3:30 PM - 5:00 PM
[MZZ43-P21] History of the water use and the topography of the Aso-4 pyroclastic flow in the Oita Bungoono Geopark, Japan
Keywords:Oita Bungoono Geopark, Aso-4 pyroclastic flow, water spring, water use, cultural landscape
Oita Bungoono Geopark is located in the southern part of Oita Prefecture, and east of the Aso caldera, where the pyroclastic flow plateau formed by the Aso-4 pyroclastic flow that erupted approximately 90,000 years ago. The plateau was dissected by the Ono River and its tributaries, and became a gorge with rock walls of welded tuff with well-developed columnar joints.
In order to live in this area, it is difficult to obtain the water necessary for daily life because the rivers flow through deep valleys, even though the flat land necessary for living and farming is located on the plateau or on the slopes of the plateau. Therefore, spring water was used first. Stratified volcanic ash and pumice layers are often distributed between the Aso-4 pyroclastic flow deposit and the Aso-3 pyroclastic flow deposit, and below the Aso-3 pyroclastic flow deposit. Among them, the relatively coarse-grained pumice layer and the base of pyroclastic flow deposits become permeable layers, and when they are exposed on cliff faces, they become springs. In the vicinity of the Bungoono City, the welded tuff of the Aso-4 and Aso-3 pyroclastic flow deposits are often carved with carved Buddha statues, and almost all of them in the city have spring water beneath them. Most of the Buddha statues are carved on weakly welded tuff with relatively few cracks in the lower part of the pyroclastic flow deposits, so there is often a permeable layer underneath. Conversely, in order to protect the spring water and use it sustainably, it is also possible to think of it as a sacred place where carved Buddhas were enshrined.
However, it was difficult to obtain enough water for large-scale paddy cultivation with such spring water. For this reason, irrigation canals have been built in this area since ancient times. Especially around Ogata-machi in the western part of the city, water intakes are set up upstream of major rivers, and water is supplied to the plateaus and terraces through waterways called 'Iro', and paddy fields are cultivated over a wide area. The origin of 'Iro' can be traced back to the Heian period, and in the Edo period, a policy was adopted to separate the paddy field area from the residential area on the boundary of the 'Iro', creating the base of the present-day village landscape. Furthermore, after the Meiji period, long-distance waterways were constructed to supply water to the tops of the hills, and terraced rice paddies were constructed at the tops of the valleys that erode the hills. These terraced rice fields are called 'Shinta' to distinguish them from the old 'Kota'.
In the Bungoono Geopark, which was laid out in this way, people took advantage of the topographical and geological features of the Aso pyroclastic flow, and at the same time overcame difficulties while making efforts to use water sustainably and stably. One of the results of that effort is the current landscape of villages and paddy fields. The landscape along the Ogata River and in the Ogata basin is selected as a national important cultural landscape in December 2022.
In order to live in this area, it is difficult to obtain the water necessary for daily life because the rivers flow through deep valleys, even though the flat land necessary for living and farming is located on the plateau or on the slopes of the plateau. Therefore, spring water was used first. Stratified volcanic ash and pumice layers are often distributed between the Aso-4 pyroclastic flow deposit and the Aso-3 pyroclastic flow deposit, and below the Aso-3 pyroclastic flow deposit. Among them, the relatively coarse-grained pumice layer and the base of pyroclastic flow deposits become permeable layers, and when they are exposed on cliff faces, they become springs. In the vicinity of the Bungoono City, the welded tuff of the Aso-4 and Aso-3 pyroclastic flow deposits are often carved with carved Buddha statues, and almost all of them in the city have spring water beneath them. Most of the Buddha statues are carved on weakly welded tuff with relatively few cracks in the lower part of the pyroclastic flow deposits, so there is often a permeable layer underneath. Conversely, in order to protect the spring water and use it sustainably, it is also possible to think of it as a sacred place where carved Buddhas were enshrined.
However, it was difficult to obtain enough water for large-scale paddy cultivation with such spring water. For this reason, irrigation canals have been built in this area since ancient times. Especially around Ogata-machi in the western part of the city, water intakes are set up upstream of major rivers, and water is supplied to the plateaus and terraces through waterways called 'Iro', and paddy fields are cultivated over a wide area. The origin of 'Iro' can be traced back to the Heian period, and in the Edo period, a policy was adopted to separate the paddy field area from the residential area on the boundary of the 'Iro', creating the base of the present-day village landscape. Furthermore, after the Meiji period, long-distance waterways were constructed to supply water to the tops of the hills, and terraced rice paddies were constructed at the tops of the valleys that erode the hills. These terraced rice fields are called 'Shinta' to distinguish them from the old 'Kota'.
In the Bungoono Geopark, which was laid out in this way, people took advantage of the topographical and geological features of the Aso pyroclastic flow, and at the same time overcame difficulties while making efforts to use water sustainably and stably. One of the results of that effort is the current landscape of villages and paddy fields. The landscape along the Ogata River and in the Ogata basin is selected as a national important cultural landscape in December 2022.