Sri Lanka has one of the oldest traditions of irrigation in the world. The economy and human settlements of early Sri Lankan society were organized around the ancient water tank (water storage reservoir) irrigation systems. These traditional tank systems thus form a vital component of both the natural and manmade landscape in Sri Lanka. This study of the Kala Oya Basin, carried out collaboratively by the IUCN Sri Lanka Country Office and the Mahaweli Authority of Sri Lanka, quantifies the benefits associated with tank goods and services, and underlines their high economic and livelihood value to local communities. It also looks at the economic returns to different options for restoring and maintaining the traditional tank system. The study concludes by making recommendations about economic and financial instruments to strengthen tank management.