Heat and Dissolved Oxygen (DO) Transfer Phenomenon in Ponded Paddy Field

Abstract

An understanding of the transfer of heat and gases into and out of water is critical to understand the function of surface water layer in a low land paddy field. This study is intended to clarify the phenomena of heat and dissolved oxygen (DO) transfer in the paddy field water layer. Field experiment was conducted for temperature profile and DO concentration. The result was then verified with a lab-scale model under natural solar and artificial infrared radiation which were convective and non-convective system, respectively. The continuous higher temperature at the soil surface indicated that the soil surface acts as the plane from which the heat is conducted upward and downward to water and soil, respectively. The heating at the bottom of water layer imposes the buoyancy forces that brings the convection current. Rayleigh and Prantdl numbers also confirms the turbulent convection in the water layer. The heat that is accumulated in the soil layer during the heating phase of the day is transferred to the soil surface during the cooling phase. Then the heat transfer from the water to the atmosphere is regulated by convection. The diurnal variation in DO concentration indicated that there is an excess supply of oxygen in the day time and an over consumption in the night time. In the day time the DO generated by micro algae is transferred from the water to the atmosphere. In contrast, during the night time the oxygen transferred from the atmosphere into the water layer by reaeration. But, there was no distinctive concentration gradient in the ponded water either from the water surface or from the soil surface throughout the day. The corresponded temperature data confirmed the existence of a convection current in the water layer. This convection resulted in a uniform DO concentration throughout the profile regardless of reaeration or deaeration. The resulting temperature and DO profiles generated under the convective system was compared with the profiles generated in a non-convective system. This comparison lead to the conclusion that convection plays a significant role in the heat and DO transfer in the ponded paddy field.