In the semi-arid environment of the valleys of the lowlands of Yemen, agriculture is dependent upon the harnessing of seasonal floods which transport, right up to the desert edges, both monsoon rainwaters coming down from the mountains and fertile silts. The progressive mastering of water diversion and water distribution techniques led to the development of vast agricultural areas and population centres which formed the basis of the ancient kingdoms of Southern Arabia. The different phases in the evolution of water acquisition strategies and of the exploitation of the valleys - the origins of which date back to the prehistoric period – are described in this article in the form of an hypothetical evolutionary model.