The Teradani Irrigation System was built in 1590 and pioneered innovative irrigation technology, which integrated flood control and water utilization in large rivers. The technology had a significant impact on the development of irrigation in Japan. The system consisted of an intake, the main canal, some lateral canals, and an operating organization.

The irrigated areas served by this canal system were along the Tenryu River, which had sufficient flow throughout the year. However, the farmers could not use the water and suffered from shortages. They had to take water from a small stream at the foot of the hill, which restricted the expansion of farmland due to the frequent flooding of the Tenryu River that changed course almost every year, depositing large amounts of sand and gravel.

The project to build the canal began at the behest of Tokugawa Ieyasu, the regional ruler who later became the commander-in-chief of the Edo shogunate (military government), to achieve economic growth through agricultural development. Under his orders, his vassal Ina Tadatsugu made the plan, and Hirano Shigesada, a magistrate, started the construction. They built a levee to separate the farmland from the floodplain, and the main canal, 4 m wide and 12 km long. The canal was completed in 1590, two years later after work started. It irrigated 2,000 ha of rice paddies, including 400 ha of newly opened rice fields. In general, overflow frequently occurs at canal intakes from large rivers during flooding. The intake in the project was a combination of a levee and a culvert buried across it, a new method in Japan. The levee was built over the culvert to protect the farmland from floods. The installed culvert was 4 m wide, 2 m high, and 21 m long, with sufficient capacity to irrigate the target area.

After its completion, the revolutionary system of combining levees and culverts received high acclaim, and the Edo shogunate applied the method in many other projects.

With a series of minor improvements, this design concept was widely used for about 300 years until cement and bricks from Europe became popular in Japan. The Teradani Irrigation System pioneered the construction of large wooden culverts that accelerated Japan's agricultural development.

Hirano Shigesada also organized Igumi, a farmers' cooperative, to ensure smooth water distribution to 73 villages and maintain and manage the canal. Igumi spent a great deal of money and labour to relocate the intake upstream after the river changed its course drastically due to flooding. Today, the irrigation system takes water from a hydropower reservoir and is free from flooding problems. The Teradani Irrigation System Land Improvement District, the successor of Igumi, manages the irrigation canal, which has played a vital role on the same route since 1590 while earning respect for its contributions and history from the residents.

The structure represents a milestone / turning points in development of irrigated agriculture and bears an exceptional testimony to development of agriculture and increase in food production along with the improvement of economic condition of farmers;

Teradani Irrigation System attributes its success to the construction of a levee to control flooding of the large river and the installation of a large culvert that allows water intake while maintaining the function of the levee. Combining these two structures enabled steady water intake for the first time. The Edo shogunate highly valued the technology developed by Ina Tadatsugu and used it in many important projects throughout eastern Japan. The application of the new technology contributed to the development of rice paddy farming along large rivers, and it also greatly improved farmers' economic conditions.

The structure was ahead of its times in terms of project formulation, engineering design, construction techniques, dimensions of the structure itself, quantum of water diverted, and size of the command;

This irrigation system adopted a large wooden culvert buried across a levee to protect farmlands from the flood-prone large river. The design was state-of-the-art. The wooden culvert installed at Teradani irrigation was 4 m wide, 2 m high, and 21 m long, an unprecedented structure in Japan at that period.

The structure made outstanding contribution to enhancing food production, livelihood opportunities, rural prosperity, and poverty alleviation in a region;

Teradani Irrigation System did not only solve the constant water shortage in the existing 1,600 ha of rice paddies but also opened 400 ha of new rice paddies, irrigating a total of 2,000 ha of arable land. It led to the establishment of 7 new villages, significantly improving the food production capacity and stability of 80 villages. Although the surrounding areas experienced food crises due to drought every ten years, in the 17th century, the irrigated area did not have a record of famine. The farmers in the irrigated area did not suffer from drought at all.

The structure was innovative in its ideas at the time of its construction;

At the time of the construction of this irrigation system, most rivers in Japan had flood problems. People did not have the means to utilize water stably despite the presence of a large river nearby. It hindered the development of agriculture. This irrigation system was the first to apply the innovative combination of preserving farmland with levees and installing a large-sized wooden culvert to ensure sufficient water intake and levee functionality. Many examples of its application followed in Japan.

The structure contributed to the evolution of efficient and contemporary engineering theories and practices;

The beneficiary area of the Teradani Irrigation System used to be the floodplain of the Tenryu River. There were many oval-shaped and irregular-shaped rice paddies because of the undulations on the ground surface. Moreover, some farmland could not get irrigation due to differences in elevation. These problems resulted in a large area of unused land and low productivity in farming.

In 1891, Suzuki Urahachi, the secretary-general of the Teradani Irrigation System, persuaded farmers to readjust 43 ha of farmland, the first to be done in the country. It involved levelling the ground, rearranging the canals and adjacent roads in a grid pattern, changing the shape of fields into rectangular ones, and expanding the plots through an exchange of property rights. The readjustment not only increased the cultivated land area but considerably improved productivity. The central government officials visited to observe the improvements. They later enacted the Farmland Readjustment Act and used the highly-regarded readjustment in the area as an exemplary model for the nationwide expansion of farmland readjustment.

Generally, farmers will not exchange ownership of farms if water availability differs from place to place. One of the success factors in this farmland readjustment was the sufficient water supply to every farmland provided by the canal system. We could say that the productivity improvement would not have been possible without the Teradani Irrigation System.

The structure is an example of attention to environmental aspects in its design and construction;

The canal was constructed by reshaping and enlarging existing small streams. This method minimized impacts on the environment and reduction of farmland. The design and construction of the system contribute to preserving the beautiful rural landscape.

The structure was an example of engineering marvel or excellence at the time of its construction;

The culvert adopted for the intake had an outstanding feature in terms of size. The cross-section was 4 m wide, 2 m high, and 21 m long, something that never existed before based on historical records. The culvert was constructed without nails, using an elaborate combination of numerous wooden pillars and boards. It was an outstanding and unprecedented work to build a structure that withstood the currents of a large river using wood alone in 16th century Japan, where modern engineering technology was still not developed.

The structure was unique in some positive and constructive way;

The intake location was carefully selected to ensure a stable water intake. To avoid sedimentation at the opening of intake structures during flooding and maintain the intake function even during drought, the appropriate intake location was chosen at the cut bank closest to the streamflow on a wide river. The main canal was constructed on the route that allowed maximum channel gradient to minimize alluvial deposits. The first intake constructed in 1590 remained functional for 168 years. However, the intake location needed to be relocated upstream as the Tenryu River changed course. The farmers spent a lot of money and labour to move the intake in 1758, 3 km upstream from the original location. In 1884, it had to relocate again 5 km upstream. The selection of the appropriate intake location used the same criteria as the initial construction in 1590.

Engineering utility vis-à-vis designed utility

This irrigation system was constructed approximately 430 years ago to supply water to 2,000 hectares of rice paddies. Its capacity could fully meet the water demand according to existing historical documents. Even 430 years ago, they could measure the gradient accurately and build the canal properly.

At present, the beneficiary area is approximately 1,500 ha. Although the earthen canal was replaced with a concrete-lined structure after World War II, it remains on the original route maintaining its 7.4 m3/s capacity. It continues to have a vital role in local agriculture by providing a water supply. One of the success factors is the appropriate route selection during its construction.

Present State of Conservation

In the 20th century, dams were constructed upstream of the river, and sediments were trapped in the dam reservoirs, resulting in a significant drop in the riverbed downstream and making it difficult to draw water from the river. Therefore, the intake structure was relocated twice upstream, and water is now taken from the dam reservoir for power generation. It has restored stability to the water intake. During the second relocation, some upstream sections of the canal became a shared structure with domestic and industrial water. Teradani Irrigation System Land Improvement District currently outsources the operation and management of the shared sections to a local public corporation that supplies domestic and industrial water.

Since its establishment 430 years ago, the farmers' cooperative, an independent and democratic organization, has been operating and maintaining the irrigation system. Farmers share the necessary costs equally according to their acreage. The canal was rebuilt from an earthen canal to a concrete-lined structure which ensured safety and reduced maintenance costs. The farmers are satisfied with the results immensely. Though the Teradani Irrigation System Land Improvement District is an organization for irrigation, it also strives to work with the local community. Every year, many high school students studying agriculture are invited to learn about irrigation. The staff gives lectures on water management for irrigation and demonstrates the facility's operation. This activity is appreciated highly by high school students aspiring to become farmers as a practical, hands-on learning experience.

Also, the Iwata City government, which has jurisdiction over the beneficiary area, created a supplementary textbook in cooperation with the organization. The history and accomplishments of the Teradani Irrigation System are taught at elementary schools every year. It has significantly helped residents recognize the value and importance of the irrigation system. The farmers in the beneficiary area never forget their gratitude to the forested area upstream, the water source. The organization donates a portion of the harvested rice to public organizations in the forest area every year. The donated rice is used in school lunches and other purposes.

Furthermore, Teradani Irrigation System Land Improvement District has been paying voluntarily for the conservation cost and maintaining 45 ha of forest for more than 60 years. In 2003, it received the prestigious Merit Award in Water Resources from the Minister of Land, Infrastructure, Transport, and Tourism for its conservation efforts. Moreover, we received the Prime Minister's Award for the 2022 Greenery Promotion Movement. The local people created and passed on a unique tradition of expressing gratitude to the founder, Hirano Shigesada. Celebrations such as the Iwata City designated intangible folk cultural property, Kamo Dainenbutsu, a ritual of playing taiko drums accompanied by flutes and gongs and singing songs dedicated to the founder and offering prayers to the local deity for a good harvest, have been held for more than 250 years. In addition, the Teradani Water Festival is held annually on the founder's death, and the local people offer prayers and present offerings to him.