As many as 33% of water used is lost through evaporation and transpiration.
In particu-larly, nearly 70% of water used for agriculture is lost into the
atmosp here. For this rea-son, the use of water for field irrigation is considered
as consumptive water use and has significant impact on the downstream region.
This impact is particularly conspicuous in rivers that flow through arid and
semiar id regions. In the Yellow River that flows through North China, it has
been rep orted that water volume drops drastically in the downstream region
in summer an d that the river dries up for several hundred kilometers. Contraction
of the Ar al Sea at the Kazakhstan and Uzbekistan border was caused by ex-cessive
intake of water from Amu Darya and Syr Darya for field irrigation upstream.
Its has be come 10 meters shallower than it was 30 years ago, and wider salt
deserts have appeared in areas where lakeshore receded and deterioration of
the environment is ad-vancing around the lake. A problem related to water resources
utilization that will become serious in th e future is the impact of global
wanning caused by increase in concentration of greenhouse effect gases in the
atmosphere. According to a recent IPCC report ( 1996), an increase of 0.5 to
1.℃ and 1.0 to 2.0℃ from the present temperatur e may occur by 2050 and 2100,
re-spectively. The changes in hydrological condit ion that may take place as
a result of this warming can be sumuarized as follow s:
1. Increase in heavy rainfall
2. Shortening of snow cover period and reduction in snow fall
3. Increase in evapotranspiration from ground and plant cover
These changes are likely to bring about lowering of decrease in renewable water
resoures and reduction in effective soil moisture. In addition, wanning will
r aise the snowline in mountain regions and cause water shortage at many oases
th at depend on mountain gla-ciers for their water. For instance, nearly a third
o f oases are said to have disappeared in the arid regions of western most China
due to increase of temperature that has occurred in the last 40 years.
Meanwhile, increase in temperature is known to bring about increase
in water us e for municipalities. The relationship between temperature, volume
of water use and electric power use according to data from Tokyo Metropolitan
Water Bureau and Tokyo Electric Power Company is shown in Figure 7. As such
relationship is likely to be found in any region of the world, global wanning
will intensify wa ter shortage in many regions. As concentration of population
in urban areas and birth of many so-called megalopolises are predicted for the
21st Century, securing water for municipalities use will become a
very important issue.
