Lake Balaton hydrology and climate change

Situated in the western part of Hungary, Lake Balaton is the second largest shallow lake in the European Union after Lake Peipus and the largest one laying entirely within the borders of the EU. Despite of its large surface area of some 600 km2, its average depth is only 352 cm. Lake Balaton is an exorheic lake on multiannual basis, but dry spells in the last 2 decades resulted in a 64-month period without outflow. Studies by the authors and others proved that human impacts of global and local nature resulted in the significant decrease of the natural water balance (NWB) in the last 3-4 decades. Climate change is already manifested in the reduction of the discharge of most of the tributaries including the largest one, Zala river. The statistically significant decrease in the last 3 decades of the discharge of Zala river and Kiskomáromi-canal corresponds to a deficit of 67 lake mm/year. The impact of further human interventions including reconstruction of Balaton Minor, a vast wetland and mining resulted in a further deficit of 119 to 154 lake mm/year. The average of the annual NWB of the last 30 years is only 63% of the long-term average. In addition to the decrease in the average discharges and NWB, variability of these values increased considerably. Some of these phenomena can be attributed to climate change. Future impacts of climate change are evaluated and it is concluded that the Lake Balaton watershed may turn into an endorheic basin in the second half of the 21st century