ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

S treamf 1 ow
A &foot H-flume was installed immediately below the study area to provide general
streamflow pattern and baseline hydrograph data for Abbott Basin on Coram Experimental
Forest. The flume began operating at the beginning of the 1975 water year (October 1,
1974). This coincides with the completion of most of the treatments ; thus no preharves t
streamflow data was generated. The nature of the watershed did not permit differentia-
tion of treatment effects, so effects, if detectable, would reflect only, a composite
of all treatments. Streamflow hydrograph data (appendix A) and general watershed
parameters (appendix B) are presented as general reference material . These hydrographs
are similar to some other northwestern Montana hydrographs (Montana DNR 1978).
RESULTS AND DISCUSSION
These results i 11 us trate how si 1 vi cul ture and residues treatments di fferenti a1 ly
affect precipitation, soil water content, and amount and periodicity of water used
during the growing season. Additional axillary relationships are also presented.
Precipitation
Annual precipitation on the immediate study site averaged about 5 percent below
normal during the period of study compared to the 30-year average annual precipitation
of 25.3 inches (64.3 cm) (USDA, SCS 1977). Precipitation averaged 24 inches (61.0 cm)
(for both calendar year and water year), but varied annually between 29 inches (73.7
cm) during 1976, and 23 inches (58.4 cm) during 1977 (water year October 1 to September
30). About 50 percent of the total precipitation came in the form of snow.
SILVICULTURE TREATMENTS
Harvesting under the three silviculture treatments differentially reduced the
density of the overwood, and consequently i ncreased the amount of preci pi tation
that reached the forest floor. However, these differences were also a function
of storm size (intensity and duration) and elevation.
Rain
Precipitation reaching the forest floor was greatest on clearcuts and group
selections, intermediate on shel terwoods , and least in the uncut mature forest
(table 1). This, however, was directly related to rainfall per 24-hour period.
As shown in table 1 and appendices C and D, storms that produced only 0.1 inch
(0.25 cm) in the clearcut openings in a 24-hour period, resulted in only about one-
third of it reaching the forest floor in the uncut mature forest and about three-
fourths in the shel terwood. However, if the storm exceeded 0.5 i nch ( 1.27 cm) ,
two-thirds or more reached the forest floor in the mature forest and nearly 90 percent
reached the floor in the shel terwoods. As indicated in table 1 and appendix D, the
amount of interception "losses" was directly related to the standing tree vol urne.
The prediction equations (precipitation in 0 .Ol-i nch i0.03-cm] increments) describing
this relationship are:
Rain reaching forest floor
per 24-hour period in mature forest
Rain reaching forest floor
per 24-hour period in shel terwood
= -0.03826 + 0.75019 (rainfall in clearcut)
= -0.01709 + 0.91103 (rainfall in clearcut)