Water is the most essential component for the life of all beings. Hardly any economic activity can be sustained without water (Haddadin, 2001).

In view of past failures, a better idea is now needed of the roles and functions of water in the global life support system. For example, the conventional conceptions of water related to plant production are increasingly unsatisfactory from a hydrological perspective. Through its physical, chemical and biological involvement, water has fundamental balancing functions in the water cycle (Ripl, 1995). It dissipates solar energy variations in space and time. This is performed through three main interactive processes with mutually balancing components:

1 Physical processes. The interaction between evaporation and condensation is of major importance for the distribution of energy around the planet.