Sri Lanka's tank cascades chain reservoirs so each catches the runoff of the one above

BBC Future reported in September 2023 on Sri Lanka's ellangawa, or tank cascade systems, built from the 4th century BC up to the 1200s. Interconnected man-made reservoirs capture runoff from the hard-rock landscape, which has poor permeability, and share it downstream. The village tank at Maeliya still supplies 202 farmers irrigating 155 acres, with water released in the dry months through a sluice gate.

Researchers say linking tanks reduces drought risk: a study measuring vegetation density found tanks in cascades retained more water in the dry season than small isolated reservoirs. Tree belts reduce wind and evaporation, community forest in the catchment supports the groundwater table, and a soil ridge with grass-like plants filtered sediment before it reached the tank.

Researchers say there were once 18,000 to 30,000 small tanks, 90% in cascades; today an estimated 14,421 active tanks and 1,661 cascades remain.

Link small reservoirs down a watershed so water is stored, passed along and reused, then protect each with trees and silt ridges.

Tanks capture runoff that would otherwise flow to rivers and the sea.

Water passes from one tank to the next, so it is recycled across the local area.

Trees and sediment ridges protect water quality, groundwater and flood buffering.

Village communities historically maintained the tanks collectively.

Designing the whole watershed as a chain, not isolated reservoirs, can add drought resilience and clean water without new technology.

Decline came from abandonment, invasions and, after the British abolished community maintenance labour in 1832, centralised neglect. In 2017 Unesco and FAO recognised the cascades as a Globally Important Agricultural Heritage System. One project restored 325 tanks in 30 cascades and another 1,700 tanks in 280 cascades; a UN project helped Kurunegala farmers keep growing rice in the 2017 drought. Restoration has limits, since many tanks cannot supply the larger area a growing population needs.