How it began
The system, called ellanga gammana in Sinhala, grew up in the dry zone of Sri Lanka. Rainwater reservoirs were being built on the island as early as 300 BCE, and the Food and Agriculture Organization names Abayawewa, of the third century BCE, as the first large reservoir in recorded history there. An estimated 15,000 tanks were built between 300 and 1300 CE under the Anuradhapura and Polonnaruwa kingdoms. King Mahasena, who reigned from 276 to 303 CE, built 16 large reservoirs.
How it works
A cascade is a series of tanks set one after another on a natural drainage line in a small watershed. Each tank is made by building an earthen dam across the stream. Rain fills the tanks, and excess water from a higher tank spills to the one below. Paddy fields lie below each tank and a sluice releases water to them, with the drainage passing on downstream. Around each tank are a tree strip upstream, a reed meadow that filters inflow, and a strip of vegetation below the dam that intercepts seepage and salts.
How it spread
The method spread across the island's dry zone. About 30,000 tanks are estimated to have been built over Sri Lanka's history. From the third to the thirteenth centuries large systems were run by a central bureaucracy, and in the ninth century a committee known as the Twelve Great Reservoirs helped organize irrigation.
Who pushed back
The system later declined and was partly abandoned. Its upkeep between the 1200s and 1700s is poorly understood. Dutch administrators concentrated on the coast and on crops such as cinnamon. Under British rule the compulsory labor system that had maintained village tanks was abolished, and the tanks suffered. In the late 1800s water sluices were replaced on several hundred tanks and larger works such as Kala Wewa were restored.
Where it stands today
Sri Lanka has about 18,000 tanks today, and between 10,000 and 14,000 are in use for irrigation. The reservoirs cover 2.7 percent of the country's surface. In 2017 the Food and Agriculture Organization designated the Cascaded Tank-Village System a Globally Important Agricultural Heritage System. It notes that land pressure and deforestation threaten the forests that form part of each cascade.
