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Centre-pivot irrigation. Aerial view of a field watered by centre-pivot irrigation. This is a system of watering crops in which a metal arm on powered wheels revolves around a central point. Sprinkler nozzles are attached at irregular distances, with the water source at the centre. The sprinkler nozzle size is varied to give low water application near the centre, where the arm travels slowly over the crops and greater application towards the edges where the arm travels faster. Photographed in Wyoming, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER)