Changes of Water Temperature, pH, Dissolved Oxygen, Inorganic Nitrogen, and Phosphorus Concentrations in Flowing Irrigation Water on Paddy Surface

Abstract

The changes in the quality of irrigation water were investigated while water was flowing on the surface of planted and bare paddy fields during the day and at night. The temperature and pH of the water on the surface of the planted field increased with the flowing process in the daytime, during the early growth stage of the rice plant, but varied very little after the middle growth stage as a result of shading by rice plants. In the bare field, the water temperature usually increased while water was flowing, throughout the growth period of the rice plant. At night, the temperature of the flowing water decreased in both fields and the pH changed little during the flowing process. The concentration of dissolved oxygen in the flowing water increased when enough sunlight reached the surface of both the planted and the bare field. In the planted field shaded by rice plants and at night, the amount of oxygen disssolved changed little while water was flowing. The contents of ammonium and total inorganic nitrogen in the flowing water decreased on the surface of both fields during the day and at night. In the planted field, the rates of decrease of the ammonium nitrogen content during the flowing process at 150 m varied from 20 to 38% in the daytime and from 8 to 37% at night. In the bare field, the average rate of decrease of the content of ammonium nitrogen was 76% in the daytime and 20% at night. The amount of inorganic phosphorus in the water decreased while water was flowing on the surface of the paddy fields. The rates of decrease of the amount of inorganic phosphorus were more or less the same between day and night in both the planted and the bare fields.