Flood-related hydrological impacts of reforestation in the Čižina River catchment, Czech Republic

The Čižina River catchment was selected as a representative area affected by large-scale bark beetle–driven deforestation and subsequent reforestation with native broadleaf species. The aim of this study was to quantify the hydrological effects of afforestation with modified forest species composition on flood generation processes at the catchment scale, with emphasis on peak discharge reduction, runoff volume changes, and seasonal variability across different flood return periods. Flood-related hydrological effects of reforestation were evaluated using the model HYDROG. The influence of changes in forest species composition on runoff dynamics was analysed across a wide range of rainfall–runoff scenarios derived from three calibration flood events and was extended using stochastic precipitation series to represent return periods from 1 to 100 years. Afforestation with modified species composition reduced peak discharges and runoff volumes, particularly during frequent to moderately frequent flood events (T < 10 years). The effect was strongly seasonal, reaching its maximum during summer (peak discharge reduction exceeding 20% for T = 5 years), while winter benefits remained substantially lower (peak discharge reduction <7% for T = 5 years). The results highlighted the limitations of afforestation as a standalone flood mitigation measure. For effective flood protection, especially in larger municipalities, nature-based measures enhancing infiltration should be combined with technical flood protection infrastructure to ensure resilience against more extreme flood events. • Forest species composition change reduces peak discharge and runoff volume. • The effect decreases with increasing flood return period. • The effect strongly depends on antecedent catchment saturation. • Benefits are lower in winter. • Infiltration-based NBS require combination with grey infrastructure.