The Honjukuyousui irrigation Canal is an irrigation system consisting of a tunnel of 540 m in length (with an internal height of 1.5 m and a width of 1.8 m) and an open channel of 480 m in length that takes water from a diversion weir (with a length of 70 m and a drop of 2.0 m) located 30 m upstream from a waterfall at the Kise-gawa River.

The Kise-gawa River easily floods due to a shallow riverbed and rapid increases of a water level when it rains. For that reason, taking water from the river posed technological difficulties, and it would not be feasible to use the river for irrigation canal until the beginning of the 17th century. Additionally, the land in Honjuku could not be expected to have a high crop yield due to soil’s inability to retain water. The construction of the Honjukuyousui Irrigation Canal made it possible for the village to expand and possess its own paddy fields and, as a result, alleviate poverty.

After a three year-long series of trial and error, examinations, adjustments and surveys on where placing the diversion weir would result in finding the most stable and efficient water intake, the village settled the location of the weir on the area upstream from the waterfall. In order to overcome the topographical challenge posed by the multi-tier drop of the fluvial terraces, the village made the bold choice of using a tunnel, which involved difficulty in construction, for the first section (This tunnel had a natural gradient and did not employ a siphon system.) and an open channel for the latter section. It is believed that if the village built an open channel along the terrace precipices located along the Kise-gawa River, flood damage caused by the river would have required that the channel be repaired over and over again.

The combination of intake, water flow and water distribution techniques that went into the canal represent outstanding civil engineering for irrigation that were the most advanced technology back in 1603. That engineering was utilized as a model for the Fukarayousui Irrigation Canal (which was registered as a WHIS in 2014) completed 67 years later. Even now, 420 years after the completion of the original in Honjuku, irrigated water is being stably supplied to the rice paddy zone of the Honjuku region. Even today, 420 years following its completion, the canal supplies irrigation water to 2.2 ha of agricultural land in the Honjuku area. As such, it is classified under “List A.”

As the tunnel area of this canal was constructed using unsupported excavation, in 1990 and again in 1998, concrete lining with a length of 247 m was added to the tunnel area, and repair and improvement work was implemented using regional government subsidies to make one of the sluices electronically operational. In this manner, efforts have been made over time to reinforce and enhance the functions of the canal and its maintenance systems.

In order to develop rice paddies, the placement of a “diversion weir” to take in water from the Kisegawa River that flows west of the Honjuku region, the excavation of a “tunnel” to convey irrigated water and the development of a “channel” to distribute that water within the area were all essential. In the early 17th century, the then-ruler of the region devised an “irrigation structure project” that made full use of civil engineering for irrigation purposes in order to dam the Kise-gawa River 30m upstream the waterfall in the river and draw river water into the tunnel of the Honjukuyousui Irrigation Canal.

The new distribution of water to the Honjuku area by the canal made it possible to convert 9.2 ha worth of rain-fed upland fields into irrigated rice paddies, resulting in a 16-ton increase in production. Additionally, new paddy development also expanded to yield a 6-ha increase in the rice paddy area. This caused dramatic increases in the food production of the Honjuku area.

In the 1600s, the then-ruler of the Honjuku region established a revolutionary “irrigation structure project” to build a diversion weir in the Kise-gawa River in order to draw river water into the canal’s tunnel, which was constructed with the full use of the most advanced civil engineering and excavation techniques at the time.  The canal’s development resulted in the installation of a gate at the water intake, making it easier to control water volume through water distribution supervisors who handled the irrigation management. This made possible the operation of a “water management system” that could be used to shut the sluice upon flooding and tailor diverse demand for required volumes of irrigation water. Additionally, water could now be gradually distributed from the upstream region to the downstream region with precision.

The development of this irrigation system resulted in the stable distribution of irrigation water to the entire Honjuku area, enabling the region to be reborn as a rice paddy zone and contributing tremendously to the improvement of livelihood of a rural village and the reduction of poverty.

The most remarkable points characterizing the canal’s system are its highly unique conception of the “excavation of the water tunnel” through very hard lava layers that extend to the upstream of Honjuku village area and the full utilization of the latest techniques available at that time, which materialized that concept, based on superior hydraulic engineering and innovative structures for water “intake, conveyance and distribution.”

As many as three years were spent before determining the location, height and positional relationship of the weir and intake, from the aspects of engineering and construction. The result was worth this huge effort, as it made possible to take water with the greatest stability, efficiency and certainty for over 4 centuries until today. Also, the importance of the project was explained to landowners and riverside communities over and over again to form a consensus. Furthermore, the successful establishment of a “water distribution system” that enabled natural water conveyance and realized the securing of flow velocity played a considerable part in the forward development of modern theory and practice.

The development of the canal resulted in irrigated water being used for the likes of daily life, motive power, cleaning, fire prevention, and the dilution of gray water as well. It also resulted in the creation of an environmentally-conscious irrigation structure which is rich in biodiversity.

The characteristics of the civil engineering for irrigation purposes that went into the construction of the canal include: “(1) Technique through which strong water flows were buffered and conveyed through the slanted placement of the weir to line up with flow of water while effectively utilizing the uneven area of the lava spreading across the riverbed,” “(2) Technique through which leveling rods were used for measurement and measuring apparatus such as levels with water placed in bamboo split from the inside, magnets, rulers, protractors and rope for measuring land area were used to measure elevation difference based on the method of combining right-angled triangles and technique through which traditional lamps were lined up at three or more points on dark nights to extract the locations of straight lines,” “(3) Excavating bedrock by using chisels to dig them by hand with  manpower, excavating particularly hard rock by dousing them in oil, lighting them on fire to heat them and dousing them with water to cool them off and make them brittle, and constructing skylights and vent holes to serve as ventilation ducts and preliminary tunnels,” and “(4) Building a “water cycle system” by placing a new river to collect irrigation water following distribution and draining remaining water to the Kise-gawa River to secure the water volume of the weir downstream, thereby preventing ‘water disputes’.”

Amid a bevy of imposing challenges, the construction methods employed in the canal, which involved fully utilizing “(1) Water intake techniques, (2) Survey techniques for excavation, (3) Excavation and (4) Water supply techniques” to successfully realize the construction of an irrigation structure, were nothing short of unique.

In the area nearby the canal’s water intake and its outlets, there is a statue of a “water god” and a stone plaque expressing gratitude to Honjuku’s predecessors. For a long time now, the traditional culture of the canal has been passed on through holding various events to remember and learn about the vestiges of the region’s past civilization, including gatherings to pay respects to the efforts of those ancestors and collective efforts to properly maintain the channel by the citizens of Nagaizumi Town.

Even today, 420 years after its completion, the Honjukuyousui Irrigation Canal accurately functions as an irrigation structure to distribute irrigated water to 2.2 ha of agriculture land in the Honjuku area located downriver. At the same time, up to this point, the canal’s tunnel has suffered from collapse with every earthquake, and has been repaired each time. While a massive earthquake in 1854 caused the tunnels to collapse at various points, in the year 1855 that followed, rehabilitation work was implemented to install a 246-m long channel and a 72-m long tunnel to convey the water from the sluice and light ports (for skylight and ventilation) at three points along the tunnel. Large earthquakes also occurred subsequently in 1923 and again in 1930, causing the tunnel to collapse. This, too, was subject to repair work. In 1990 (Phase I Construction) and 1998 (Phase II Construction), the regional government carried out rehabilitation work to line the tunnel with concrete, as well as electrification work to enable the safe operation of the sluice.

The Honjukuyousui Irrigation Canal is an irrigation structure that was built 420 years ago with the full use of superior civil engineering techniques. Following its completion, the tunnel and irrigation channel suffered from collapse due to repeated earthquake disasters. However, each time, the Honjuku Common Property Management Committee and the Honjuku Regional Agricultural Association, which manage the structure, have spearheaded efforts to appropriately preserve and manage it over time with the help of funding by the Honjuku area, subsidies from the regional government, donations from corporations, and so forth.

In recent times, for the purpose of recalling the toil of Honjuku’s predecessors and learning about the historical, cultural and technical value and innovation of the Honjukuyousui Irrigation Canal, “A Walk along the Honjukuyousui Irrigation Canal to Inquire about Its History” targeting residents of Nagaizumi Town and other local residents and a “community friendship event with resident/child participation set at a beneficiary rice paddy,” which took place in a rice field as a hands-on event to remind participants of the importance of rice fields, are held.

Moreover, seminars on the Honjuku region and the event named “Walk along the Honjukuyousui Irrigation Canal to Inquire about Its History” are planned and held on a near-annual basis. Residents of Nagaizumi Town, other local residents and people from outside the city are invited to learn about the role of the Honjukuyousui Irrigation Canal as an irrigation structure, its value as historical heritage, the importance of preserving and managing the structure, the importance and significance of its continued existence, and more as they make their way around the diversion weir, tunnel, main channel and other key components of the irrigation structure.

Furthermore, the Honjuku, the community newspaper features reports on the “Dorolympics (Mud Olympics) in Honjuku” held nearly every year, and communicates and provides notice of the importance and preciousness of the Honjukuyousui Irrigation Canal as an irrigation structure. The history of the hardships that the region’s ancestors experienced in securing irrigation water, the importance of safeguarding and passing on that water and other related information are conveyed to children largely by adults and farmers through diverse events and community activities.

In early April, Nagaizumi Town residents and farmers mobilize to perform dredging and cleanup work for the Arai Weir on the Kise-gawa River and for the water intake and irrigation channel. The smooth distribution of irrigated water continues to be maintained without fail today.

During irrigation season, the rice fields become filled with water. As this is an effective measure to prevent the warming of urban areas, which increase in temperature in the summer season, as well as to counter the heat island effect, this water is preserved and managed with care as “a regional treasure,” “an irrigation structure heritage” and “local water” that is absolutely essential to the Honjuku area.