groundwater runoff rates (L3T-1), respectively. The term, et, represents the evapotranspiration rate (L3T-1).

Under average annual conditions, we would be able to assume that water stored on or within the continents does not change its volume significantly; accordingly, the term (dV/dt) in Equation 15.2 become negligible. Hence, Equation 15.2 becomes:

where p is the average annual precipitation rate (L3T-1), and r and rg are the average annual surface water and groundwater runoff rates (L3T-1), respectively. The term, et, represents the annual average evapotranspiration rate (l3T-1).

Equation 15.3 represents the flow of usable freshwater from atmosphere to land through precipitation as a part of the hydrologic cycle, divided into three components: surface runoff, groundwater runoff, and evapotranspiration. Freshwater resources that we can utilize are both surface and groundwater runoff (F and rg) in Equation 15.3. This equation implies that the quantity of available water (i.e., both surface and groundwater runoff) is highly dependent on not only the quantity of precipitation but also that of evapotranspiration. For example, a region may have a significant amount of precipitation, but it may not have enough water resources if evapotranspiration is also significantly high.

Spatial and Temporal Variability

Considerable variation in both the spatial and temporal distribution of available freshwater (i.e., water resources) occurs due to the significant variations of precipitation and evapotranspiration in time and space.

Temporal variation of these processes occurs on a wide range of timescales, from hourly storm events to interannual changes. Most countries depend on seasonable rains for their freshwater supply, and the distribution of seasonal precipitation is not uniform in many countries. In India, for example, 90% of its annual precipitation occurs during the summer monsoon season (June to September), and the country receives barely a drop during the other eight months (Clarke 1991). In the Moulouya basin of Morocco, annual rainfall is scarce and concentrated over a few days.

A variation in spatial distribution of available freshwater is also significant among the continents. Table 15.2 illustrates precipitation, evaporation, and runoff by region. It can be seen that the percentages of runoff and evaporation have a wide range of variation. For example, Africa shows 20% runoff whereas Asia and North America show 45% runoff. Accordingly, the evaporation rate changes from 55-80%. This implies that we would be able to use 20% of precipitation as a freshwater resource in Africa, whereas 45% of precipitation could be used for water resources in countries in Asia and North America as described in Equation 15.3.

Available freshwater per capita depends not only on availability of freshwater but also population, and it greatly varies among the continents and countries. Figure 15.2 illustrates the availability of freshwater through average streamflows and groundwater recharge, in cubic meters per capita per year, at the national level in 2000. As can be seen from the figure, Egypt and the United Arab Emirates have the least freshwater resources: 26 and 61 m3/capita/yr, respectively. By contrast, Suriname and Iceland have the most, with 479,000 and 605,000 m3/capita/yr, respectively, i.e., Iceland's freshwater resource is 2,300 times larger than that of Egypt.

Watershed Water Balance

As discussed in the previous section, there is a great deal of variation in the water budgets of each continent (Table 15.2). On this basis, we can expect that the

Table 15.2 World surface water: precipitation, evaporation, and runoff by region (adapted from UNEP 2002a).
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