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"-I l I I I I I Also, the seeding is usually conducted during only part of a storm episode. For example, if a storm produces precipitation over a six hour period, seeding may have ., occurred over only three of those hours. Therefore, over the period of a seeding program, seeding is likely to have occurred well under 100 percent of the time rainlsnow was falling. These and other limitations lead us to use monthly or seasonal average values in our analyses, which indicate overall increases which are less than what occurs in individual seeding cases. Thus a "dilution" of the seeding effect occurs. I I I , I I I I I I , I J 5.2 Evaluation of Snownack in the Tar2et Area The Natural Resources Conservation Service (NRCS) routinely measures the mountain snowpack once or twice per month, often starting in January and continuing until about June 1. Measurements were formerly made by visiting each site, taking a core sample of the snow, determining the depth of the snow and its water content. In more recent years (since about 1980) the advent ofNRCS's SNOTEL system has allowed frequent (daily) measurements of snow water content. By use of a sensing system called a snow pillow, the water equivalent of the snowpack can be determined by remotely monitoring the weight of the snow on the snow pillow. Snowpack water content is important since it ultimately determines how much water will be available to replenish the underground and surface water supplies when the snow melt occurs. Hydrologists routinely use snowpack water content to produce forecasts of streamflow during the ensuing spring and early summer months. Some problems with snowpack data must be recognized when using snow water content to evaluate seeding effectiveness. One problem is that not all winter storms are cold and sometimes rain as well as snow falls in the mountains. This can lead to a disparity between precipitation totals, which measure everything that falls, and snowpack 5-3