Lowland catchment runoff response to climate change under CMIP6 in the Baltic region

Record or near-record high or low river flows are more often observed in different regions of the world. A thriving society must understand the magnitude of these changes in the future, mitigate their negative impacts, and be prepared to live in a different world. That is why qualified, constantly updated scientific projections of future changes are essential. Neither Lithuania nor the other Baltic countries have yet assessed runoff changes according to the latest climate change projection tools outlined in the IPCC 6th AR on climate change. In this study, the HBV model was used to project potential changes in river runoff. The ranking procedure was developed and used to select the best-fit GCMs that most accurately reproduced the climate conditions of Lithuania. Due to the anticipated changes in climatic factors affecting the studied rivers, the average annual discharge is projected to decrease by 12 to 42%, depending on the hydrological region (i.e., the conditions of river runoff formation) and the selected future period. High flows (Q5) are likely to decline very similarly to the annual ones, while low flows (Q95) are expected to decrease by approximately two-thirds compared to the reference period. An uncertainty analysis of the projections revealed that GCMs contributed up to two-thirds of the total uncertainty in the final results.