Abstract
To assess the water environment of irrigation reservoirs in Nara City and to improve the understanding of the basic characteristics of the reservoir and other factors affecting water quality, including land use of the catchment area, water quality analysis and the collection of basic characteristic data were conducted. Analysis revealed that the water quality in the irrigation period is worse than that in the nonirrigation period. In addition, the principal component analysis carried out using 8 water quality parameters (pH, EC, transparency, DTN, DTP, COD, Chl.a, and SS) showed that the water quality characteristics of the irrigation reservoir can be explained using two major components (the levels of organic and inorganic contamination). Furthermore, the Water Quality Index (WQI) formulated using 5 water quality parameters (DTN, DTP, COD, Chl.a, SS) showed that 29% of irrigation reservoirs indicated water quality deterioration, and the water qualities of reservoirs in the western region were worse than those in the eastern region of the study site. The results of the analysis of the relationship between WQI and the basic characteristics of reservoirs, suggested that the water quality of reservoirs is affected by 5 main parameters; proportion of forested area per catchment area, proportion of residential area, proportion of farming area, dam height, and use of reservoir for culturing fish.
