WMOD00312 (2) - Laserfiche WebLink

Home Browse Search

EXECUTIVE SUMMARY This feasibility study was conducted to examine the meteorological feasibility and potential cost effectiveness of increasing runoff into reservoirs in the Shasta and Trinity Watersheds through application of winter orographic cloud seeding technology. A primary goal was to combine results from past seeding experiments with climatological data analyses for these watersheds to quantify the range of suitable seeding conditions available for low, average and high precipitation years. Additional goals were to d,evelop a general design for an operational seeding program incorporating sufficient measurements to document long-term results, and to estimate the probable benefit-cost ratios for the conduct of the program. A brief discussion is given regarding seeding of winter orographic clouds. These clouds form when moisture laden air is forced up and over a mountain barrier. When temperatures are below freezing, small water droplets may exist as liquid at temperatures below freezing. These droplets are known as SLW (supercooled liquid water). Successful seeding involves conversion of a portion of these drops to ice crystals. Given appropriate conditions, these ice crystals can grow and settle to the ground, thus augmenting the snowpack. However, for this augmentation to occur, several critical links in a chain of physical events must occur. First, suitable clouds must be present where SLW exceeds what is being naturally converted to ice. Second, seeding material must be reliably produced and transported in an adequate concentration to the desired area of the cloud at an appropriate temperature. Finally, the resulting crystals must remain in a suitable growth environment for sufficient time to grow and fall prior to being carried beyond the mountain barrier to a region where they may evaporate or sublimate as the cloud descends and warms. The considerable body of experimental seeding data collected during the SCPP (Sierra Cooperative Pilot Project) is extensively reviewed. The results of this project, operated by the Bureau of Reclamation in the American River Basin of north em Califomia between 1976 and 1987, greatly improved understanding of winter mountain clouds over the Sierra Nevada and provided much of the basis for formulating a seeding strategy for the Shasta-Trinity Watersheds. Extensive measurements of SLW in the SCPP have produced a consistent and encouraging pattem. SLW, although highly variabl,e, is observed during many Sierra winter storms. Several hundred hours of SLW per season can be anticipated, both in wet and dry years. This SLW generally occurs during light precipitation periods within a storm, ~nd when clouds are shallow. The SLW is concentrated within the first kilometer above the highest terrain, in low concentrations, and at relatively warm temperatures. These findings document the existence of significant seeding potential and provide a basis for estimating SLW hours in the Shasta-Trinity area. Seeding experiments were conducted using a variety of agents and dispensing techniques during the SCPP. Tests results for ground and aircraft seeding trials, during which extensive physical measurements were made in the intended target area, are summarized. Proper targeting of ground-released agents such as silver iodide (AgI) was difficult to achieve, and cloud temperatures were often too warm for the nucleating agent to be effective. Ground seeding using AgI is estimated to be appropriate for only about 20 pet of westem Sierra storms. Aircraft seeding trials indicated that a single aircraft could not reliably dispense sufficient seeding agent to affect a large cloud volume, but offered the potential of better targeting at more appropriate cloud temperatures. Results of SCPP ground-based physical measurements from both the ground and aircraft seeding trials are used to estimate the probable magnitude of seeding effects possible for an operational program. IX