Case Study on the Salinity Behavior in the Alluvial Farmland near the Naruto Straits.

Abstract

Japanese alluvial area has been an excellent agricultural zone but subjected to complex problems such as drainage, salinity and water quality due to its natural low-lying and flat characteristics. These problems are worsened by the modern economical development and severe relations between water supply and demand press the shortage of water for agricultural use continuously. Consequently, traditional water order in the coastal field which controlled skilfully the uprising of salt from salinized underground layer by the intrusion of seawater to surface layer is getting worse and remarkable risk of salt injury would be increased. The objective of this study is to present a proposal for necessary countermeasures to maintain agricultural productivity in the alluvial area. Two fields were selected near the Naruto Straits with salinity problem in underground layer originated from tidal river. In first case at the Magota River improvement project on western Awaji Island, it become clear that some device like upper and lower layer not to be mixed at construction works are necessary to avoid the uprise of saline groundwater in a range from the river mouth to upstream 600m. In second case at Kawauchi district on the most downstream of the Yoshino River, we considered it is necessary a high level of countermeasure that distributes sufficient fresh water as multipurpose use of irrigation water that include the customary salt exclusion water.