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I I I I I I I I I . . ~ ~ ~ . . . . . . . . I t Development of Target/Control Evaluation Procedures for the Denver Board of Water Commissioners Winter Snowpack Enhancement Project 1. Background/Overview The Denver Board of Water Commissioners (Denver Water) is participating in a cloud seeding project for snowpack enhancement in the central mountains of Colorado, scheduled for operations during the period of November 1, 2002 through March 31, 2003. Denver Water requested that a seeding effects assessment technique be developed by a weather modification consultant not involved in the seeding project operations. North American Weather Consultants (NAWC), a firm with a long-standing track record of winter project design. operations and evaluation, was awarded the contract. Denver Water, at NAWC's recommendation, specified that the evaluation method be established as early as possible, in this case, by December 31,2002, to avoid any question of potential bias based on storm occurrence during the first season of operations. Historical TarQeUControl ReQression Evaluations vs Randomization A means of estimating the effects of cloud seeding operations is important to any precipitation enhancement project. Most evaluation techniques have a statistical basis and most precipitation enhancement projects focus on precipitation measurements. More research oriented projects have included additional physical measurements, such as silver in snow, radar-derived (inferred) precipitation characteristics, or cloud microphysical measurements made by instrumented aircraft. Randomization of the cloud seeding decisions over a given project area adds statistical power to evaluation attempts, but at the expense of half the seeding opportunities. This technique is used in pure research projects with costly specialized measurements and observations, Most operational project sponsors are not willing to give up half the seeding opportunity, and hence half the potential value of the project. espeCially since new projects are typically initiated due to a specific (current) need for additional usable water, Evaluations or assessments of the effectiveness of cloud seeding projects are recommended for all projects whenever practical (ASCE, 1995). The Weather Modification Association (WMA) states that "Evaluations of projects are strongly encouraged. Any limitations to evaluation should be reported. Procedures to be used in evaluations should be specified in advance" (WMA, 2002). For operational projects, the most widely used approach of evaluations is the comparison of events in the target area to events in one of more control areas which are assumed to be unaffected by the seeding (Dennis, 1980). NAWC has, for decades, incorporated targeUcontrol