WMOD00593 - Laserfiche WebLink

Home Browse Search

The second set of potential impacts derives from the changes in snowfall and snowpack produced by c+oud seeding. Artificially induced increases in snow would be imposed upon seasonal snowpacks which are normally highly variable. Snowpack augmentation could act to reduce this variability, especially in years of low snowfall, and would increase the average snow accumulation. An artificial increase in snow- fall would not be uniformly distribu~ed, but, like the natural snowpack, would be redistributed by wind drifting, especially in alpine areas. Where snow- drifts persist into the late summer (1 percent of the alpine area and 0.05 percent of the total area of the San Juan Mountains), an increase in the snowpack of more than four time~ the average is possible. All increases would be reflected in delayed snowmelt, reduced soil temperatures, increased soil moisture, and additional snow loads. It is through these kinds of environmental changes that ecological impacts are to be anticipated. On the average, a 30 percent increase in snowfall is equal to 16 cm w.e. on the peak accumulation at 3500 m, the approximate elevation of treeline in the San Juan Mountains. Such a change in snow depth could delay spring snowmelt by 8 days on sites which normzlly clear of snow in May and by 4 days when clearance occurs in July. Where snow redistribution occurs~ the effects would be even greater but the delay is unlikely to exceed 15 days on more than 5 percent of the alpine area (0.25 per- cent of the total area). In 1973, following a winter with 43 percent above-normal snow accumulation, observed delays in snowmelt on our study sites in both forest and tundra areas varied between 2 and 8 weeks compared to melt dates in the previous 2 years, both of which were years of below-normal snowfall. In 1973 and 1975 the snow cover survived into the following winter. In alpine situations, equivalent delays have been produced experimentally in winters of below-normal snowpack by the use of snow fences. The microclimatic effects of an increased snow cover have not been studied in detail in this project. However, microclimatic observations made as part of SJEP studies of mountain vegetation suggest four in- fluences of the microclimate where snowmelt occurs before mid-July. 1) A late snowmelt delays soil warming. Warm- ing of the uppermost 10 cm of the soil to ambient temperatures occurs within 2 or 3 days after snowmelt but at depths of about 100 cm, requires up to 20 days. No longer term cooling of the soil has been observed. 2) Additional snow accumuation gives an in- creased soil moisture flux, and the return to "normal" levels is up to 12 days later than it is with 'soil temperature at 10 cm depth. 3) The effects of air temperature and humidity are slight because of advection, but in forest stands where advection is reduced, such effects are relatively more important than on the alpine tundra. 4) While the ground remains snow covered, the available energy or net radiation (R ) at the ground surface is much reduced dne to the reflection of short-wave radiation, but no influence on R is retained after snow- melt. n No microclimatic observations have been made on sites from which the snow clears in late summer, when solar radiation is reduced~ and it is not known whether these conclusions can be extrapolated to those areas. Erosion and sedimentation have been studied in de- tail in the alpine areas of the San Juan Mountains. Less detailed observations made elsewhere allow some estimates of possible effects at lower elevations. In general, an increase in snowfall of about 30 per- cent will influence erosion and sedimentation only as a second order effect, i.e. through prior changes in vegetation and/or fauna, and through more signifi- cant topographic and soil controls. An important exception to this rule is the sediment-transporting work of' avalanches and streams. Avalanches directly influence erosion by transporting debris to lower elevations and to drainage channels, and so are included in Figure 1 (below) as a snow loading influence. In the San Juan Mountains, avalanches appear to be more important geomorphic agents below treeline rather than above it. This is because the ground surface at lower elevations is more likely to be exposed in avalanche tracks when the wet snow avalanches of spring occur. Supporting this conclusion is the fact that most avalanche- related erosion seems to occur in winters of low snowfall, i.e., when larger areas Qf soil are exposed. In this case, snowpack augmentation seems likely to reduce avalanche erosion by adding to the snow cover during years of below-normal snowfall, even if avalanche occurrence is increased. Empirical evidence suggests an increase in the rates of rock weathering, nutrient release, and plant litter decomposition in response to increased water movement. In concert with the acceleration of these processes, solute transport in stream flow is ex- pected to increase by about the magnitude of the change in snowfall or streamflow with no deterioration in chemical water quality. The impact on clastic sediment transport will also be slight, at least at h",gher elevations where there is little evidence of channel or bank erosion at the present time. Indirectly, a slight general increase in sediment yield from upland source areas is to be expected from an increase in snowfall, although this effect may require decades to develop (Table 1, below). The more immediate response may be a reduction in sedi- ment delivery rates as a more extensive snow cover protects bare soil areas from summer rainstorms. The longer term increase in erosion in the alpine is contingent on a change in the vegetation cover around snowdrifts, and in the population size and distribution pattern of burrowing animals. Such changes involving the surface soil may give a mechanism for mechanically concentrating silver iodide into sedimentation areas. Such a mechanism has not been studied in the project. SEEDING AGENT At present, silver iodide is the most commonly used nucleating agent in cloud seeding and has the poten- tial, particularly through the silver ion, to impact the ecosystem independently of any additional pre- cipitation it induces. In the San Juan Ecology Project, studies of silver have centered on its disposition in the ecosystem and its toxicity to organisms. '8