Temperature stratification in a long-term underground water reservoir was studied experimentally. The cold water storage tank, which was selected for this study, is an underground water reservoir with a domed shape roof and equipped with wind towers (Baad-Gir) which are responsible for capturing wind from any direction and inducing airflow over the water surface. These historic reservoirs were used as a source of drinking cold water in hot arid central regions of Iran during hot and dry summer season. The cylindrical shape underground reservoir, with 12m in height and 12m in diameter, was filled with 15°C water from a nearby well in winter. Temperature data were taken every ten days from late April until mid-October. To obtain accurate experimental temperature data, water layers temperature was measured in vertical direction whilst cold water was extracted from bottom of the tank on a daily basis at a rate corresponding to the regional inhabitants water consumption. It was observed that stable thermal stratification was developed after charging the reservoir. The temperature of extracted water was in the range of 11.9–13.1 °C during the entire summer period whilst the outside ambient temperature was reached upto 42 °C. It is believed that the radiation heat exchange between the water surface and the storage ceiling, as well as the convective heat and mass transfer from the surface of water induced by airflow were primarily responsible for temperature profile change. However, the discharged water flow rate had a secondary effect on thermal stratification.

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