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OOU170 and opportunity recognition systems will be tailored to the specific terrain and meteorological conditions of the subbasins. a. Seeding system. - Alternative seeding agents that could be used in the demonstration program are dry ice and silver iodide, both of which can be released into clouds from aircraft. Silver iodide can also be dispersed from ground-based generators. Ground-based seeding is preferred because it is least costly, but terrain and atmospheric conditions are not always appropriate for its use. Airborne seeding is usually more effective in placing the seeding material where and when it is needed, but it is more costly and dispersion of the seeding material is harder to achieve. Dry ice is less costly than silver iodide, and it is more effective at rela- tively warm temperatures, but it must be injected into a cloud to be effective. All seeding modes and agents will be considered as they apply to each subbasin's terrain and storm conditions. An extensive plume tracing program will be conducted in each sub- basin to document the region of the atmosphere that can be affected by ground-released silver iodide and to optimize aircraft deliveries of seeding material. The plume tracing program will use airborne continuous ice nucleus and ice crystal counters to determine the T&D (transport and diffusion) of seeding material over the barrier. The airborne sampling will be supplemented by limited surface measure- ments where access permits. The T&D program will guide the placement of ground-based generators in each demonstration subbasin to ensure that they are generally well above stable layers or blocking flow to overcome the opera- tional problems encountered in previous programs. It will guide generator placement with respect to specific terrain features to use terrain-induced mechanical mixing to enhance the diffusion process. Further, the horizontal spacing between generators will be finalized by analysis of plume tracing data. It is important that plumes from adjoining generators intersect within a reasonable downwind distance to minimize "gaps" in seeding coverage. Additional generators may be needed downwind of the first "upwind line" for very broad barriers like the San Juan Mountains. Investigations into the rate of diffusion of airborne AgI releases have revealed very limited mixing rates that are terrain dependent in stable free atmospheres. Seeding aircraft may need to be posi- tioned 1 or more hours travel time upwind of the target barrier for sufficient diffusion. T&D work is needed at each terrain specific site with airborne releases, particularly to determine the extent to which mechanical mixing can enhance the diffusion process near the mountainous terrain of each subbasin. While site-specific studies need to be accomplished concerning the targeting of seeding material, a network of remote-controlled AgI generators, located high on the windward slopes of the mountain barriers, will be developed that provide an adequate means of treating many storms during CREST. The T&D program will optimize 29