What happened to the karst spring Ombla water temperature?

The paper focuses on analyzing the water temperature of Ombla, the largest karst spring in the Dinaric Karst. Hourly measurement data were available from January 1, 2007, at 0:00 a.m. to December 31, 2021, at 11:00 p.m. It has been established that, within that period, there were two sub-periods with different characteristic (minimum, average, and maximum) water temperatures. In the first sub-period, which lasted from January 1, 2007, at 0 a.m. to January 24, 2012, at 8 a.m., the minimum, average, and maximum water temperature values were 12.5, 13.83, and 15.0 °C, respectively. In the second sub-period (January 24, 2012, at 9 a.m. to December 31, 2021, at 11 p.m.), there was a sudden drop in the characteristic temperatures, which were 10.9, 12.91, and 14.0 °C. At the same time, a strong increasing trend of the regional air temperature is noted. It has been detected that the water temperature reacts to the air temperature with a delay of about 1 month. Due to the enormous importance of this abundant karst source, it is necessary to determine the reasons that led to the sudden drop in water temperature at the end of January 2012.