<論説>清濁灌漑方式が持つ水環境問題への対応力 : 中国山西呂梁山脈南麓の歴史的事例を基に (特集 : 環境)

Abstract

中国山西省西南部の河津三峪地区においては、呂梁山脈中に湧き出る泉水と、山々の雨水を集めて流出する濁水という異なる水源を併用する清濁灌漑方式が用いられた。そこには、清水の稀少さと降雨の時期的偏りに由来する濁水の量的、時期的な不安定性という問題が存在した。これに対して、清濁両水の利用村庄を区別し、水源と村庄との対応関係を決定する地域・水源の別という人為的制限を加えることで、同一の扇状地における水資源の共有が図られた。また、村庄間、個人間など様々なレベルで清濁両水をめぐる水争が勃発する一方、清水や水利用地の売買・貸借の契約が取り交わされ、これにより時期的・量的な過不足が補われるとともに、限られた水資源の有効利用がなされた。三峪清濁灌漑方式の長期持続性を支えたものは、地域・水源の別という制限規定と水利契約による融通性にあり、そこに不安定な自然環境への適応のあり方が見いだせる。

A system that combined spring irrigation and flood irrigation, which was known as the qingzhuo guangai (clear and muddy water) method was used in the Sanyu 三峪 district of Hejin 河津 located in the southwest of the Shanxi 山西, China, This irrigation system used two sources of water, one was clear spring water from Mt. Luliang 呂梁 and the other was the collection of muddy rainwater runoff from the mountain. The former source maintained a steady discharge, but the latter, dependent on general trends of rainfall in northern China, was unstable in terms of timing and quantity. On the other hand, the wide catchment area of the flood irrigation system supplied water abundant enough to make up for deficiencies to some degree, and by supplying water with rich mineral components to cultivated areas, the cultivated soil was fertilized and salinization prevented at the same time. The water problem at the Sanyu district was derived from the deficiency in the discharge of spring water and quantitative and temporal instabilities of the flood waters. Therefore, two kinds of deliberate restrictions were prescribed for sharing the scarce water resources on the alluvial fan; one of them was to distinguish villages and estates that used the spring water and those that used flood waters; and the other was to stipulate the relationship between these communities and the water sources. These distinctions were not intended to confirm the privileged rights of upstream villages to the use of water, but to curtail their advantage. In the case of spring water, water was first drawn for downstream communities in order to prevent monopoly use by the upstream villages and to secure a regulated amount for each village, and after the allotted portion of the water for the downstream villages was drawn off into cultivated land and reservoirs, water was drawn to upstream communities by turns from below. In contrast, the method of drawing off a deliberately calculated amount of flood water from the upstream villages functioned effectively in responding to the two extremes of water shortage and flood. Fights over spring and flood water broke out at various levels in the Sanyu district. This was particularly true in regard to the flood runoff water. Due to its abundance and rich mineral component, there were disputes not only between communities that depended on flood water and those that depended on spring water but also between communities that depended on flood water themselves. Meanwhile, in regard to the spring water for which the order and amount to be used by each village was strictly stipulated, people sold and leased water for irrigation and irrigated land to create more favorable conditions. Therefore, the content of disputes over spring water was more complex than fights over flood runoff water. From the point of the adaptation to the environment, excessive diversion of spring water was limited by strictly stipulating water sources and the amount to be used by communities relying on spring water and prohibiting communities dependent on flood runoff water from using spring water. Villages in the Sanyu district were able to avoid the depletion of water resources and also to share scarce water resources. Moreover, trading in water that was alienated from land was done relatively openly. As a result, the regulated amount for each village was maintained and excesses and deficiencies in the water supply were compensated for. In other words, it was the deliberate limitations which distinguished areas and water sources and the flexibility which originated in contracts over water rights that supported the long-term durability of the system of water use in the Sanyu district. Villages using flood runoff water were able to overcome the instability of the rains by buying spring water. Yet, purchase of spring water by communities relying on flood water was prohibited in the 19th century. In addition the usage relationship between cultivated land and water sources was also stipulated in the 20th century, and regulations distinguishing areas and water sources increased in severity. The increasing severity of its management obstructed the flexibility of the system, and it is thought to have brought the system of spring-flood irrigation to a state of immobilization. The long-term durability of the spring-flood irrigation system that was supported by deliberate restrictions and contracts on water usage can convey to us today a harmonious way to deal with the natural environment that has been lost due to the attempt to eliminate instability.