"A line of volcanoes dominate the landscape of Bali and have provided it with fertile soil which, combined with a wet tropical climate, make it an ideal place for crop cultivation. Water from the rivers has been channelled into canals to irrigate the land, allowing the cultivation of rice on both flat land and mountain terraces. Rice, the water that sustains it, and subak , the cooperative social system that controls the water, have together shaped the landscape over the past thousand years and are an integral part of religious life. Rice is seen as the gift of god, and the subak system is part of temple culture. Water from springs and canals flows through the temples and out onto the rice paddy fields. Water temples are the focus of a cooperative management of water resource by a group of subaks . Since the 11th century the water temple networks have managed the ecology of rice terraces at the scale of whole watersheds."
Learn more at the UNESCO site: http://whc.unesco.org/en/list/1194