DYNAMICS OF DISSOLVED-OXYGEN (DO) IN PONDED WATER OF A PADDY FIELD

The transfer of heat and dissolved oxygen (DO) through water is import ant to understand the phenomenon of ponded water in a paddy soil. The heat from solar radiation is absorbed at the soil surface and transfer red into the ponded water by convection. This study clarified the dyna mics of DO, as well as the role of convection in mater in DO transfer in the ponded water of a paddy field. DO concentration in the ponded w ater of a paddy field was measured in situ in the daytime and during t he night. The results were confirmed in lab-scale model experiments. T he DO concentration and temperature profiles in the ponded water of a lab-scale paddy field model were investigated under convective and non -convective conditions using solar radiation and infrared radiation, r espectively. Under the ponded condition, solar radiation was absorbed at the soil surface whereas infrared radiation was absorbed at the wat er surface and thereby convective and non-convective conditions were g enerated, respectively. The diurnal variation in DO concentration was closely related to the intensity of solar radiation. Oxygen generation by micro-algae and its subsequent circulation by convection resulted in uniform DO concentration profiles, with super-saturated values in t he ponded water in the daytime. Eventually oxygen was released to the atmosphere by deaeration until DO in water was depleted to the saturat ed level. During the night the oxygen moved from the atmosphere into t he water surface by reaeration which depends on the oxygen deficit rel ated to saturation. The oxygen deficit is caused by the respiration of microorganisms. The oxygen, that moved from the atmosphere to the wat er surface, was transferred to the soil surface by convection in the w ater layer. Thus convection plays an important role in the DO transfer in the ponded water of a paddy field. The DO dynamics is correlated w ith biological processes in the ponded paddy soil.