Proposal of New Return Flow Analysis of Replacement in Order for Paddy Irrigation Water and Application to the Actual Site, Japan

-For dual-purpose of irrigation and drainage main canal system in the Shichika irrigation district-

Abstract

Return flow and repeated use of irrigation water for paddy is the most important issue in Asian monsoon region because the water sometimes applied more than evapotranspiration plus percolation. New return flow analysis, replacement in order method, is proposed for main canal which aimed to the dual-purpose of irrigation and drainage. The method contains unique numbering procedure for irrigation and drainage system. After calculation of return flow from the drainage block, the block replace in order from small to large number as the same of the irrigation system. Through this replacement, the return flow to the receive weirs can be systematically conducted. This method applied to the Shichika irrigation district when water right discharge, resulting the return flow ratio is 59% for entire area, in which 33% is available for irrigation again in the area and the other 26% does not available, because the return flow discharged directly into the downstream canal which have no receive weir or the Japan Sea.