Kala Oya (කලා ඔය)
Sri Lanka has one of the oldest traditions of irrigation in the world, dating back as far back as 500 BC. The famous dictum of the epic hero King Parakrama Bahu I (1153-86) states “let not even one drop of water that falls on the earth in the form of rain be allowed to reach the sea without being first made useful to man”. It was around these ancient tank (water storage reservoir) irrigation systems that the economy and human settlements of early Sri Lankan society were organised into a “hydraulic civilization”. Unlike in the case of most ancient civilizations, which grew in fertile river valleys and floodwater retention areas, Sri Lankan hydraulic societies were based on reservoir systems and control devices or biso-kotuwas for the release of irrigation water. It has been reported that at the peak of its development, the ancient Sri Lankan hydraulic engineers were even called upon to serve in other countries. Today’s map of Sri Lanka, especially the Dry Zone, is dotted with literally thousands of ancient tanks of varying sizes and shapes, some operational and others long abandoned. These ancient tank systems have both ecological and biological importance. A key issue is seasonality and duration of water retention, which has a significant influence on their biodiversity and ecology. Due to natural processes water levels are very low during the dry season, and many tanks dry out completely before being filled again in the rainy season. Their use for grazing cattle during the dry season maintains high levels of nutrients in the tanks – which in turn supports high levels of aquatic biodiversity.