Tank – cascade systems in the dryzone of Sri Lanka - A combined rainfall and runoff harvesting system
In the north-central drylands of Sri Lanka the storage of rainfall and surface runoff in human-made reservoirs is applied since ancient times and provides water for irrigation throughout the year. During the two monsoonal periods in May-September (southwest monsoon) and December-February (northeast monsoon) surface runoff is stored in these reservoirs, the so-called tanks or wewas, and is distributed during the dry period to the downslope located paddy fields. The tanks are in general located cascade-like along shallow valley courses. They are connected by canals and spillways and build a complex system of floodwater harvesting, water storage and water distribution (Jayatilaka et al. 2003).
Relevance for the development of urban societies
The majority of these tanks were constructed during the period of the Anuradhapura kingdom (377 BCE to 1017 BC) (Deraniyagala 1998). The introduction of this innovation is one of the most important preconditions for the development of early urban societies in north central Sri Lanka, as the knowledge of storage and distribution of water for irrigation purposes was necessary to obtain the agricultural surplus for the growing population (Deraniyagala 1998).