Currently, water demands are in continuous growth, which increases the pressure, in quantity and quality, on the available water resources globally. At the same time, the expected reduction of the natural water contributions, due to global change, depicts a new level of uncertainty. In this context, hydrological models are important tools for planning and managing water resources. Its correct application is essential in areas with high levels of water scarcity (arid and semi-arid regions). The southeast of Spain, the field work of this doctoral thesis, presents one of the highest rates of water scarcity and intensive exploitation of water resources in the European and global context. In addition, the frequency and severity of extreme events (floods) in this study region has increased during last decades as a direct consequence of the global change (climate change and land use changes). This doctoral thesis aims to analyse the intrinsic and complete problem of water resources in arid and semi-arid regions, that is, as a need (in case of scarcity and global change) and as a threat (in case of occurrence of extreme events or floods and global change) while studying certain management and adaptation strategies (technologies of wastewater reuse and water desalination) in view of the dual nature of water resources in arid and semi-arid climate areas, in this case in the southeast of Spain. Based on the above mentioned objective, five manuscripts have been performed and published as scientific articles in journals with high impact factor. These papers analyse certain paradigmatic study areas (hydrological basins, aquifers and municipalities) that belong to the southeast of Spain. Main results obtained in the mentioned scientific works can help local authorities responsible for the management and planning of water resources in this region. / Subprograma de Formación de Profesorado Universitario (FPU15/00636). Convocatoria 2015 del Programa Estatal de Promoción del Talento y su Empleabilidad del Ministerio de Educación, Cultura y Deporte.