The paper attempts to document the ontology of socio-hydrological systems (SHSs), propose approaches of delimitation of SHSs' (territorial) boundaries, and investigate operational aspects of their adaptation to drought including repercussions on sustainability. To this end, a series of hypotheses are tested: (a) SHSs contain social subsystems with different expectations regarding water resources, different adaptive capacities, adaptation limits and prospects of sustainability, (b) SHSs do not adapt homogenously; some of their subsystems manage optimum adaptation, others fail to adapt and (c) territorial transformation of SHSs (e.g., through expansion of SHSs) may be the result of differential adaptation and sustainability potential within the SHS owing to power relations. After testing above hypotheses in the SHS of Messara Plain, Crete, the authors found out that powerful and dynamic sub-SHSs expand or break the boundaries of the initial SHS by establishing new relationships with other SHSs for the sake of resilience resources. Conversely, powerless sub-SHSs incapable to adapt descend and disappear. Therefore, territorial transformation of SHSs comes about from a combination of successful and failed adaptations, or in other words from different adaptation limits within SHSs. Consequently, water management and local development planning to guarantee adaptability to drought for all should be based on SHSs' analysis and management, not on jurisdictional areas or hydrological basins.
Keywords: Adaptation limit; Drought; Resilience; SES; Social-hydrological systems; Sustainability.