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Last modified
7/28/2009 2:40:31 PM
Creation date
4/24/2008 2:51:47 PM
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Weather Modification
Project Name
Weather Modification Programme - Precipitation Enhancement Project
Title
A Review of the Hydrological Aspects of Evaluation of Precipitation Enhancement
Date
5/1/1977
Weather Modification - Doc Type
Report
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<br />--~,.~ <br /> <br />4 <br /> <br />A series of short-term sb:eamflo"\'1 observation" will or;ly rarely exhibit the rr-eedom <br />from serial co:erelation that "\'10uld permit randorpized seeding of individual storms to <br />be successfully evaluated by comparing the streamflow associated with each precipi- <br />tation event. <br /> <br />IV <br /> <br />BASIC CONSIDERATION FOR 11EJ.'HODS OF EVALUATION <br /> <br />15. I'lethods of statistical evaluation of precipitation enhancement by means <br />of streamflo"\'1 should have the following desirable properties (Harkovic, 1966): <br />applicability, generality, detectability, simplicity, and practicality. Of these <br />detectability is the most desirable. Detection of a small change in streamflow due <br />to seeding effects may be required. <br /> <br />16. Since instantaneous flow' rates are integrated over a period of time to <br />provide streamflow volumes, a decision is required as to what time units should be <br />employed in the evaluation process. Flow records may be available as averages of <br />daily, weekly, monthly, seasonal and annual exper.ience. The flow resulting from <br />a storm can bo computed with the required accuracy under some circumstances, and <br />when this can be done the time required to accumulate the desired number of exper- <br />imental units is sho:ctened. <br /> <br />17. The circumstances "\'1hich permit use of comparisons of seeded and u...'1seeded <br />flows on a storm-by-storm basis are: <br /> <br />(i) <br /> <br />Precipitation occurs either as rain, or as snow that melts shortly <br />after falling. <br /> <br />(ii) <br /> <br />The drainage basin has a shallo"\1 or' nonexistent soil mantle" so <br />that the portion of the preci})itation that infiltrates the surface <br />can move promptly to the stream and not become part of the slo'l-lly- <br />moving grolmd wc;,ter. <br /> <br />(iii) <br /> <br />The drainage basin is characterized by a compact shape and <I.ecided <br />slope so tha.t rurof'f concentrates rapidly at thE.- gauging point <br />before the flo"\1 can be complicated by subsequent precipi tat:Lon. <br /> <br />(iv) <br /> <br />High-quality observations are made "\1hich permit accurate determi- <br />nation of' the volu.me of runoff. <br /> <br />18. These conditions are not commonly met in practice, but "\1ere :found to exist <br />for Bear Creek, a headwater sub-basin of the San ,Joaquin River Basin in California <br />(\'lilliams and Lehrman, 1970). Seeding of summer cumulus 0 ccurring near the ,cldge <br />of the Sierra Nevada Range, on a total of 12 days in 1967 and 1968, produced <br />sufficient evidence of seeding effectiveness to p,ersuade the sponsors to change what <br />had previous 1;)' been a long-established oper'ational programme of Hinter cloud seeding <br />in the Kings Hiver Basin into one .,hich DO"\1 features both summer and \"inter seeding. <br /> <br />19. 47 test pairs (a total of 94 clouds) "\1ere randomly seeded. 'rhe seeded <br />clouds produced rainfall that reached the ground in 21 cases. 13 cases of virga <br />were observed and 10 cases of no precipitation effect. For the 47 unseeded cases, <br />the breakdo"\'m ,vas three cases of observed rainfall reaching the ground, no cases of <br />virga, and 44 cases of no effect, <br /> <br />20. Hydrological evaluation WDS accomplished by estimating the expected runoff <br />from the (139.1-km2) basin using a regression equation developed from 13 years of <br />record (1952-1964), relating the flo\'1 from four neiGhbouring basins to the flo"\1 from <br />Bear Creek. 'rhe actual runoff fro.m three test cases in 1967 was 34 per cent groater <br />than predicted and the nino test cases in 1968 ex.perienced 266 per cent more runoff <br />
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