Thermal structure and variability of a shallow tropical reservoir

Abstract

The structure and mixing dynamics of shallow tropical reservoirs was investigated using data collected in the Kranji Reservoir in Singapore. Water temperature data spanning a 2 month period in 2007 shows that diurnal cycles of stratification and destratification were formed in various locations in the reservoir. Temperature stratification of 0.5°C to 3.5°C was formed during the daylight hours and reduced nightly when cooling occurred. Substantial horizontal redistribution of heat was also observed between the side arms and the main body of the reservoir. The Kranji's dynamics can be summarized in terms of three physical forcing regimes: a solar radiation–dominated regime, a windy regime, and a cold inflow regime. We delineate the three physical forcing regimes by two potentially useful dimensionless numbers that quantify the relative influences of surface heating, wind stirring, and inflow buoyancy fluxes. For most of the measurement period (88% of the data record), the solar radiation–dominated regime characterized the hydrodynamics. In spite of the dominance of solar radiation in setting local stratification, it is shown that Kranji Reservoir is a three-dimensional system in which there can be significant variations in temperature in the vertical and along-reservoir directions, as determined by cold inflow events, differential heating, and reservoir releases. Moreover, the data suggest that the dynamical balance of the Kranji system is sensitive to small forcing events, with the timescales of stratification and mixing as short as a day or less.