Effect of convection on the exchange coefficient of oxygen and estimation of net production rate of oxygen in ponded water of a paddy field

Abstract

Dissolved oxygen (DO) in ponded water is essential to biological activities in paddy fields. DO concentrations results from the balance among photosynthesis, respiration, and exchange of oxygen between the atmosphere and water, all of which show daily changes. For the estimation of the net production rate of oxygen that is the overall response of photosynthesis and respiration, it is necessary to determine the exchange rate of oxygen between the atmosphere and water. The exchange of oxygen occurs in the form of reaeration and deaeration in the paddy fields. The exchange coefficients of reaeration and deaeration were determined in the laboratory considering field conditions. The coefficients of reaeration and deaeration were not identical. Both coefficients increased as the convective velocity in the water increased. It was considered that the convection in bulk water reduced the thickness of the water film at the air-water interface and increased the exchange coefficient. The reaeration and de aeration coefficients ranged from 2.1 to 2.4 and 5.7 to 6.3 d^?1, respectively. The rates of reaeration and de aeration were estimated using these exchange coefficients. The exchange rate was not constant in the paddy field throughout the day but varied with the convection, water temperature, and the degree of saturation in the ponded water. Taking these factors into consideration, the net production rate was estimated from the DO mass balance. The net production rate was positive from 5 : 30 to 15 : 30 and negative in the other part of the day. These results and analytical methods would contribute to the understanding of the physical, chemical, and biological processes in a paddy field.