Snow melts from a watershed in a predictable pattern. Melt begins earlier in the season at lower elevations and proceeds upslope. Snow has generally disappeared from the lower elevations some time before the spring stream flows peak. During peak flow, snow is beginning to disappear from the mid-elevations and is actively melting at the higher elevations of a watershed.
After an area has been harvested, both winter snow accumulation and spring melt rates increase. This effect is less important at lower elevations, since the snow disappears before peak flow. At mid-elevations, the additional melt may or may not be important, depending on seasonal variations. Harvesting at high elevations will have the greatest impact and is, therefore, of most concern.
The changes in snow accumulation and melt brought about by forest harvesting are reduced as new forests grow. This is commonly referred to as hydrologic recovery.
The degree of canopy closure is determined by tree species, height, and stocking density. Since tree height data is readily available and is closely correlated with canopy closure, it is the variable used to evaluate hydrologic recovery.
The first approximation of hydrologic recovery (Table 8-1) for the southern interior is based on theoretical estimates of the effects of canopy closure on radiation penetration and snow interception, stand growth curves relating tree height and canopy closure, and snow data from studies in the Okanagan and Kootenays. The recovery estimates apply to fully stocked stands that reach a maximum crown closure of 50-70% and height of 20-30 m when mature. The growth curves used to convert crown closure to tree height assume a stand density of 1500 stems per hectare when the main canopy is 3 m in height. Tree heights refer to the average height of the main canopy (that is, co-dominant and intermediate trees, not dominant and suppressed stems).
Table 8-1. First approximation of snow recovery in the southern interior for fully stocked stands in the snow zone that reach a maximum crown closure of 50-70%
Low flows in summer or winter can harm fish populations by reducing the amount of available habitat. During the summer, this is exacerbated by the added stress of higher oxygen needs of fish and lower dissolved oxygen concentrations when the water is warmer. During the winter, low flows cause less oxygen stress, but overwintering eggs can be damaged by freezing or ice movement.
Both summer and winter low flows result from long periods during which the water being discharged from soils and bedrock is not replenished by rain or snowmelt. Trees affect low flows by intercepting rain and snow, by reducing the amount of water entering the soil and, through transpiration, by removing water from the soil.
Transpiration, however, is directly related to moisture availability. Consider what happens in a clearcut under different conditions. During a wet summer, interception loss in a clearcut is low, resulting in more water entering the soil than would occur under a forest canopy. In addition, the water that would have been transpired from the soil by trees is available for groundwater recharge and stream flow. As a result, under wet conditions, the summertime low flow after clearcutting is greater than the low flow that would have occurred in the forest.
In contrast, during a summer without rain, water input to the soil is zero regardless of whether the site is forested or not. Transpiration losses in the clearcut would probably be less than in the forest, but the forested site would have very low transpiration losses anyway. Consequently, stream flow from both sites would be very low and clearcutting would have little effect on the water balance.
There is a general public perception that clearcutting dries out soils. This is probably because the top layers of soil do, in fact, become drier upon exposure to stronger sunlight and wind. However, the deeper soil layers in the rooting zone of trees have been shown to have higher moisture content after clearcutting. The net effect is that total soil moisture tends to increase after clearcutting. This effect diminishes as a site becomes revegetated until there is no detectable difference within 10 to 15 years after logging.
Low flows may occasionally also be observed to decrease as a result of channel aggradation. In some cases, water continues to be discharged from a basin. However, it moves below the surface through the stream bed where channel aggradation has occurred.
Watershed studies have shown that tree removal tends to result in increasing mean monthly flows in August, September, and October by a moderate amount during the 10- to 15-year revegetation period. This is probably beneficial in cases where water can be impounded for human use or for delayed release downstream. However, in most cases, there may be no benefit to fish, since the very lowest flows are not increased by harvesting.
In summary, timber harvesting appears to have a negligible, or slightly positive, effect on summer low flows in most cases. Winter low flows are probably not affected by forestry activities.
In the Alberta Rockies and the interior of British Columbia, research has also shown increases in water yield after timber removal. In an Alberta study, harvesting 50% of the forested area resulted in a water yield increase of 27%, or 40 mm. In a paired watershed study in British Columbia’s southern interior, clearcutting 30% of a watershed resulted in a 21% increase in yield.
The 1973 Eden fire near Salmon Arm burned 50% of a watershed and caused a 24% increase in the April to August runoff. The effects of this fire on water yield are assumed to be similar to those that would result from timber harvesting.
One difference between the studies in the U.S. and the ones in western Canada is that most runoff in the British Columbia interior and Alberta Rockies occurs during spring snowmelt. Because of the snow-dominated regime in these areas, tree removal effects on the annual water balance are not limited to changes in evapotranspiration, but include increases in snow accumulation and spring discharge levels.
In summary, timber harvesting can be expected to produce the largest increases in water yield in areas that have an ample supply of moisture during the growing season. In areas where runoff is dominated by snowmelt, a large part of the annual yield increase can be associated with increased snow storage in openings, faster snowmelt, and thus an increase in spring runoff volume.
The following assumptions can be made for the ECA calculations:
NSR (not sufficiently restocked): - clearcut with 0% recovery
Partial cutting:
<30% basal area removal - expect 100% recovery
30-60% basal area removal - clearcut x 0.5
60% basal area removal - clearcut with 0% recovery
clusters of trees - apply appropriate recovery to area occupied by clusters
Private land[2]:
<15% of total sub-basin area - exclude from total sub-basin area (Form 1) and ECA calculations (Form 2)
>15% of total sub-basin area - include in total sub-basin area and ECA
Cultivated land: - same as for private land
Open range: - include in total sub-basin area (Form 1) but exclude from ECA calculations (Form 2)
Burn sites: - clearcut with 0% recovery; extrapolate if regeneration
Large slides: - clearcut with 0% recovery
Hydro line: - clearcut with 0% recovery
Tally all opening information (as shown in Table 8-2) and summarize it in columns A and D in Form 2.
Example ECA calculation:
Q: What is the ECA for a 0.85 km2 fully stocked stand with an average canopy height of 4 m?
A: ECA = area of opening x (1 - appropriate percent hydrological recovery)
ECA = 0.85 km2 x (1 - 0.25)
ECA = 0.64 km2
ECA below the H60 line total sub-basin area (column B): Divide the value obtained for ECA below the H60 line (column A) by the area of the entire sub-basin.
Weighted ECA below the H60 line (column C): After an area has been harvested, both winter snow accumulation and spring melt rates increase. This effect is less important at lower elevations, since the snow disappears before peak flow. Directly transfer results from column B into column C (ECA weighting is equal to 1).
ECA above the H60 line (column D): To estimate this value, determine the height of regeneration in each polygon above the H60 line on the 1:20 000 forest cover map. Heights may need to be extrapolated if reference material is not up-to-date. The area of each opening will then have to be reduced by the appropriate percent hydrological recovery (see Table 8-1 and ECA assumptions listed above). Tally all opening information and summarize in Form 2.
ECA above the H60 line total sub-basin area (column E): Divide the value obtained for ECA above the H60 line (column A) by the area of the entire sub-basin.
Weighted ECA above the H60 line (column E): During peak flow, snow is beginning to disappear from the mid-elevations and is actively melting at the higher elevations of a watershed. Therefore, harvesting at high elevations will have the greatest impact and is, hence, of greater concern than at lower elevations. This value can be obtained by multiplying column E by an ECA weighting factor of 1.5.
Peak flow index (Indicator #1): The peak flow index is derived from estimates of the area which are equivalent to clearcut (ECA). Add the weighted ECAs from column C and column F to obtain the peak flow index in Indicator #1.
Table 8-2. ECA tally sheet
Use this sheet to tally individual results for column A and column D in Form 2.