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Tank Cascade System

Rich heritage

Irrigation concepts of our ancient tanks remarkably in tandem with modern water management concepts

The advanced irrigation concepts of the Tank Cascade System (TCS) which are remarkably in accordance with the modern sustainable water management concept of Integrated Water Resources Management (IWRM) could provide a strong base to sustainably address the current irrigation and water management and agrarian issues in Sri Lanka.

IWRM is a widely recognised concept among water scholars and practitioners which emphasises the coordinated development of water, land and related resources to boost socio-economic welfare while sustainably protecting the associated ecosystems. Along with the prevailing complex irrigation and water management issues faced by Sri Lanka which are further aggravated by challenges such as population growth, climate change, destruction of ecosystems, surging demand and competition for energy and natural resources, IWRM can be used as a promising concept to overcome the water-related issues of the country.

Sri Lanka is privileged to have a rich hydraulic civilisation recognised as one of the outstanding rice-based irrigation systems in Asia. Among the diverse irrigation techniques developed over time that have been used and are still being used particularly in the Dry Zone areas, the Tank Cascade System (TCS) can be regarded as one of the phenomenal and sophisticated developments of irrigation history. TCS is a connected series of tanks organised within a catchment of the Dry Zone landscape that has been designed to store, convey and utilise water for diverse purposes.

Since this system has been developed over centuries, it is dynamic, time-tested and well adapted to the landscape, soil, climatic and socio-economic conditions of the Dry Zone. It is a perfect example for a mutually beneficial symbiotic system of human and natural systems. Therefore, this system could ensure the food and livelihood security of the Dry Zone community for centuries while preserving the ecosystem. A study conducted by the Hector Kobbekaduwa Agrarian Research and Training Institute (HARTI) shows that the principle ideologies behind TCS complement the fundamental principles of the IWRM concept.

Therefore, learning the underlying scientific wisdom behind these advanced ancient concepts will provide a promising base to introduce solutions for prevailing issues in irrigation and water management as well as the agrarian sector by positively contributing to implement modern irrigation concepts such as IWRM.

TCS consists of diverse components which have specific functions to perform. However, in-depth analysis of advanced thinking embedded within the system illustrates that all the specific functions are interconnected and interdependent which has made the entire system a multifunctional, holistic and sustainable one.

Therefore, the proper functioning of each component of TCS is important for the desired and continuous functioning of the rest of the components. This has been the secret behind the thriving of TCS over centuries and generations, irrespective of climatic upheavals and socio-economic changes.

How can TCS address the major issues prevailing in the water, irrigation and agrarian sectors?

Protection from wild animals

Even though TCS has mainly been designed to provide irrigation water for lowland paddy cultivation, all the tanks in the TCS do not necessarily provide water for irrigation purposes. Instead, the TCS has focused on supplying water for the entire system which fulfills the water needs of humans as well as other living beings. Satisfying the water needs of the entire ecosystem reduces the competition among the different water users and thereby, indirectly minimises the wild animal damage to crops. Particularly, the ‘forest tanks’ helped to fulfil the water needs of the wild animals while minimising the probability of crop damage. Other than this, the Kurulupaluwa, which is the area dedicated for birds to feed, helped minimise crop damage while indirectly minimising pest damage as well.

Mitigating frequent water shortages

The TCS treated water as a finite and vulnerable resource and the whole system has been designed to get the maximum use of available water. The whole idea of the TCS is to harvest, store and conserve the water received during the rainy season for use during the dry season. Especially, in order to get the maximum use of this limited resource, a considerable emphasis has been given to recycling and re-using water. For example, before water drains away from the system, water is re-used multiple times in the upperparts of the TCS.

Other than harvesting, storing and conserving the water received during rainy season, the system has practised several different water management concepts to ensure controlled and regular supply of water during the dry period. One of the key principles is maintaining the proper balance between the command area of the tank and water storage capacity of the tank. Therefore, unlike in the present context, the available water volume in the tank was sufficient to cultivate the available land extent.

Another key feature of the TCS is that the structures of the system have been constructed to ensure water security not only for humans, but also for the entire ecosystem during the scarce water period. For example, the Mandakaluwa section of the tank always retains some water by fixing the lowest level sluice allowing the fish species to survive even in muddy water until the rains arrive, while cattle and other animals could obtain water from the Mandakaluwa during the dry period.

Flood damage prevention

While the TCS has been planned to ensure water security during the dry season, it has also emphasised on reducing flood damage during the rainy season. For example, silt trapping small tanks (kulu wewa) and waterholes help minimise flood damage while preserving water for drought periods.

Fighting climate change

Since the embedded technologies of the TCS have been developed over centuries, they are time-tested and have adapted to the dynamic changes of nature. The major reason why the tank cascade system could withstand climatic and natural system dynamics is that it has been a well-balanced system by integrating human and ecological systems. Unlike modern irrigation and agricultural practices which mainly focused on supplying water to the root zone of the crop, the TCS considered irrigation and water management as a part of the functioning of the ecosystem. Hence, all components of the TCS have been designed in a way to maintain the system at its equilibrium which in turn ensures the water security for the entire system during the whole year, irrespective of the season.

In addition, by staying one step ahead of the current water management prioritisations, the TCS has focused on providing not only sufficient quantities of water, but also focused on the quality of the water supplied. Therefore, all the structures of the TCS such as filter, interceptor, soil ridge and tree belt have been designed to conserve both the quantity and quality of water, soil and the entire ecosystem.

Addressing accountability issues

One of the key issues prevailing in today’s irrigation and water management is the issue of ownership and maintenance obligations. In contrast, the participatory approach of water management was one of the salient features of the TCS. This traditional system was successful since participatory and inclusive approaches were used for water distribution in the irrigable area, according to community interest rather than the individual self-interest that exists in many modern schemes.

In ancient times, the management of TCS was mostly fulfilled by the social system with well-developed institutions and leadership structures. Tanks were well maintained as the ownership was with the community. In addition, making collective decisions was another important characteristic of water governance system that ensures the sustainability of the entire system while minimising the conflicting situations. In this way, the community could save water even during severe dry periods in all the tanks of the cascade.

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