Isotopic response of run‐off to forest disturbance in small mountain catchments

Stable water isotopes were applied to trace hydrological processes in an undisturbed (mature spruce forest) and a nearby disturbed (deforested from a bark beetle outbreak) lake catchments in the Czech Republic. Both catchments are situated above 1,000 m a.s.l. within the Šumava National Park and have similar environmental conditions. The isotopic compositions of precipitation, creeks, springs, and lakes were sampled at 3‐week intervals over one hydrological year. Water inputs to catchments were derived from isotopically similar local precipitation, whereas run‐off was found to have different isotopic signatures. Creeks in the undisturbed catchment had ~1‰ and ~7‰ higher δ18O and δ2H with ~2‰ lower d‐excess than in the disturbed catchment. The d‐excess in creeks of the undisturbed catchment was more pronounced, particularly during snowmelt, and highly heterogeneous as compared with the disturbed catchment. Creeks in the undisturbed catchment were mainly fed by precipitation during the warm period (May–October), whereas creeks in the disturbed catchment were mostly fed by precipitation during the cold period (November–April). Estimated mean transit times of creeks and springs were ~6 months, except for two creeks in the undisturbed catchment, which had residence times of ~1 year. Although evaporation and transpiration fluxes were apparently reduced in the disturbed catchment, transpiration ratios were similar for both catchments. The difference in isotope signatures between catchments was attributed to the altered role of the forest canopy in temporal water distribution, which produced changes in the water cycle, potentially influencing important biogeochemical processes.