[Community Watershed Guidebook Table of Contents]
In community watersheds, maintaining water quality and quantity and the timing of flow are the primary management objectives. Harvest scheduling, including rates of cut, cutblock size and green-up, should meet those objectives.
Water quality, quantity and timing of flow in a community watershed are affected by harvest scheduling in a number of ways.
- Forests intercept rainfall, particularly at low or moderate rainfall intensities. The larger the clearcut area, the more rainfall may produce runoff and erosion.
- Clearcuts accelerate the rate of snowmelt and hence increase the runoff rate. Large clearcuts can increase peak flow to streams and may accelerate surface erosion, landslides and channel-change processes. All these have significant short- and long-term impacts on water quality.
- Where harvesting covers an extensive area, there is greater potential for flow over skid trails, roads and compacted areas, and for soil disturbance. Overland flow can increase surface erosion.
Requirements:
- Cutblock design, including size, shape and pattern must be consistent with the objectives for an area contained in any higher level plan. Where a community watershed is designated as part of a landscape unit, the objectives for that area can include specifications for cutblock size and green-up requirements that are more stringent than those in the Operational Planning Regulation.
- In the absence of a higher level plan, the district manager may require a smaller maximum cutblock size than what is specified in the Operational Planning Regulation where he is of the opinion that it is necessary for hydrological reasons. The district manager may also specify taller green-up requirements before harvesting a contiguous block, exceeding the 3 m specified in the Operational Planning Regulation, where he determines that the area does not support a stand sufficient in height to meet hydrological concerns.
Equivalent clearcut area (ECA) describes a second-growth block in terms of its hydrological equivalent as a clearcut. As second growth develops, the hydrological impact on a site is reduced. The rate of reduction is expressed in proportion to the height of the second growth (Figure 3). For example, a 20 ha block with 6 m tree heights is 50 per cent recovered so the ECA of the block is 10 ha (20 ha x 50 per cent). On average, a stand must be at least 9 m tall before it can be considered 90 per cent hydrologically recovered (Figure 3). The ECA is used in calculating the peak flow index.
Peak flow index describes the risk of a change in peak flows for an entire watershed. It is calculated as part of the watershed assessment procedure (WAP), one component of the peak flow hazard. All the cutblocks in a watershed are tabulated with their ECAs. A weighting factor then is applied, based on the elevation band of the cutblock. All the weighted ECAs are added to arrive at the peak flow index for the watershed. This number is further increased depending on the amount of road in the watershed.
In summary, the ECA is a measure of the site hazard for peak flows in a cutblock. The peak flow index is a measure of the watershed hazard for peak flows. A stand that is 9 m tall is 90 per cent recovered – the maximum that a second growth stand will attain in a normal rotation (Figure 3).
Figure 3. Per cent hydrologic recovery as average stand height increases.
The following steps can be taken to determine what rate of cut and cutblock size are appropriate within a community watershed.
The rate of cut in community watersheds must always be within the limits indicated by the watershed assessment procedure (WAP). This procedure assesses the past rate of harvest and roadbuilding in a watershed and the associated risks to water quality and quantity. Excessive past harvest rates may constrain plans for future harvest.
The WAP determines a peak flow index limit for each watershed. This limit is a function of the unique area-to-elevation distribution, snowpack distribution, mass wasting hazard and channel stability of a watershed. As a result, no one number can be specified. Instead, a peak flow index should be calculated for each watershed. From that index comes the rate of cut limits for the watershed. An example of this calculation is presented for a fictitious watershed in Figure 4.
Figure 4. Calculating the peak flow index for a 1000 ha watershed in the interior.
The H60 line is the contour elevation above which is 60 per cent of the watershed area. Consult the Interior Watershed Assessment Procedure or Coastal Watershed Assessment Procedure Guidebooks to interpret peak flow index scores.
In order not to exceed the maximum peak flow index, the total area of new clearcuts and roads in the forest development plan must be measured and assessed through the watershed assessment procedure.
High peak flows in small tributary sub-basins can be controlled by limiting the amount of recent clearcut area in the tributary watershed. The maximum equivalent clearcut area (ECA) in any drainage basin larger than 250 ha, that has not had a separate WAP completed, should not exceed 30 per cent of the area. If a separate WAP has been completed on the sub-basin, then the results of the WAP override this guideline.
Maintaining a cut level in the small sub-basins that does not exceed five per cent in five years normally ensures that these ECA levels are not exceeded.
This guideline limits the amount of run off and groundwater flow to any high or very high erosion hazard area or any terrain stability class IV or V area. If cutblocks are proposed within the drainage area contributing to sensitive sites, the ECA must be calculated for the catchment area upslope of the sensitive site. See Figure 5 as an example of how to define the contributing drainage area above a sensitive site.
Figure 5. Measuring the contributing source area above an unstable slope. Numerals refer to terrain stability classes.
Figure 6. ECA limitations in a community watershed. A WAP 1 is conducted on the entire watershed above the POD. WAP 2 and WAP 3 are conducted on the sub-basins one stream order less than WAP 1. Any lower order sub-basins greater than 250 ha have an ECA restriction of 30 per cent. Slopes above sensitive (class V) terrain have an ECA restriction of 20 per cent.
- Where silviculturally appropriate, tree species and site hydrological objectives are best met by small clearcut, small openings or selection harvesting systems.
- The average cutblock size for clearcut, seedtree or shelterwood systems in a community watershed should be less than 20 ha, with a maximum individual cutblock size of 40 ha. In the ESSF biogeoclimatic subzone, the average cutblock size should be less than 10 ha, with a maximum individual cutblock size of 20 ha.
- Link cutblock size to the soil erosion hazard. Use the soil erosion hazard map, which must be produced for all community watersheds, to determine the maximum cutblock area that can be harvested for each soil erosion class in one cutblock. See the following table.
| Surface soil erosion hazard | Maximum cutblock area (ha) |
| Low or very low | 40 |
| Medium (cable or aerial harvesting) | 20 |
| Medium (ground-based harvesting) | 10 |
| High (cable or aerial harvesting) | 5 |
| High (ground-based harvesting) | partial cut harvesting only |
| Very high | no harvesting |
- Cutblocks with two or more soil erosion classes should be controlled by the most limiting guideline. For example, a proposed cable-harvested 25 ha cutblock with 15 ha on a high surface erosion hazard site and 10 ha on medium erosion hazard site should be reduced so the high hazard area is no more than 5 ha, for a total cutblock size of 15 ha.
- There is no limit on partial harvest cutblock size provided that at least 40 per cent of the original basal area is retained.
- If more than 40 per cent of the basal area is harvested, the site will not be a partial harvest cutblock. Remaining stands should have at least 40 per cent crown closure and codominant tree heights of at least 9 m.
- Under the Forest Practices Code, all blocks must meet a three-metre green-up criterion. There are no additional adjacent block green-up requirements in community watersheds. Hydrological recovery is managed by ECA limits instead (which can translate into enhanced green-up requirements).
- Adjacent block green-up requirements would produce a highly dispersed harvesting pattern with an extensive active road network. This can worsen water turbidity. Equivalent clearcut area limits allow concentrations of harvesting, within specified limits.
- Cutblocks should be concentrated in sub-basins of watersheds to minimize the extent of roads.
- As much as possible, distribute cutblocks evenly by elevation band to minimize changes to the timing of peak flow. Within each elevation band, however, it is better to concentrate harvesting in a smaller area than to disperse it over the watershed.
- Recent cutblocks should not be concentrated along streams. Also try to reduce livestock concentrations around streams by not concentrating cutblocks next to mainstem streams.
- In interior watersheds, extensive high-elevation logging in a short period should be avoided. High and mid-elevation sites are the most significant snowmelt-producing areas. Peak flow timing and quantity may be significantly changed if a large area is in young seral stages or clearcut.
- In coastal watersheds, concentration of harvesting in the "rain-on-snow" zone (500–800 m elevation band) over a short period of time should be avoided.
