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opportunities would likely be reduced. The remaining area of the rim is the vicinity of Happy Jack, located 20 km south of Mormon Lake, where Reclamation conducted a field program in early 1987. As discussed by Super et al. (1989), flight restrictions are minimal in that area. An all-weather road passes through the region (Lake Mary Road), generally along the crestline of the rim, which would provide practical access for project personnel and equipment. Based on the considerations above, the region around Mormon Lake and Happy Jack is proposed as the most suitable for experimentation. 1.4.2 Aircraft Sampling Over the Selected Experimental Area. - Normal IFR flight procedures established by the FAA (Federal Aviation Administration) require aircraft to stay at least 600 m above the highest terrain within 8 km of the flight path. Special waivers may be obtained in certain circumstances for reduced separation; for example, only 300 m separation and a 3-km horizontal separation were required for a specific flight path over the valley upwind of the Tushar Mountains in early 1989 (Utah, 1989). Such special waivers require permission by the FAA and can be sought only when the specific terrain, aircraft, navigational aids, and pilots in question have been established. However, discussions with FAA personnel in the Scottsdale Flight Standards District Office suggested that a waver for flight within 300 m of the highest terrain within 3 km of the flight path may be obtainable for the region. of interest, especially if navigational aids are upgraded (e.g., improved Loran-C coverage, addition of a nondirectional beacon). For the purpose of this design document it will be assumed that such a waiver will be obtained. A more detailed examination of the Happy Jack region indicated that two minor peaks would most interfere with low-level aircraft sampling. These are Mormon Mountain immediately northwest of Mormon Lake, which has a tower rising to 2666 m, and Hutch Mountain at 2601 m located 18.5 km southeast of Mormon Mountain. In practice, when calculating minimum terrain clearance for aviation, the highest ground elevation is rounded up to lthe next 30-m contour. For the higher of the two peaks, this results in a minimum flight altitude of 2990 m. I r The general airflow during SLW periods over the Happy Jack area is known to be from the southwest with a secondary maximum from the northeast [fig. 5-5 of Super et a1. (1989)]. Therefore, it would be practical to fly along-the-wind passes lower than 2990 m by staying at least 3 km crosswind of these two peaks, that is, along a 12-km-wide corridor centered between them. Flight along a southwest-northeast path would be practical down to about 2800 m in this corridor. Crosswind passes would have to stay above 2990 m when over the crestline of the rim, but could descend as terrain elevation decreases to the southwest or northeast. Surface observations could readily be made near 2270 m within the corridor, or 530 m below the lowest flight altitude, The only alternative for lower flight over this portion of the rim would be to fly along-the-wind passes at least 6 km southeast of Hutch Mountain. However, that region is atypical because the terrain rapidly decreases in elevation towards the south. 1.4.3 Ground Survey for a Target Site. - The region between Mormon Lake and Happy Jack was extensively surveyed on the ground during September 1989. The limited number of existing large clearings within about 2 km of the highway were examined for suitability as the well-instrumented target site. Factors considered included clearing size, orientation and slope, proximity to existing roads and powerlines, stability of the surface for bearing instrumentation systems (many of the clearings become shallow lakes during snowmelt periods), and nearby small sheltered clearings for ice particle and precipitation observations. It was also considered desirable to locate the target within the Verde River Drainage. The best overall site was determined to be 11