The climate in the dryzone of Sri Lanka is characterised by two distinct dry periods throughout the year. To cope with the semi-arid climatic conditions c. 2000 years ago a sophisticated water harvesting system was implemented consisting of human made reservoirs, called tank or wewas, being used to collect, store and distribute rainfall and runoff for paddy irrigation. The so called thaulla upstream of the tank bed serves as sediment trap. In Sri Lanka thousands of tanks were constructed in ancient times, many modern tanks have ancient ancestors, therefore they are understood as part of the cultural heritage of the country.