Cascade systems in Sri Lanka

A cascade system is a connected series of tanks organized within a micro-catchment of the dry zone in Sri Lanka. The tanks are used to store water from a seasonal stream. The stored water is conveyed to other tanks downstream and used for a variety of purposes.

Irrigation tanks are often not isolated tanks but are part of a larger interconnected system of tanks called a ‘tank cascade system’. It has connected with series of tanks organized within a micro-catchments.   A ‘cascade system’ is the traditional unit used in the management of tanks. From ancient times it is referred to as ‘Ellangawa’. The term is made up of the Sinhalese words ‘ellan’, meaning hanging and ‘gawa’, meaning one after the other. The image on the left shows the organization of tanks and their connectivity within a typical tank cascade system. The image below shows the micro-catchment of a typical village tank, which is usually the main tank of a cascade system. Different river basins have different numbers of cascade systems within them. For example, the Mee Oya river basin has only one associated cascade system, while the Malwathu Oya river basin has 179 associated cascade systems.

Traditionally, there is one village tank (Maha Wewa/Pahala Wewa) for use by each village. The village tank is the main component of the tank cascade system. Water from all other tanks in the system drain into the village tank. This tank is used for agriculture, as well as other community activities. A typical village comprised of the village tank and a field below it.

Although the main purpose of the tank cascade system was to provide water for irrigation purposes, the tank cascade system included tanks that had other purposes too. The purposes of the other tank types are connected in different ways to provide a good and constant supply of water to the village tank. The proper functioning of the village tank is therefore dependent on the condition of all the other types of tanks that are found in the tank cascade system. The ecosystem services and support to biodiversity varies considerably between each type of tank.

·   Kulu Wewa (Forest tank): constructed in the upper catchment of the village in order to provide water for wild animals, filter debris and silt, and capture the rainwater that will enter into the village tank through seepage. By providing water for wild animals, these forest tanks reduce the likelihood of these animals coming into the village to look for water and damaging crops.
 
·   Kayan Wewa: built where the upper catchment has been cleared or degraded. It is used         to trap sediment and controls salinity.

·   Olagam Wewa: lies close to the village, but is not associated with a permanent settlement       or cultivation. It is used as a source of water for seasonal cultivation.

·   Goda wala (Water hole): constructed for the trapping and deposition of silt, to avoid             siltation and sedimentation of the main village tank.
 
·   Ihala Wewa (Storage tank): constructed for the storage of water, and is associated with       paddy cultivation and other community activities.

In addition, cascade systems found in mountainous regions also include a Mountain Tank, constructed to provide water for ‘chena’ or slash-and-burn agriculture.  This is not observed in dry zone cascade systems. 

Sources: Madduma Bandara (1985); Panabokke (2001); Dharmasena (2010); Senanayake et al. (2010); Someratne et al. (2005).