[H-CG25] Cultural Hydrology

[HCG25-03] Detection of Unknown Tanks in the Middle Mahaweli River Basins

*Shinya Suzuki¹ (1.National Institute of Technology, Tokyo College )

Keywords:Sri Lanka, Tank Cascade Systems, Dry Zone, GIS, DTM

Sri Lanka is characterized by a tropical monsoon climate, and in the Dry Zone covering the northern half of the island, about 70% of the annual precipitation is concentrated during the rainy season. This being the main reason for the difficulty in cultivation using only rainwater during the dry season, many tanks and waterways have been built since ancient times. The Tank Cascade Systems, composed of such tanks and waterways, was a revolutionary means of making it possible to reuse drainage from upstream as irrigation water in the lower course of a stream. Through this system, the Dry Zone prospered as a major granary for more than a thousand years, but the system suffered an unexplained collapse in the 13th century. In recent years, this system has been reevaluated as a sustainable irrigation system due to its water purification effects and abundant ecosystem, and efforts to restore and renovate the group of water storage tanks are being promoted as national policy. However, the realization of truly sustainable irrigation cannot be achieved without clarifying the cause of the collapse. To clarify the cause of the collapse, it is first necessary to clarify the situation at the time the group of water storage tanks was abandoned. However, due to irrigation policies, many existing tanks have been repaired or renovated since the British rule in the 19th century and since independence, making it difficult to determine the situation when they were abandoned. In such circumstances, surveys and research on tanks in nature conservation areas and national parks which have not undergone such reparation and renovation can be said to be the only way to clarify the situation at the time. This study endeavors to identify previously unknown tanks in the middle Mahaweli River basins by leveraging microtopographic maps generated from a high-precision Digital Terrain Model (DTM). The analysis resulted in the detection of over 100 tank candidates in the middle Mahaweli River basins, an area with a low density of tank distribution in comparison to other regions. This study highlights the existence of numerous overlooked tanks in the region and sheds light on their development status, which has remained obscure.

Acknowledgment: This study was conducted with the support of the Kurita Water & Environment Foundation's domestic research grant in the fiscal year of 2021.