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3. SUMMARY OF THE SCPP RESULTS RELEVANT TO THE SHASTA-TRINITY WATERSHED 3.1 Introduction During the winters from 1976-77 through 1986-87 (excluding the 1980-81 winter), Reclamation sponsored the SCPP. Extensive physical measurements were conducted in winter mountain clouds over the ARB (American River Basin) to develop a cloud seeding technology for increasing precipitation on the watershed. SCPP involved the use of remote- sensing devices, in situ observations, and application of numerical models in conducting exploratory seeding trials. Reynolds and Dennis (1986), and Reynolds (1988) describe the SCPP and the major results obtained. Although funding was terminated before a complete analysis of the data or a final report of the SCPP could be completed, analyses performed provided some general conclusions relevant to this Feasibility Report. These results follow. 3.2 SLW Studies . SLW is necessary for seeding to affect precipitation. SCPP applied various methods for determining SLW in clouds over the ARB. Aircraft mounted liquid water probes provided the magnitude and areal distribution of SLW; however, these results were restricted in both space and time. Safety considerations eliminated aircraft sampling at low elevation near the mountain where extensive SLW water existed. Required pilot rest periods limited aircraft hours of operation. The highest liquid water concentrations were found by aircraft 30 to 65 kIn upwind from the crest in cumulus clouds forming in the post cold-frontal airmass (Heggli et al., 1983). Analysis of aircraft data suggests that some of this liquid water is used in the natural precipitation process prior to passing over the crest of the Sierra. A better method of sampling SLW is vertically integrating the SLW content using a ground- based platform as the air passes out of the watershed. A dual-channel microwave radiometer, capable of remotely integrating the amount of SLW throughout cloud depth, provides these measurements. The radiometer frequencies are specifically tuned to measure the emitted radiation from both water vapor and condensed liquid. The radiometer is positioned at elevations above the freezing level so that the liquid water measured is supercooled. Given these conditions, the radiometer provides continuous observations ofSLW throughout a storm instead of being confined to a single aircraft flight track. The radiometer normally operated during the SCPP 3-mo field program, January through March. With the exception of March 1983, when the radiometer was located at BLU (Blue Canyon) (1500 m), it operated about 5 kIn west of the crest at Kingvale (1750 m), figure 3.1. During the period March 1983 through early January 1987, about 12 winter months ofliquid . water data were collected near the crest of the Sierra Nevada. For this 12-mo period, 775 h of acceptable SLW were observed (radiometer data were excluded ifthe temperature at the observation site was above 0 OC). These observations were summarized by Heggli and Rauber (1988). The main conclusions were that SLW develops during the passage of large- scale weather features. The combination oflarge-scale vertical motions associated with a cold front, and the forced ascent of this moist air over the mountain barrier, causes condensation and SLW production. Periods of SLW production vary considerably depending on storm trajectories (west to east moving storms vs north to south moving storms) and position with respect to the cold front. In general, SLW was concentrated in the lowest kilometer aboveb 4