THE International Hydrological Programme (IHP) of the UNESCO will work extensively in the field of rainwater harvesting, not only to consolidate existing knowledge, but also to develop cheaper and more efficient technologies for water harvesting, said Minja Yang, Director, UNESCO, New Delhi. "About 40 per cent of the world pop ulation is suffering from acute water shortage,' said Yang, whose message was read out at the International Symposium on Water Harvesting at the Tamil Nadu Agricultural University (TNAU) here on Monday She said . poor people still relied on subsistence agriculture. Although it was considered the only option to improve crop productivity, development of irrigation was not always possible due to climatic constraints in the arid and semiarid regions of the world, she said.