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ASPECTS OF PRECIPITATION ENHANCEMENT 29 Two biological communities, subalpine meadow vegetation and mountain hemlock stands, were identified and studied as candidate sys- tems that could change as a result of increased snowpack. These commu- nities were thought to exist in microenvironments that received substantially more water than indicated by regional precipitation amounts. Hemlocks, for instance, occupy siiteswith cool and moist north- ern exposures, cold air drainage, and abundant snow, often immediately below snow cornices. Meadows are often in drainage sumps that concen- trate moisture. Among the findings were the following: . Mountain hemlock stands would increase at the expense of other subalpine vegetation types if snowpacks increased over the long term. . Some types of hemlock stands would become increasingly less spe- cies rich and the size-structure patterns of hemlocks would be al- tered. . Snow cover duration was a principal control of subalpine meadow plant development, reproductive success, and existing vegetation patterns. . Long-term effects of snowpack increased by 30% every year might result in dramatic vegetation changes, but longer-term studies would be needed to substantiate this assertion. The hydrologic effect of an extended snowcover was investigated in a field experiment by sprinkling water on 1000 square meter plots in a small basin for 11 days after natural snowmelt ended (MacDonald 1986). Rises in ground water were observed, but once the simulated snowmelt ceased, piezometric pressures appeared to decline more rapidly than before. Other data supported the contention that increases in streamflow resulting from snowpack augmentation would not extend appreciably beyond the time of soil desaturation. Although the aim of snowpack augmentation projects is not to enhance rainfall, potential impacts from increased rainfall stood out in the SEP as the single most critical concern. In uncommon circumstances (e.g., in- creased rainfall on steep, disturbed, saturated soils), an increase in soil erosion by splash, sheet, or rill erosion is possible. Similarly, if snowpack augmentation resulted in a thin snow cover on ground that would other- wise have been bare, snowmelt during subsequent rainfall would lead to greater runoff than would have occurred naturally. Although these con- cerns are real ones, routine adherence to suspension criteria that guard against rain-induced flooding would reduce the likelihood of appreciable negative impacts. SEP recommendations included monitoring environmental situations anticipated to concentrate the effects of snowpack increases. Subalpine meadows and mountain hemlock stands were identified as candidate biotic communities. Snowbanks and spring/seep communities were also candidate index sites for long-term monitoring.