Cascade systems in Sri Lanka

Clusters of Cascade systems in Sri Lanka, a World Heritage Site

Eng. Lanka Ramanayaka BSc.(Eng.), PG Dip.(BSE), CEng., MIE(SL), MEC(SL)

PUBLISHED ON June 25, 2021

Cascade systems are an invention of tank-built tanks built by ancient kings for maximum efficiency in water management and sustainable agriculture. This is not just a single irrigation industry but a group of interconnected tanks.

Fig. 1 Cascade system

More than 80% of the lakes in the dry zone of Sri Lanka belong to the Cascade system. Due to the undulating nature of the geographical features of the dry zone, these suspension systems are made up of a group of lakes that are connected to each other by naturally formed waterways.

These tank systems, which are arranged to allow water to flow gradually from the upper tank to the lower tanks, which feed from the upper catchment areas, feed the agricultural lands in the middle. These tank systems as an interconnected network are important for several reasons.

1.Supply of water required mainly for agricultural lands. 2. Assisting in fish farming

3. Supply of raw materials required for various industries

4. Provide habitat for animals and conserve biological diversity

5. Flood control

6. Drought tolerance

7. Runoff control

8. Groundwater recharge

9. Water Management and Water Purification

10. Environmental beauty

For the above reasons, these systems are an excellent product that provides the basis for sustainable agriculture. An ideal solution to the acute water crisis in dry climates. Tank systems like the lifeblood of agriculture contribute generously to land farming as well as mud farming. Furthermore, this system consists of a number of small irrigation systems.

Cascade systems belong to 3 zones according to their location.
- North Central Region
- Northwest Zone
- Ruhuna Zone

Cascade systems today are gradually deteriorating. Repairing small irrigations individually has led to the breakdown of the interconnections in the elliptical systems. Therefore, all tanks in the suspension system should be considered when managing tanks (Figure 1: Cascade system)

The agrochemicals currently used extensively in moving water through several tanks in the Cascade system move from the upper lakes to the lower lakes. Many tanks have become environmentally polluted due to the movement of these pesticides and chemical fertilizers into the lower lakes. Therefore, the water quality of the upper tanks of the Cascade system increases and the water quality decreases as it descends.

Its adverse effects can be mainly water pollution, threat to fish population, nutrient uptake, and as a by-product of water pollution, making irrigation water unsuitable for agricultural purposes, causing various diseases (kidney disease, cancer, gastritis, etc.). In addition, damage to watersheds and a lack of attention to soil conservation practices can lead to siltation of tanks and waterways in embankment systems. As a result, the area under cultivation is gradually reduced due to lack of optimal water management.

To study the difference in water quality between the upper tanks (head tanks) and the lower tanks (tail tanks) of the cascade system, a study was carried out on the cascade systems in the Thanamalwila and Wellawaya areas of the Moneragala District. (Figure 2: Thanamalwila and Wellawaya Cascade systems used for the study) Post Images

Fig. 2 Thanamalwila and Wellawaya Cascade systems used for the study

For this, various chemical parameters of the water were measured and mapped by GIS. Accordingly, it was confirmed that the pH of the water, EC, nitrate, ammonium, phosphate and carbonate ions increase as the tank system goes down.
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