Infiltration Rates of Various Vegetative Communities within the Blue Mountains of Oregon

Mean infiltration rates differed among several natural vegetation communities with ponderosa pine (Pinusponderosa) exhibiting the lowest mean infiltration rate of 6.0 cm/hr and larch (Larix occidentalis) demonstrating the highest at 8.8 cm/hr. A trend toward increasing infiitration rates corresponded to increasingly mesic sites. Alpine, Douglas fir (Psuedotsuga me&e@), mountain meadow, and larch types demonstrated the greatest vegetative cover, occupied the most mesic sites, and exhibited the highest infiltration rates. Infiltration differences within vegetative communities,based upon changes in condition and productivity were also noted. The forested sites were more dependent upon condition class than productivity class, with higher infiltration rates being exhibited on pole sites than on timbered sites, apparently in response to higher plant densities associated with the pole thickets. Nonforested sites were responsive to both productivity and condition class with higher infiltration rates being exhibited on these sites with the more productive or better condition classifications. Water is a primary factor limiting production on many western rangelands. Many of these areas are subject to low annual precipitation and high potential evaporational losses. Activities which disturb the soil surface or vegetative composition and cover have the potential for reducing soil water intake, thereby reducing productivity which in some instances may be minimal at best. A prime concern of land managers is to maintain or enhance those factors within managerial capabilities which affect soil water intakeand to identify those areas most susceptible to disturbance. Blackburn and Skau (1974) studied infiltration rates and sediment production fo 29 plant communities and soils in central and eastern Nevada. The highest infiltration rates occurred on sites with weliaggregated surface soils free of vesicular porosity. Williams et al. (1972) used multiple regression analysis to determine relationships between vegetative and soil factors and infiltration rates and erosion from 550 infiltrometer plots at chained pinyon-juniper sites in Utah. Factors found to be most important in predicting infiltration rates were: (1) total porosity in the O-8 cm layer of soil, (2) percent bare soil surface, (3) soil texture in the O-8 cm layer of soil, and (4) crown cover. Raindrop impact on bare soil tends to rapidly close the natural channels of percolation by degrading soil structure. Studies at the Manti County watershed in Utah showed that runoff varied inversely with the total amount of ground cover (Orr 1957). Vegetative cover tends to reduce the energy of rainfall by reducing velocity and by breaking the large drops into a fine spray which can then enter the soil without damage to the soil surface. Gifford (1972) reported that the ability to predict infiltration rates, using cover characteristics alone varies with time, both within a given storm The authors are teacher, Monument Elementary School, Monument. Ore., and associate professor, Oregon State University, Corvallis, Ore., respectively. This report was submitted as Technical Paper No. 5884. Oregon Agricultural Experiment Station. The work upon which this report is based wassupported byfunds provided by the USDA-FS, Range Evaluation Project. Theauthors wish to thank Jon Skovlin and Reed Sanderson for the cooperation and help which they extended at all stages of this effort. Manuscript received June 8, 1981. event and on a seasonal basis. He further stated that measured cover characteristics may helpexplain hydrologic behavior of a site at one time, yet be of little value at another time. The objective of this study was to determine and compare infiltration rates within and among 10 different ecosystems found in the Oregon Range Evaluation Project Work Area.