The effects of water diversion and climate change on hydrological alteration and temperature regime of karst rivers in central Croatia

Karst rivers in Croatia have been heavily dammed in the past. However, long-term impacts of dams, water diversion and climate change on karst rivers in Croatia have rarely been examined. This study analyzes long-term trends of hydrological and temperature conditions, prior to closure of new large dam, in five connected karst rivers of central Croatia: the sinking rivers Gornja Dobra and Zagorska Mrežnica that have underground connections with the Gojačka Dobra, Tounjčica and Mrežnica Rivers. It was established that hydrological conditions of these rivers have been greatly changed after the Gojak HPP, located at spring of Gojačka Dobra, became operational in 1959. Water diversion from the reservoirs on Zagorska Mrežnica and Gornja Dobra to the Gojak HPP has increased mean annual discharge (52–106%), causing great subdaily fluctuations and overall modification of natural flow regime along the whole course of Gojačka Dobra. By cutting the connection between Zagorska Mrežnica and Tounjčica, water diversion has caused a reduction of mean annual discharges of the Tounjčica (−60%) and Mrežnica (−29%) Rivers. Analyses of long-term trends in water temperature over the last 50 years have revealed significant positive trends in average annual temperature at three gauging stations on three rivers with increase of 0.17, 0.44 and 0.48°C per decade and climate change as the primary cause. The highest significant increasing trend was established at the lower course of Mrežnica where discharge was reduced by water diversion. Mitigation measures for the improvement of hydrological conditions of affected rivers and underground systems are proposed and discussed.