Forest Investment Account

Abstract of FIA Project Y051293

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Hydrologic decision making tools for sustainable forest management in rain dominated Coastal BC watersheds

Author(s): Alila, Younes
Imprint: Vancouver, B.C. : University of British Columbia, 2005
Subject: Forest Investment Account (FIA), Hydrology, Forest, British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program


A physically-based distributed parameter model was used to investigate the impacts of forest management on streamflow from the rainfall-dominated 9.8 km2 Carnation Creek watershed located on the west coast of Vancouver Island, British Columbia. The model used accounts explicitly for the effects of vegetation upon runoff production and is also unique in its explicit portrayal of preferential subsurface runoff processes common to forested watersheds. Flood frequency analysis was used to quantify the impact upon the peak discharge regime as a result of both forest removal and road construction. The relative change in peak discharge is scale invariant and ranges from -4 to 30% for cut rates ranging from 10 to 100%, drainage areas ranging from 0.88 to 9.8 km2 and return periods of 0.17 to 20 years. The relative change in peak discharge is strongly correlated to both rate of cut and return period, increasing with increasing rate of cut and decreasing return period. However, statistically significant (alpha = 0.05) increases in peak discharge are restricted to cut rates greater than 30% and return periods less than 2 years. The effects of roads alone is to generate relative increases in peak discharge that range from -2 to 11%, an effect that is analogous to changes attributed to forest harvesting at cut rates ranging from 10 to 40%, depending upon return period. Relative changes in peak discharge increase with decreasing return period; however, changes are statistically insignificant (alpha = 0.05) at all return periods. The combined effect of roads and forest harvesting is to increase peak discharge more so than would have occurred with either forest harvesting or roads in isolation. However, the effect of roads and forest harvesting is only additive at the lowest return period (0.17 years); the combined effect is increasingly less than additive with increasing return period. Changes to peak discharge due to forest removal at return periods of 0.17 years and less are related to increased runoff efficiency arising from higher soil moisture whereas impacts at return periods higher that 0.17 years are affected by decreasing rainfall interception efficiency with increasing storm size. The weakening response of peak flow change to roads with increasing return period is linked to the higher subsurface flow rates associated with preferential runoff and the upper limit on the water table response to precipitation once preferential flow is activated.

For further information, please contact Younes Alila, University of British Columbia (

Updated September 08, 2005 

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