Evaluation of Reused Water Effect on Irrigation Water Quality of Low-lying Paddy Area

This paper presents results of a case study on reused water effect on the irrigation water quality in a low-lying paddy area with creek networks. The study was conducted through the procedure of observations on the increment values of water quality indices (Δ COD, Δ EC and Δ T-N) at arbitrary points in creeks from their primary values of the original irrigation water in the main canal. The analysis was carried out by plotting the increment values of those water quality indices corresponding to the reused water that was stated in term of content ratio of total return flow (CTR). The value of CTR was defined as the ratio of total return flow to the total amount of water in a creek network. The results show that in fine days, Δ COD and Δ EC perform the same tendency of the relation to return flow, the value increases in line to raise in CTR. In rainy days, the increase of CTR will also increase Δ COD and Δ EC. But if CTR over 70%, the change value of both water quality indices tend to decrease. For Δ T-N, the plotted data are very scattered and the value fluctuates negatively and positively showing the purification process and the polluting action occur in paddy fields. Among the considered water quality parameters, it can be observed that the most severe index is COD, which recommends the value of CTR should not be higher than 55%.