A combination of high temporal variability and spatial heterogeneity of rainfall, soil surfaces, and plant cover is the cause of the complex hydrological response in arid/semiarid regions. Under these premises, long-term monitoring is necessary to capture drivers controlling the response of these areas and to be able to model and predict their reaction. A succinct, up-to-date review of the databases and results produced by two representative micro-catchments in the most arid extreme of Europe, Almería (SE Spain), is presented with the aim to show how the different precipitation patterns, during a 20-year period, influence the hydrological behavior on different lithologies and soil surfaces. The problems encountered about the functioning of these experimental stations, including the generation and maintenance of long-term databases, is also reviewed.