Search for water and its management is a significant factor of complexity. The adoption of a technique for water supply can overturns the structures of a society. In ancient Yemen, the settlement pattern in the lowlands valleys was based on irrigated agriculture. The gradual development of hydraulic technics led to an increasing agricultural activity from upstream to downstream in the valleys. The sharing of the water along a same river bed was significant to the cohesion of the communities. The building up of a network of villages and towns along each valley resulted in the formation of the ancient kingdoms, Awsan, Saba', Qataban and their capitals on the edge of the desert. Each of those kingdoms bordering the Ramlat as-Sab'atayn had a strong identity, made of common religious and social practices on a well limited territory. The unification in the Hadramawt was of a different level. There, the seasonal floods in the main central course were never diverted, bringing about any sharing practices along the whole valley. As a consequence, irrigation practices did not facilitate the gradual integration of the different communities in a single political entity.