September 28, 2016
Some people get water out of the ground by digging a well, a vertical shaft down to the water table, then hauling the water up. But another way is to dig a gently inclined tunnel into sloping terrain until the water table is reached. Then water simply flows out of the tunnel entrance. This works because the water table generally follows the slope of the surface (see diagram).
Such irrigation devices are called qanats (puquios in Peru) and were first made in the first millennium BC to supply water to communities in arid regions. They later spread over Europe, Asia and North Africa. Vertical access shafts allow for construction, maintenance and when properly arranged, can be used to cool buildings. Construction of qanats is a major undertaking requiring organization, significant manpower and cultural foresight. The water is used communally for daily living and irrigation.
In many places, qanats are still in use, though some have dried up because ground water pumping has lowered the water table. The photograph above shows a Google Earth image of a vast array of access shafts near Fezna Ouled Jellal, Morocco, for qanats built in the 11th century. They run for dozens of kilometers through the desert, a testament to their durability and amount of effort that went into their construction. A recently built dam has lowered the water table below them so they are no longer functioning. Near Merzouga, however, qanats are still in use. Here two lines join each other just before the water surfaces.
Qanat diagram from Wikimedia Commons by Samuel Bailey