By Rita Winkler and Sarah Boon
Introduction: Forest disturbance, whether natural or as a result of timber harvesting, directly affects stand-scale hydrologic processes through changes in interception, evaporation, and transpiration. When disturbance occurs over a large enough area, hydrogeomorphic processes at the watershed scale can also be affected. In British Columbia, statistically significant shifts in the timing and magnitude of snowmelt- dominated streamflows and in the frequency of peak flow events of all magnitudes have been measured in watersheds where more than 5% of the area has been clearcut. Streamflow regimes may also be affected following less extensive changes in forest cover where disturbance location and watershed attributes synchronize melt runoff timing and delivery to stream channels. These increases may have subsequent downstream effects on channel morphology, aquatic habitat, alluvial fans, floodplains, infrastructure, and community water supplies.
The potential effects of forest disturbance on streamflow are often evaluated by examining the total area disturbed and the location(s) in a watershed where forest cover has been (or will be) altered. The assumption is that the greater the disturbed area, the greater the potential for hydrologic change. It is also assumed that these changes will diminish over time as the forest regrows (i.e., recovers). The extent of disturbance, accounting for regrowth, is referred to as the equivalent clearcut area (eca). This note describes eca, including its origin, development, and use, how it is calculated, and its applicability to forest development planning and watershed assessment. This note focusses on eca in snow-dominated interior British Columbia watersheds where spring peak flows are a key hydrologic concern. However, the discussion applies wherever eca is used, although the methods of calculation and seasons considered may vary.