Water turnover in Honjo aea of Lake Nakaumi was estimated from salinity change rate. Reduced major axis method and principal component analysis were used instead of least squares method, to avoid artifact in parameter estimation. 2.26% of surface water in Honjo area were exchanged every day with the surface water of central area of Lake Nakaumi before new channel construction. 0.0403%o day^<-1> of background salinity increase was also detected. This may be caused by mixture with saline bottom water. After new channel construction between north area of Lake Nakaumi, water inflow through the new channel was although not detected. From the analysis assuming the absence of inflow through the new channel, daily inflow from central part of Lake Nakaumi was 3.42% of surface water in Honjyo area. Background salinity increase was 0.0461%o day^<-1>. Long term experiment is necessary to avoid the effect of seasonal change and episodic weathers. Comparison between the result of other method may be also helpful.