Coastal Plain Research Station Research Articles

A physical analysis of diurnal temperature regimes in clear and turbid water layers: A problem in rice culture

Previous work has shown that water temperature is an important factor in the establishment of a rice crop by broadcasting pregerminated seed into flooded fields. In the tropics water temperatures often exceed the optimum for this method of establishment. At Coastal Plains Research Station (12°33′S 131°20′E) near Darwin, N.T., Australia, a rice field environment was simulated using tanks. Diurnal temperature profiles were measured in 10- and 30-cm layers of clear water with and without an evaporation suppressant, and turbid water, for the months of December and June. The patterns of temperature in clear and turbid water were consistently different at both times of year. Marked temperature gradients (up to 1°C/cm) occurred by day in turbid water, with maxima at the air-water interface, but gradients in clear water were invariably very small. The physical theory of radiation absorption and thermal transport in a water layer is presented and used to interpret the observations of temperature. A crucial factor in the interpretation relates to the conditions which govern the onset of convective motion. The analysis indicates that the temperature in clear water layers will be uniform throughout at all times and for all depths used in rice culture. The same will be true in turbid water at night, but during the day temperature will decrease with depth.