**3. Water temperature in the reservoir**

The water temperature and its fluctuations are important characteristics of storage reservoirs. In the areas characterized by well-defined cyclic temperature changes, the diurnal and seasonal temperature changes of surface water are typical. Thermal conditions of water reservoir are determined by climate (temperature of air, insolation, direction and speed of wind) and reservoir morphology, subsoil type and vegetation [3]. Water temperature can also be influenced by anthropogenic factors.

In the case of the Turawa reservoir, the water temperature changes in a wide range. It depends mainly on air temperature. In 2004–2006, the average water temperature was relatively well correlated with the changes of average monthly air temperature (correlation coefficient of 0.9).

In a year cycle, there are two periods of relatively stable temperature, winter season and summer season, and also two periods characterized by large water temperature changes, spring and winter seasons. The monthly minimum temperatures in the range of 0.6–0.8°C were observed in the period from December to February. The maximum temperatures ranging from 23.1 to 25.4°C were measured in July and August. In the period discussed, the lowest temperature was 0.3°C (January 2004) while the highest was 26.7°C in July 2006.

The changes of the limits of thermal layers and periods of water circulation and stagnation were also studied. Spring circulation starts after ice melts off and is characterized by temperature increase (from 0°C). Heavier water moves downwards while colder water from the bottom layer moves up resulting in water mixing and constant temperature in a vertical profile [4].

The period of summer water stagnation is characterized by temperature drop with depth. This is typical water stratification whereby the upper layer of the highest temperature close to air temperature is epilimnion and the coldest bottom layer is hypolimnion. The water between these two layers is metalimnion. Due to the small depth of the reservoir (in summer rarely more than 10 m in the deepest point) and intensive wind mixing in summer stagnation period thermal stratification was not observed—vertical temperature difference did not exceed 4°C (**Figure 4**).

**Figure 4.** Changes in water temperature in the Turawa reservoir in the hydrological year 2004—vertical profile: 50°44′10.28″N, 18°05′30.22″E.

At the beginning of September, the period of autumn circulation starts in the Turawa reservoir. Due to cooling of surface layer, wind and convective mixing, the difference between the water temperature of epilimnion and hypolimnion in storage reservoirs decreases [5]. In the case of the Turawa reservoir, the process of decreasing difference in temperature was accelerated by intensive water outflow in the period from September to December. The water level dropped by 2.5 m and the water volume approximately by 30 million m3 . The process of water cooling ended when the temperature dropped to 4°C in the whole thermal profile, what for the Turawa reservoir was at the turn of November and December. From December to February (hydrological year 2004) or to March (hydrological years 2005 and 2006), winter stagnation occurred. In the deepest parts of the Turawa reservoir, water temperature was in the range of 2–2.7°C, while directly under ice layer it was 0.8°C.

In fact, summer stratification did not occur in the Turawa reservoir—the average water temperature was high and ranged from 20 to 21.8°C, whereas in winter it was in the range of 0.8–2.0°C. The annual average water temperature for the whole reservoir changed from 10.1 to 10.7°C. Thermal variability of the reservoir or the ratio of maximum temperature to minimum temperature reached the value of 26. In the hydrological years 2004–2006, the thermal conditions of the reservoir were strongly influenced by fluctuations of water level and the corresponding large changes of the volume of water from about 16 to 91 million m3 . The relatively small average depth of the Turawa reservoir favours wind mixing.

The interesting characteristics of the Turawa reservoir are water-freezing phenomena. The earliest time ice cover appeared was the end of November while the latest time its disappearance was the beginning of April; on average, it was the second half of December and the middle of March, respectively. The ice cover disappeared first in the coastal zone, most likely due to higher absorption of heat by land masses and intensive supply of surface and ground water.

The annual length of time the Turawa reservoir was covered with ice varied within a wide range. In the studied period, it changed from 18 days in 1994 to 106 days in 1984, on average 61 days. There were also large differences in the thickness of ice cover, an annual average cover ranged from 6 cm in 1988 to 26 cm in 1996. Due to the formation of ice cover, anthropogenic storage reservoirs, including the Turawa reservoir, can be used for recreation, too.
