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<br />period. None of the correlation coefficients was better than 0.3. The <br />morning rawinsonde data for Oklahoma City (April through September 1977) were <br />used to determine 12 cloud and thermodynamic variables (temperatures, pre- <br />cipitable water, stability indexes, specific humidity, and wind velocities) <br />which were correlated with the areal extent and mean intensity of radar <br />echoes within 140 km of Oklahoma City. The best correlation (minus 0.4) for <br />both area and intensity was with the temperature rise required to reach <br />convective temperature. <br /> <br />Proper integration of weather modification into a State's water resources <br />program requires an understanding of the natural variability of those <br />resources. To study the impact of climatological variability on surface <br />water supplies, the frequency and duration of surface water deficits in two <br />major ri ver bas i ns of western Olkl ahoma were exami ned for 30 years (1951 <br />through 1980). The North Canadian River Basin was divided into six sub- <br />basins, and the North Fork of the Red River Basin was divided into four <br />subbasins. Deficit occurs when demand (composed of evapotranspiration, stream <br />evaporation, lake evaporation, and channel loss) exceeds storage (composed of <br />soil moisture, lake contents, and channel contents). For this study, poten- <br />tial deficit is defined as the intersection of the 75 percent empirical <br />envelopes of storage and demand (i.e., when the curves marking the bottom <br />12.5 percent of the storage values and the top 12.5 percent of the demand <br />values intersect). A study of interrelationships among variables showed, for <br />example, that unless storage in soil moisture and reservoirs is sufficient by <br />late May, there is a potential for surface water deficit during the summer. <br />This is because most of the rainfall after May is used to satisfy evapotrans- <br />piration demands directly, rather than for soil-moisture recharge or runoff. <br />The study showed that potential deficits existed in 14 of the 30 years. The <br />potential deficit period averaged 17 weeks, ranging from 2 weeks for a <br />portion of the North Canadian River Basin to 29 weeks for part of the North <br />Fork of the Red River. There is a wider range in the starting date of the <br />potential deficit period (late J~pril to mid-September) than in the ending <br />date (late September to early November). When averaged oyer all 10 sub- <br />basins, the deficit period begins in late June and ends in late October or <br />early November. <br /> <br />25 <br />