Clarification of Temporal Variations in Water Temperature of Drainage Canals in a Paddy-Field District Implementing Cyclic Irrigation

概要

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Cyclic irrigation, which is the reuse of drainage water as irrigation water, is practiced in paddy-field districts near Lake Biwa to conserve water quality. The water purification thus achieved, such as denitrification and adsorption of phosphorus to the sediment, is considered to depend significantly on storage capacity and water temperature of drainage canals. A paddy-field district where cyclic irrigation was being implemented was investigated and temporal variations in the vertical profiles of water temperature in the main drainage canal were measured during the period 2006 - 2008. In addition, a water temperature model for drainage canals was developed. Measurements confirmed that under cyclic irrigation, the temperature of drainage water was 2°C greater than that of the water in Lake Biwa, which is the original source of irrigation water. Although cyclic irrigation increased the suspended solids (SS) in the drainage canal, no significant relationship between water temperature and SS was observed. The results of simulation of water temperature in the drainage canal were improved by considering surplus irrigation water flowing into the drainage canal. It is suggested that operation of the irrigation pump and the inflow of surplus irrigation water has a more pronounced effect on water temperature than SS during the irrigation period.