ABSTRACT
One third of the world’s land surface is classified as arid or semi-arid. In most of the world easily developed land is already exploited, and therefore increasing attention is turning to arid areas to relieve population pressures and to provide more food. Soil and water natural resources of arid and semi-arid regions are often in a delicate environmental balance. Arid regions cannot be developed along lines that have been successful in humid areas. These regions are under severe and increasing water stress due to expanding populations, increasing per capita water use, and limited water resources. The world’s total population has increased four times in the past 150 years and may double again in the next 30 years. Future projections indicate that by 2025, five billion people will live in countries experiencing moderate or severe water stress (WHO, 1997; Arnell, 1999). Evidently, conditions will be most severe for the driest regions of the world. Effective management is essential, and this requires appropriate understanding of the hydrological processes in arid and semiarid areas (Wheather and Al-Weshah, 2002).
