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<br />Overall, the northwest quarter of the target area has received the most seeding material and the <br />southeast quarter the least (see figures 2.2 to 2.4). This is true for all seeding modes and for most <br />years. There are several possible reasons why the northwest quarter has been the preferred <br />seeding area. Our study of samples of radar data shows two major tracks for storm echoes, one <br />from the northwest and the other from the southwest. It is possible that the northwest storms were <br />identified as more suitable for seeding. Another possibility has to do with the deployment of the <br />seeding aircraft. Although aircraft have been -based at several airports during the course of the <br />project, the most used bases are in the northwest quarter (table 2.2). It is natural to expect project <br />personnel to seed the nearest suitable event, which would usually be in the northwest quarter. <br /> <br />3. STATISTICAL ANALYSIS OF RAINFALL DATA <br /> <br />The purpose of the rainfall analysis is to describe the effects of operational seeding on rainfall <br />patterns in the target area and the surrounding study area (see figure 3.1). The Western Kansas <br />Weather Modification Project was not designed to employ evaluation techniques based on <br />randomized seeding and fixed target and control areas. This operational project was designed to <br />use the latest techniques for cloud seeding, and the operational procedures were subject to revision <br />each year (as noted earlier, the seeding rates, seeding devices, and seeding materials varied from <br />year to year, as did the "target" area). We reasoned that, although daily rainfall data are available, <br />the overall pattern of monthly and seasonal rainfall1 in the study area would reveal the most <br />information about those effects of cloud seeding of interest to the sponsors of the project. <br /> <br />We recognized at the outset of the study that timing of rain (and hail) is important to agriculture, <br />but, in general, the most important factor is total seasonal rainfall (see, for example, James and <br />Eddy [5]). We also recognized that the current technology has essentially no control over the <br />timing of cloud seeding opportunities. Furthermore, data on individual clouds were not available, <br />and the information on seeding flights was rarely adequate to relate seeding location to specific <br />gauges. The characteristics of the data available for this study did not lend themselves to separate <br />analyses of rain-increase events and hail-suppression events. For similar reasons, no attempt has <br />been made to partition events by storm size, seeding agent, or seeding mode. Thus, the monthly <br />and seasonal effects of seeding are the subject of this study. <br /> <br />The project operational period varied from 4 months (5/1-8131) in 1979 to 1983 to 5.5 months <br />(4/1-9115) in 1976 and 1977. Although operations in April and September were conducted in 6 of <br />the 11 years, relatively few seeding missions were conducted during those months. Therefore, <br />only the May-thro1igh-August rainfall is considered in "seasonal" analyses. <br /> <br />3.1 Oimatology <br /> <br />~ <br /> <br />The study area is located in the High Plains region of the United States. Rainfall generally <br />increases from west to east. Average annual precipitation in the target area ranges from about 380 <br />mm (15 in) in the west to about 530 mm (21 in) in the east, most of which falls in the <br />May-through-August period. There are about 35 to 38 days during the May-to-August period with <br />rainfall of 0.25 mm (0.01 in) or more (these are not necessarily the same days in all parts of the <br /> <br />1 The "rainfall" statistics used in this study include all forms of precipitation (i.e.,rain, <br />drizzle, and hail). It is estimated that only 1-2 percent of the total precipitation is hail <br />melt. <br /> <br />8 <br />