Assessing the impacts of climate change on hydrological regimes and fish EQR in two Danish catchments

Study region: Two large-scale catchments in Denmark: Ringlobing fiord and Mid-Zealand catchments. Study focus: Future changes in hydrological regime, extremes, groundwater levels and ecological quality ratio (EQR) for fish were assessed. Based on an ensemble of 17 bias-corrected regional climate models forced by the emission scenario RCP8.5, a dry, a medium and a wet model were identified for the future period 2071-2100. Changes in water balances for catchments, impacts on groundwater level and fish ecological quality ration (fish EQR) were evaluated. The impact changes were based on an integrated, transient, coupled groundwater and surface-water flow model, set up with a horizontal resolution of 200m x 200m for the two catchments. New hydrological insights for the region: Strengths and weaknesses of EQR assessments based on symbolic regression methods compared to expert assessments based on flow calendar and max flow changes in percent exceedance thresholds (Q95) were identified. While the modelled impact of climate change on monthly flow and flow calendar follows the same pattern at all monitoring stations, the impact on groundwater level and fish EQRs are more complex, and somewhat ambiguous across realisations and catchments.