氏   名
モハメド モウジュード
M.I.Mohammed Mowjood
本籍(国籍)
スリランカ
学位の種類
博士(農学)
学位記番号
甲 第157号
学位授与年月日
平成12年3月24日
学位授与の要件
学位規則第4条第1項該当
専  攻
生物環境科学専攻
学位論文題目
Heat Transfer and Dissolved Oxygen (DO) Dynamics in Ponded Water of a Paddy Ffeid.
(水田の湛水層における熱輸送と容存酸素の動態)

論文の内容の要旨

  A comprehensive understanding on diverse processes that take place in a ponded paddy field is necessary to increase the productivity and to preserve the paddy field eco-system. Ponded condition in paddy cultivation creates unique characteristics with regard to the energy and mass dynamics, which are critical for the growth and development of rice plant and crop environment. In order to elucidate the phenomena of ponded water on heat and mass transfer in the paddy f~eld, the heat transfer and dynamics of dissolved oxygen (DO) were studied in the field as well as in the laboratory.

  A vertical temperature profile was measured in the paddy field, and the results were confirmed with model simulations under infrared and solar radiation, which are regarded as non-convective and convective systems, respectively. The temperatures at different depths of water layer were almost the same. The temperature of the soil surface was higher than the temperature of water throughout the day. Solar radiation is absorbed at the soil surface and changed into heat. Then the heat was conducted upward and downward to adjacent water and soil layer, respectively. The heating at the bottom of water layer imposed the buoyancy forces and brought the convection current in the water. On the contrary, infrared radiation was absorbed at the water surface and resulted in decreasing temperature gradient from the water surface to the soil surface thus no convection occurred. This comparison leads to the conclusion that convection plays an important role in heat transfer in the ponded water of a paddy field. During the night the soil was warm as a result of the heat stored in the heating phase of the day. Because of the warmed soil surface and cooled water surface the convection is a continuous phenomena in the ponded paddy field.

  The temperature difference between boundaries of water layer, which is the driving force of the convective mechanism, was measured using Ni-Ni wire sensor. From the continuous monitoring of the temperature difference in the paddy field, it was observed that the maximum temperature difference was around 4°C. The temperature difference is mainly determined by the albedo of solar radiation at water surface and soil surface and the thermal conductivity of soil. The convective mechanism was further elaborated by measuring convective velocity in ponded water of a paddy field using constant current hot wire method. A long constantan wire was used as a heater and two nickel (Ni) wires used as sensors for sensing temperatures, One of the Ni wire was attached to the constant wire to detect the temperature of the heater on heating. Another Ni wire was used to detect the temperature of ambient water. These Ni wires were connected to a bridge circuit so that the output of the circuit would indicate the temperature difference between the heater and ambient water. The temperature rise in the heater depended on both the power supply and the convection flow in the water. The sensor was tested and calibrated in laboratory experiments. The output response (temperature rise) was correlated with power supply and temperature difference. In this study a 10 mW cm-1 heat source was found experimentally to be effective. The temperature rise decreased as the temperature difference between the boundaries of the water layer increased. For calibration, the temperature rise was measured in stationary water as the sensor moved at very low velocities. The temperature rise decreased as the sensor velocity increased. The temperature rise was inversely proportional to the convective velocity. The convective velocity measured in the paddy field using the sensor, showed a diel variation with a maximum velocity 0,14 cm s-1 at around noon corresponding to the maximum temperature difference.

  The role of the convection on the dynamics of DO in ponded water was studied. The DO concentration profile in the field as well as in the laboratory showed that the concentration gradient of DO was observed neither from water suface nor from soil suface. If the diffusion was the only mechanism for DO transfer, one would expect a concentration gradient between water suface and soil surface. The oxygen produced by micro algae by photosynthesis and its subsequent circulation by convection flow resulted in uniform DO concentration profiles. During the night,the depletion of DO concentration below saturation due to respiration of microorganisms cause the atmospheric oxygen to transfer Into the water by reaeration. The reaerated DO was then distributed over the water profile by convection. This effect of convection on DO concentration profile under solar radiation was confirmed with the result under non-convective condition. The DO concentration gradient was clearly found from the soil surface, where the oxygen was produced by photosynthesis, under non-convective condition. Thus convection plays an important role not only in the heat transfer but also in the mass transfer of DO in the ponded water of a paddy field.

  The diel change of DO was studied in relation to dissolved CO2 under convective condition in ponded water of a paddy field. The DO concentration is the result of the balance between the photosynthesis, respiration and the exchange with atmosphere. The diet trends of the concentrations of both DO and CO2 were opposite to each other. If the photosynthesis and respiration are the only contributors for this trend assuming the exchange remains constant for both gases, there should be a linear relation in their molar concentrations. But a hysteresis loop, found in their relation Indicated that there is a difference in the exchange behavior of these gases. The exchange coeffrcient of DO was studied in the laboratory considering the field conditions, re aeration and deaeration and convection flow.
The experimental values for coefficients of reaeration and deaeration shows that they are not identical. The effect of convection in the ponded water on the exchange rate oxygen was investigated. The exchange coefficients increased with increasing convective velocity in water. The exchange coefficients depends on the resistance by thin film of water that exists at the water surface. The convection in the bulk water reduces the thickness of water film and increases the exchange coefficient. Thus, the rate of exchange of oxygen is facilitated by the rate at which oxygen is transferred by convection in the bulk water. Taking these factors into consideration, the net production, which was estimated from mass balance of both DO and dissolved C02 agreed each other and was closer to 1:1 relation. The DO concentration in ponded water exllibited a seasonal change particularly during the early growing season in paddy cultivation. The daily maximum increased during the first two weeks after transplanting and then decreased afterwards. The minimum was decreased even to zero level one month after transplanting.

  This study could help for the development of techniques for control of thermal regime in order to maintain an optimum environment for plant and overall paddy field eco-system. The dynamics of DO and dissolved C02 as elaborated in the present study is vital in cultural and soil fertility management in ponded paddy cultivations, and environmental implications to preserve the paddy field ecosystems.