Peak flow regime changes following forest harvesting in a snow-dominated basin: Effects of harvest area, elevation, and channel connectivity

Key Points There is a strong relation between forest harvesting area and the flood regime Effects of forest harvesting on floods is related to connectivity to streams Effect of harvesting on flood magnitude increases with return period of flood Numerical modeling in conjunction with a stochastic weather generator was used to investigate the immediate impact of forest harvesting upon the annual‐maximum peak discharge regime of 240 Creek, a snow‐dominated headwater basin of low relief located in south‐central British Columbia (BC), Canada. Harvesting effects were simulated using 11 hypothetical harvest scenarios that specifically assess the impact of clear‐cut harvesting in the absence of roads. Distribution statistics show that the annual‐maximum peak flow frequency curve for hourly discharge is affected both by harvest area, AH, and by harvest elevation, ZH. Annual peak discharge magnitude tends to increase with increasing AH, but decreasing ZH. Forest harvesting does not have a statistically significant (α = 0.05) impact on the peak flow distribution until AH ≥ 20% to 30%, depending on the statistical comparison used. There is no substantial elevation gradient in snow accumulation within 240 Creek, and the sensitivity of the hourly peak discharge regime to ZH is instead related to the spatial variation of channel density with elevation. Flood frequency analysis was used to directly compare control and treatment events of equal frequency of occurrence, using return periods, T, of 1.003–100 years. This analysis indicates that, contrary to the prevalent hydrological wisdom and regardless of AH and ZH, the relative increase in peak discharge quantiles (ΔQT) increases with increasing event magnitude (increasing T) for events equal to or larger than the median (T = 2 years) event. Overall, ΔQT ranges from 7% for AH = 30% to 84% for AH = 100%. The magnitude of peak flow change is found to be a function of changes in both net 48‐h input fluxes (rainfall plus snowmelt minus evapotranspiration) and changes in snowmelt runoff synchronization, which directly affect treatment peak discharge magnitude and timing.