MW-01-071

Hydrologic Impacts of Forest Harvesting on Water Yield and Peak Flows
Sponsor: 
Millar Western Forest Products Ltd.
Lead Researcher: 
Watertight Solutions, Edmonton, Alberta
Body: 
The purpose of this project was to evaluate the impact of forest harvesting on water flows in Millar Western Forest Products Forest Management Agreement (FMA) area.

Hydrologic simulations of forest harvesting effects on water flows were carried out on three watersheds within the FMA, ranging in size from 73 to 221 km2. Three timber harvesting scenarios were evaluated in the simulations. The “business as usual” (BAU) scenario represented current harvest planning. The second scenario, “enhanced timber production” (ETP), maximized fiber recovery. The third scenario, “balanced silviculture intensity” (BSI), represented the balance between timber production and its impact on the forest. Harvest scheduling was carried out using ComPlan and hydrologic simulations using the WRENSS procedure.

Results of the simulations demonstrated annual water yield increases of 11% to 26% following harvesting. Maximum annual increases coincided or lagged slightly behind the period of maximum harvesting. The water yield increases declined steadily following the end of harvesting. The water yield increases in the BAU and BSI scenario simulations were smaller than those in the ETP scenario simulations, however the simulated increases in all three scenarios’ peak flows following harvesting were small as well as the estimated frequency of the flow events after harvesting. The Water Management Division of Alberta Environment Protection suggests that a 15% increase in annual water yield will not produce any significant changes in peak flow magnitudes or volumes. While the guideline provides a benchmark, there is need for more rational and effective guidelines or indices to gauge the impacts of forest harvesting on water yield and peak flow.