About tank-cascade systems
In the north central drylands of Sri Lanka rainfall and surface runoff have been stored in manmade reservoirs since ancient times, providing 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 paddy fields located downslope. 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).
Major tanks were under the control of the kings, while small village irrigation systems were cooperatively initiated and constructed by villagers (Pearce 1962). The traditional water reservoir system has not changed much in its functioning ever since.
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 was one of the most important preconditions for the development of early urban societies in north central Sri Lanka, as knowledge of storage and distribution of water for irrigation purposes was necessary to obtain an agricultural surplus for the growing population (Deraniyagala 1998).
Today 10,000 small so-called village tanks are still in use in the Dry Zone of Sri Lanka. Beside irrigation the stored water is used for domestic purposes, desiccated tank beds serve as pasture for the cattle, and deposited sediments are used as raw material for brick production. Additionally, wewas are spots of social interaction (UNDP 2017: 2) where villagers go for recreation, bathing, washing, cleaning vehicles, meeting friends or neighbours and exchanging information. The majority of villages in the Dry Zone of Sri Lanka have at least one wewa available, which is dedicated to the adjacent paddy fields. Wewas also serve as a source of water for the maintenance of home gardens (SAARC 2015).
Wewas and paddy fields are surrounded by buffer zones of vegetation that function as flood prevention for the village and wind breaks for cultivation.