Wet and dry season heat loss in two tropical Australian reservoirs

Abstract

The influence of the wet/dry tropical climate on the seasonal heat pattern for two Australian water bodies is examined. The heat content and volume-weighted mean temperature of the reservoirs (Darwin River Reservoir, DRR; Manton River Reservoir, MRR) were computed for an eight year period which included wet seasons of very high and low rainfall. Because of the large volume changes of the two water bodies and dependence of heat content on volume, volume-weighted mean temperature was a better parameter to assess the heat pattern of DRR and MRR. The surface energy budget for DRR, in common with other tropical water bodies, was primarily controlled by net radiation and evaporative energy fluxes. Both reservoirs experienced two annual temperature minima and maxima in most years. The water bodies reached minimum annual temperatures (22-25°C) in the cool, dry season (June-August), then gained heat rapidly during the dry-wet transition (August-October). In December, temperatures reached 31-32°C. During most wet seasons (January, February), monsoon weather caused significant heat loss, leading to a second temperature minimum of 27-30°C. Subsequent heat gain increased temperatures to 30-32°C in March. When monsoon weather was brief, however, neither reservoir experienced significant wet season cooling, resulting in a single annual minimum and maximum temperature. Between April and June reservoir heat loss was most rapid due to reduced shortwave radiation and increased evaporative energy fluxes. The seasonal heat pattern of DRR and MRR is compared to other water bodies exposed to a tropical climate comprising of distinct high and low rainfall seasons. Minimum annual temperatures and heat contents of DRR and MRR coincided with the hemispheric winter, as experienced by nearly all tropical lakes. No single seasonal heat pattern, however, is representative of these tropical climates, due to variable influences of advective and surface heat fluxes, though two annual temperature and heat content maxima and minima is most common.