Uranium/Thorium (U/Th) and 226Ra dating has been applied to spring deposits in order to determine the timing of the drying up of springs in the Wādī al-Ajāl, Fazzān, Libya. These dates are compared with results of archaeological surveys of irrigation structures and archaeobotanical studies of plant remains to further our understanding of the factors that controlled the introduction of irrigated agriculture, its nature and timing.
Though there are still important gaps in our understanding, the results suggest that by 3,100 ±125 BP surface water was either very scarce or nonexistent at the base of the escarpment, although the water table was probably very near the surface at the base of the Wādī in the vicinity of Jarma. Archaeobotanical evidence indicates that irrigated agriculture was introduced in the early part of the first millennium BC, soon after the desiccation of the springs and presumably in response to the fall in the water table and the increasingly scarce availability of surface water. No irrigation structures have been found that relate to this period, but the most likely scenario involves wells tapping the near surface water in the vicinity of Jarma. The earliest irrigation systems are foggaras which archaeological evidence suggests were probably introduced by the final few centuries BC, and definitely before the fourth century AD. The extensification of agriculture associated with extensive foggara development broadly corresponds with archaeobotanical evidence for an intensification and diversification of agriculture involving the introduction of a farming system utilising both winter and summer crops. There are likely to be strong connections between these two developments and that of the rise of the Garamantes.