Improving water productivity, especially by irrigated agriculture, is key to mitigating ongoing problems of water scarcity. This report discusses and illustrates concepts for improving water productivity within basins. Researchers applied a water accounting procedure to four sub-basins in South Asia where there are perceived problems of water scarcity: Bhakra in India, Chishtian in Pakistan, Huruluwewa in northern Sri Lanka, and Kirindi Oya in southern Sri Lanka. The accounting procedure identifies the quantities and productivity of various uses of water within a basin. This information is used to identify the water-saving potential and the means of improving the productivity of the managed supplies. There is little remaining prospect for water savings at Bhakra and Chishtian, but considerable opportunity for water savings at Huruluwewa and Kirindi Oya. in all four cases, the productivity of water currently being depleted by agriculture can be improved. The four sub-basins are representative of situations that appear typical of many other basins worldwide, and the methodology used appears thorough, robust, and applicable to other basins. (Author abstract, modified)