"For millenniums, Yemen preserved traditions of careful water use. Farmers depended mostly on rainwater collection and shallow wells. In some areas they built dams, including the great Marib dam in northern Yemen, which lasted for more than 1,000 years until it collapsed in the sixth century A.D.
But traditional agriculture began to fall apart in the 1960s after Yemen was flooded with cheap foreign grain, which put many farmers out of business. Qat began replacing food crops, and in the late 1960s, motorized drills began to proliferate, allowing farmers and villagers to pump water from underground aquifers much faster than it could be replaced through natural processes. The number of drills has only grown since they were outlawed in 2002." (Thanks D.)