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1. INTRODUCT ION 1.1 Objective In 1959, the Senate Select Committee on National Water Resources was appointed to assess the nation's water supplies in relation to anticipated demand through the year 1980. The conterminous United States was pivided into twenty-two water resource regions, and the following five principal categories of water management studied: 1. Streamflow regulation 2. Improved water quality 3. Increased water use efficiency 4. Expanded use of underground storage 5. Expanding natural water yield by desalting, weather modification, and reduction of evaporation losses. The Select Committee report in .January 1961 found that five of the twenty-two water resource regions would be using all available water supplies by or before 1980. All five regions for which short wat4~r resources were projected are in the western United States. Subsequent to the Select Committee report, November 1961, the Congress of the United States authorized the Bureau of Reclamation to explore the potential of weather modification for increasing water resources in their continuing role of water resources management. During the past twelve years, the Bureau of Reclamation has sponsored many research programs to further the undl~rstanding of cloud physics, and how the preci- pitation process may be altered by artificial means. A major goal of this effort has been to de~ermine the"feasibility of increasing water resources by weather modi- fication on an operational basis. Several operational cloud seeding programs have indicated that increases in seasonal precipitation and subsequent runoff on the order of 10 to 15% could be expected. These, or similar percentage increases have often been applied to aver- age seasonal precipitation in estimating the potential increased water resources to be attained from cloud seeding in a drainage basin. Since seasonal precipitation varies considerably from year to year, this approach does not provide information to state what the potential increased water resources, and subsequent benefits, might be in dry years as opposed to 1~et years. Such an approach also does not take into account the frequency of occurrlence of precipitation enhancement. In addition, models have been recently developed relating seeding effects to key air mass pa- rameters. Mountain station precipitation records can now be analyzed in conjunction 1-1 "lI