RAINWATER HARVEST BY TANK CASCADES IN SRI LANKA- WAS IT A TECHNICALLY ADAPTED METHODOLOGY BY THE ANCIENTS?

Since 3rd century BC, sustainable water management was portrayed in the dry zone of Sri Lanka for approximately 1500 years. Maps show more than 15000 tanks (“wew”) scattered in this rolling flat terrain. Majority were in some state of existence and either fully or partially operational. The objectives of this study are to examine the effectiveness of rainwater harvesting in the evolution of tank cascade system (TCS) and the sociotechnical criteria associated with sustainability. The study uses 4633 tanks from the Deduru Oya river basin and adjacent areas, which shows apparent increase with lower rainfall regimes. A new sequencing method was introduced with a number assigned to the tank along the longest axis of the catchments. The log (number of tanks) vs. sequence shows a negative linear relationship. This is not only true for all 4633 tanks, but also for tanks in individual catchments. The regression coefficients calculated for rainfall regimes vary from-0.210 (1200-1300mm) to-0.653 (1800-1900 mm). The results indicate that the ancient engineers did not build tanks haphazardly; instead a tank ordering method had been in use under strict sociotechnical guidelines. This was the basis for effective micro basin rainwater harvesting and sustainable water management