Tank cascade system
The tank cascade system is an ancient irrigation system spanning the island of Sri Lanka. It is a network of thousands of small irrigation tanks 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 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, which is a compound word combining and .
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. 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, 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, ecosystem enrichment, urban development, administrative boundary setting, 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 and Polonnaruwa kingdom eras. Sri Lanka irrigation engineers of this period were supposedly summoned or hired by other kingdoms for their expertise.
File:Nachchaduwa wewa.jpg|thumb|Nachchaduwa reservoir, located just outside Anuradhapura, is thought to be one of the 16 large reservoirs built by King Mahasena.
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 and the "lion rock" fortress Sigiriya, a UNESCO World Heritage Site. The only possible source of water at Sigiriya 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.
Thousands of modest tanks with hyperlocal catchment areas were built at the same time as "the larger and more impressive network of irrigation systems that …controlled and directed by the kings and other higher echelons of the irrigation bureaucracy." The extensive tank cascade infrastructure incorporated local and regional Buddhist monasteries by providing them with their own irrigation access and related incomes. In contemporary Sri Lanka, "Buddhist monks of any given village…are often consulted on water management decisions and lead agro-based cultural festivities."
Eventually the tank cascade system entered a period of decline and partial abandonment. Maintenance of the system between the 1200s and the 1700s CE, considered the "dark ages of tank civilization," is poorly understood. Very little is known of this period as the historical record is thin, but the Rājākariya labour system may have been involved. Dutch colonial administrators mostly concerned themselves with cultivation of coastal areas and lucrative crops like cinnamon and seem to have ignored the inland tank cascade systems. During the British colonial period, the Rājākariya system was abolished and the tank cascade system seemingly suffered as a result.
In the late 1800s CE an effort was made to reclaim and reorganise the surviving remnants of the tank cascade system; water sluices were replaced on several hundred tanks, and restoration projects were initiated for larger elements including Yodha Ela canal, Kala Wewa tank, Kantale tank, Giant's Tank and Minneriya-Elahara. British records also tell of village irrigation managers creating sluices from hollow tree trunks or clay pots turned pipes.
The Sri Lankan Department of Agricultural Services has overseen irrigation-management groups, called Farmers Organizations, since 1979. Sri Lanka's current water management plan seeks to preserve the ecosystem and cultural benefits of the system while making large-scale investments in drinking water systems, sewage treatment plants, and commercial-industrial water infrastructure. In addition to the tank cascade system, surface irrigation has been used on the island since the mid-20th century. One source says "the tanks have been largely untouched since the 1970s with the development of large irrigation and hydropower schemes."
Similar historic tank cascade systems can be found in Tamil Nadu state in southern India and West Bengal state in eastern India.
Hydrology and function
Village tanks and cascades are "naturalized" and generally built with permeable natural materials rather than concreted in place. Tanks can be any size from small vernal pools to huge perennial lakes "thousands of hectares in surface area."These tanks are connected into a series, the "cascade" or, so that an ephemeral waterflow can be used, stored for future use, or conveyed elsewhere. The water flows through channels and spillways within a small or medium-sized drainage area.
The cascade network draws from or serves to a variety of reservoirs: pahala wewa, kulu wewa, pin wewa, olagam wewa, ilaha wewa, et al. Tanks are edged with earthen embankments called wekandas with integrated water gates called kuto sorowwas, horowwas or bisokotuwas that release water into the canal system. The extent or expanse of water in the reservoir is called diyagiluma; the “dry lakebed” or “meadow” or parkland that the cascade potentially fills with water is wew pitiya. Village livestock congregate at the wew pitiya in the dry season. The upland stream channels are called diya para, the drainage channel exiting a village tank and paddy field is called kiwul ela.
The upstream edge of the tank is usually planted with a protective treeline called gasgommana and a reed bed for filtration, called perahana; the downstream edge is planted with biodiverse "interceptor" vegetation called kattakaduwa, intended as a bioremediation trap for salts and other contaminants. The gosgommana may be planted with indigenous species including Bassia longifolia, Terminalia arjuna, Crateva adansonii and Diosoyros malabarica. Herbs and medicinal plants are grown in the upper thaulla area of the system, and vegetables are often grown on the mounded barriers that separate paddy fields.
Some upstream elements of the system were designed to trap sediment that could eventually block the canals, while other upstream "forest tanks" serve as watering holes to keep wildlife out of the human water supply. Still other tank elements are engineered to recharge the aquifer. Studies of similar tank cascade systems in India found that they increase well recharge by 40 per cent and decrease surface runoff by 75 per cent.
The cascade network can be understood as an integrated, human-managed ecosystem "where water and land resources are organized within the micro-catchments of the dry zone landscape, providing basic needs to human, floral and faunal communities through water, soil, air and vegetation."