Water resources management in the arid and semiarid areas is a complex task, involving a large number of hydrologic, environmental, and manage- ment factors that have to be considered in order to supply sufficient water and to ensure the minimum levels of environmental protection and quality of life. Droughts, so frequent in the semiarid areas, intensify these problems even more. Since they are unpredictable phenomena (both in their occurrence and duration), prevision and preparation against droughts are key elements for minimizing their impact.