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<br />this temperature. range, frequency of occurrence, and the low elevation of a seeding <br />effect as well as crest zone specifications either windward of leeward or both if <br />applicable. <br />1.3 Hydrologic Approach <br />The objective of the hydrologic analysis phase of the Twelve Basin Study was <br />to convert incremental precipitation quantities derived in the meteorologic phase <br />of the project into surface water runoff from mountain massifs subjected to a hypo~ <br />thetical seeding program. Both the quantity and time distribution of runoff origi- <br />nating in and discharging from the pertinent massif units in selected study water- <br />sheds have been developed and summarized by study watersheds. <br />Various investigators have prepared estimates of potential incremental runoff <br />from weather modification activities in portions of the western United States. <br />Hurley ("Augmenting Upper Colorado River Basin Water Supply by Weather Modification", <br />Patrick A. Hurley, presented to ASCE annual meeting and national meeting on Water <br />Resources Engineering, New York City, October 18, 1967) estimated an average annual <br />incremental runoff in the order of 1,800,000 acre-feet from the Upper Colorado River <br />basin. This estimate was based upon a fifteen percent increase in the average <br />November-April precipitation occurring above the 9500 foot elevation throughout the <br />areas in the upper basin subject to weather modification. This increase was based <br />on average conditions, assuming the 85% of the precipitation which falls will result <br />in surface runoff to supplement water available downstream. The estimate was based <br />on an average November-April precipitation of 1~.4 inches (493 mm) over the entire <br />area above 9500 feet. <br />In 1971, Elliott and Hannaford ("A Hydrometeorological Data Appraisal and <br />Design Study in Certain River Basins in the Western United States", Report No. 15-15) <br />made similar estimates of average incremental runoff for the Snake, Missouri, Bear, <br />North Platte, and Rio Grande ?asins based upon a 10% increase in precipitation over <br />the entire seeding units or massifs tributary to the individual watersheds. Analysis <br />varied from the Colorado study in that variation in precipitation and forest cover <br />within the seeding units was considered. Elevation bands were used to define the <br />physical and hydrologic characteristics of the seeding units. Loss rates (and the <br />percentage of precipitation resulting in runoff) varied with respect to average pre-- <br />cipitation within the elevation zones, permitting identification of the most pro- <br />ductive areas within the major watersheds. <br />A simple precipitation-runoff relationship might at first appear as the easiest <br />and most straightforward way of computing incremental runoff from the study watersheds. <br /> <br />1-39 <br />