In the Second Iron Age, the Sub-Atlantic period was characterised by a significant improvement in climatic conditions, ushering in the so-called “Roman Climatic Optimum” from the 3rd century BC. Rising temperatures and increased rainfall define this period, which coincides with the beginning of the expansion of the Roman Civilisation and contrasts with previous years–the 5th and 4th centuries BC, a period of greater aridity and thermal amplitude. The protohistoric settlement of the southwest of the Iberian Peninsula shows a singular dynamism between the 5th and 3rd centuries BC: the mysterious "disappearance" of a civilisation with strong Mediterranean components, such as the "Tartessian", and the emergence of a new form of settlement with evident links with the north of the Peninsula, which coincides with the peoples that, centuries later, the Romans would call “Celts”. The disappearance of one model and the appearance of the other have been verified with certainty by archaeology, but such changes have been explained in social, economic and even linguistic terms, without paying special attention to the components essential for the habitat, especially water supply. Our research presents a comparative analysis of the water strategies among the late Tartessian peoples and the following Celtic peoples in order to draw conclusions that could allow us to understand the differences and coincidences between both peoples. Likewise, the question is posed as to how water supply was successfully resolved by the newly arrived populations, especially in two centuries of worsening climatic conditions. Both premises are analysed on the postulate that, although the supplanting of populations in the past has been proven, there are no complete replacements in these cultural and historical spheres.