The tank cascade system (Sinhala: එල්ලංගාව, romanized: ellaṅgāva) is an ancient irrigation system spanning the island of Sri Lanka. It is a network of thousands of small irrigation tanks (Sinhala: වැව, romanized: wewa) draining to large reservoirs that store rainwater and surface runoff for later use. They make agriculture possible in the dry-zone, where periods of drought and flooding otherwise make it difficult to support paddy fields and livestock. Originating in the 1st millennium BCE, the system was designated as a Globally Important Agricultural Heritage System by the United Nations Food and Agriculture Organization in 2017. Centralized bureaucratic management of large-scale systems was implemented from the 3rd to the 13th centuries. Small-scale systems continued to be well-maintained up until the abolishment of compulsory labor, following British consolidation of control over the island. Efforts since independence to rehabilitate the tanks have resulted in much of the system being restored, as well as the addition and integration of new reservoirs. The reservoirs total to 2.7% of the country's surface area and have a significant effect on the ecology of the island.
Etymology A catchment site within the system is referred to as a wewa (වැව) in Sinhala, and this term is translated into English as "tank". These tanks are connected in a series, referred to as a cascade, so that an ephemeral waterflow can be used, stored for future use, or conveyed elsewhere. The native term in Sinhala for a cascade is ellaṅgāva, which is a compound word combining ellaṁ ("hanging") and gāva ("next to one another").
Geography The tank cascade system is largely located in the semi-arid north-central section of the island, which experiences equatorial heat, limited freshwater, and erratic rainfall patterns. The monsoon cycle in the region, coupled with low water retention in the soils of the region, results in minimal groundwater storage capacity, high rates of evaporation, and low or variable precipitation, meaning that "in this hard rock region...no stable human settlement would have been possible without recourse to the storage of surface water in small tanks." Granite and charnockite underlie in this area, decreasing permeability. The "undulating topography" of the island's dry zone is also appropriate for pond or reservoir construction, with small dams being able to create large reservoirs. Overall, Sri Lanka has 80 major dams and 18,000 extant tanks, down from an estimated 30,000 tanks in ancient times. Between 10,000 and 14,000 tanks are in active use as irrigation sources; the majority of these hold water in the north-central lowland dry zone. The total surface area of all reservoirs in Sri Lanka was estimated in 1988 to be 175,774 hectares (1,758 km2), 2.7% of the country's area. Of this, 39,000 hectares correspond to just 44 major ancient reservoirs.
History
Whereas the agriculture of Fertile Crescent arose from stored water in low bottomland soil, and the agriculture of ancient Egypt was dependent on retained Nile River flood waters, ancient Sri Lankans used a chain of reservoir systems as their water source. Sri Lanka has been called a "hydraulic civilization." Similar ancient water engineering projects in tropical and subtropical climates include the qanats of Iran, oases in the Near East and North Africa, and the Gurganj Dam of Amu Darya. Researchers theorise that the evolution of the tank cascade began with rain-fed agriculture and then became increasingly sophisticated beginning with diverting rivulets, then permanent rivers, followed by a leap forward with the construction of spillways, weirs and ultimately sluices, then the construction of reservoirs, until, at the apogee of development, ancient Sri Lankans were able to successfully dam up perennial rivers and use the water as they saw fit. Historic uses of the tank cascade system included human needs (drinking water, sanitation, food production), ecosystem enrichment, urban development, administrative boundary setting ("water cordons"), and natural disaster mitigation. Rainwater reservoirs were being constructed on the island as early as 300 BCE—there are assertions that Sorabora Wewa in Mahiyangana was constructed by the yaksha spirits before the theory postulated as the Indo-Aryan migration to the island—and an estimated total of 30,000 tanks have been built over the history of Sri Lanka. The existence of what is now called the tank cascade system is recorded in the Dīpavaṃsa and the two Mahāvaṃsa chronicles, which describe tanks, ponds, water holes, dams, canals, irrigation funding grants, irrigation income, irrigation taxes, and irrigation laws. An estimated 15,000 tanks were built between 300 and 1300 CE, during the Anuradhapura Kingdom (437 BCE–845 CE) and Polonnaruwa kingdom (846 CE–1302 CE) eras. Sri Lankan irrigation engineers of this period were supposedly summoned or hired by other kingdoms for their expertise. Without these tanks, ancient Anuradhapura would not have been able to develop and flourish, as it is located in a dry area.
In the 9th century, bureaucracy to organise the irrigation system included a committee known as the Twelve Great Reservoirs. The most famous surviving exemplars of the irrigation infrastructure used by Sri Lankan elites are the Abhayavapi rainwater reservoir in Anuradhapura built by Pandukabhaya (437–366 BCE) and the "lion rock" fortress Sigiriya, a UNESCO World Heritage Site. The only source of water at Sigiriya (which sits 360 meters above the surrounding plain) is rainwater, which was cunningly managed through a network of pools, underground channels and drains.
Other historic landmarks of Sri Lanka water engineering include the lion pond of Mihinthale, the stone lotus pond of Polonnaruva, and the architecture of Kumara Pokuna, the royal baths of Parakramabahu the Great.
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