The object of this research was Taoyuan agricultural zone belonged to Taoyuan Irrigation Council. The main purposes were: 1) to investigate the irrigation water quality in the second branch of Taoyuan channel, 2) to evaluate the self-purification of storage ponds, 3) to study characteristics of domestic wastewater, and its influence on irrigation water, and 4) to find out the effects of agricultural returning water on irrigation water. The data showed that water characteristics deteriorated gradually from upper- to down-stream in the second branch of Taoyuan channel, and it was more obvious in dry season. This phenomenon may be related to that agricultural, domestic, and industrial waste water were discharged arbitrarily into the irrigation channels. In addition, the results presented that the rainwater had not only dilution function but also washout function. The results of storage ponds indicated that 2-18 and 2-24 pond could provide the natural self-purification in dry season, and the self-purification of 2-18 pond was usually superior to that of 2-24 pond. In the rainy season, the washout function of rainwater occasionally increased the concentrations of water quality in storage ponds, this phenomenon was more pronounced in initial- and middle-wet season. For 2-1 pond, it had lost the self-purification completely due to deterioration of water quality, and the dilution function of rainwater was also ineffective. Additionally, its water surface of dense algae also showed that the eutrophication condition was very serious. For domestic wastewater, the results presented that the number of community family had no any relationship with the water characteristics at all, and the primary factors of influence were the personal and community life pattern. When the dilution effects of washing cars and garden watering were negligible, the chemical oxygen demand of domestic wastewater was approximately 159 ~ 563 mg/L, total nitrogen was 15 ~ 68 mg N/L, and total phosphorus was 0.6 ~ 5.7 mg/L. If domestic wastewater was discharged arbitrarily into irrigation channels, its contamination would affect seriously the quality of irrigation water. Finally, the study exhibited that carbon, nitrogen, and phosphorus concentrations of irrigation water were generally increased after agricultural utilization.
