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<br />Atlas southwest of the target area was selected as the control area for the project. Figure 2.2 <br />shows a map of central Morocco with the target and control areas and the principal field site <br />loca tions. <br /> <br />The primary target area was that portion of the Oued Oum Er Rbia drainage basin northwest of <br />the High Atlas Divide containing the headwaters of streams that flow into three principal reservoirs: <br />Bin EI Ouidane, Ait Chouarit (Hassan I Dam), arid Moulay Youssef. For the purpose of <br />Programme Al Ghait, the High Atlas Divide within the target area was defined by a line running <br />from southwest to northeast (235 to 0550) at a perpendicular distance of 150 kilometers from the <br />project radar site, which was set at Khouribga. The target area, extending from 31.25 to 32.500 N. <br />and from 5.25 to 7.250 W., formed a rectangle 220 kilometers long by 65 kilometers wide <br />(14 300 km2 in area) northwest of the crest. Figure 2.3 shows a cross section of the mountains in <br />the primary target area along a line perpendicular from the project radar at Khouribga to the crest <br />of the High Atlas. <br /> <br />The target and control areas both have long periods of streamflow records, permitting a statistical <br />target-control evaluation of seeding effects. Since the primary evaluation of Programme AI Ghait <br />is through the application of statistical techniques capable of detecting changes in streamflow into <br />Bin EI Ouidane reservoir, cloud seeding operations have been concentrated on the northeastern <br />two-thirds of the High Atlas target area that drains into this reservoir. <br /> <br />2.3 Physical Basis of Project <br /> <br />2.3.1 Seeding hypothesis. - Initial analyses of the thermodynamic structure of winter storms <br />in Morocco indicated that ice phase seeding of clouds containing supercooled water for <br />microphysical effects, the so-called static seeding hypothesis, should be the physical basis for the <br />demonstration project. This hypothesis holds that introduction of artificial ice nuclei can lead to <br />the formation of additional snowflakes in clouds. It is called the static seeding hypothesis to <br />contrast it with hypotheses involving changes in the dynamics of the cloud systems. <br /> <br />"According to the static seeding hypothesis a cloud is postulated to be seedable [WMO, <br />1982] if it contains supercooled water that is or will be under-utilized by the cloud's <br />natural precipitation process, that will not be eroded by competitive depletion processes <br />and that will last long enough in sufficient quantities to permit growth of additional <br />precipitation particles induced by seeding to sizes that can reach the ground. If the <br />amount and persistence of supercooled water in a cloud is high, then the depletion rate <br />of water associated with natural precipitation development, cloud ice evolution, and <br />entrainment is likely to be low, and the opportunity for seeding tends to be high. The <br />coexistence 0f ice in the cloud is only a deterrent to seeding if it exists in sizes and <br />concentrations which cause the supercooled water to be depleted faster than seeding can <br />exploit it" (Silverman, 1986). <br /> <br />2.3.2 Seedablllty Of clouds in Morocco. - Clouds are seedable under the static seeding <br />hypothesis if they contain supercooled liquid water that Can be converted to precipitation with ice <br />nucleants. Because very little was known of the winter clouds over the target area at the beginning <br />of Programme Al Ghait, the defining of clouds suitable for seeding has been an evolving process. <br />It was expected originaUy that the clouds to be treated would be mainly stratiform, orographic <br />clouds. Previous work in California and elsewhere had suggested that orographic clouds with top <br /> <br />12 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />