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<br />SUITABILITY OF THE UPPER COLORADO RIVER BASIN FOR PRECIPITATION MANAGEMENT <br /> <br />Hiroshi Nakamichi*and Hubert J. Morel-Seytoux** <br /> <br />by <br /> <br />INTRODUCTION <br /> <br />1. Water needs of the basin. The Colorado River <br />system is the largest in the United States that flows <br />mainly through lands having a chronic water deficiency <br />for cultivation of crops [1]. Since the 1940's, the <br />basin's population has increased rapidly with an accom- <br />panying growth in demand upon the region's water re- <br />sources for irrigation, industrial, and domestic uses <br />[2]. Over the decade from 1951 through 1960, the popu- <br />lation of the five states comprising the Upper Colorado <br />River Basin has increased by 40 percent, while over the <br />same period the population of the nation as a whole has <br />increased only by 20 percent [3]. <br /> <br />2. Precipitation management program. In an <br />effort to reduce the severity of these demands, an <br />atmospheric water resource project is currently pur- <br />sued by the United States Department of the Interior, <br />Bureau of Reclamation, Office of Atmospheric Water <br />Resources. The goal of this project is to induce more <br />precipitation from the atmosphere by winter cloud seed- <br />ing operations over certain high altitude watersheds in <br />the Upper Colorado River Basin. In the past, there <br />was some controversy as to whether man could economi- <br />cally increase precipitation in worthwhile amounts. <br />There now exists evidence that this is possible at <br />least in high mountain areas [4]. As of February 1969, <br />plans of the Bureau of Reclamation called for a concen- <br />trated experimental effort in two pilot areas of the <br />Upper Colorado River Basin, to start in the fall of <br />1969 [5]. This study was undertaken in connection with <br />the Bureau's overall program in general and in connec- <br />tion with this pilot program in particular.*** <br /> <br />3. Criteria of suitability. In the experimental <br />or large-scale operational stage of the project, a <br />site should be selected. At this point, one needs <br />certain criteria in order to select suitable basins. <br />These criteria should be considered both from a water <br />resource and an evaluation standpoint [6]. The first <br />standpoint requires a criterion of suitability for <br />optimal water yield, and the second, a criterion of <br />suitability for minimum time evaluation. <br /> <br />Idea11y the criteria should be objective and <br />simple. That is, they should be derived easily from <br />available data rather than from theory. Though various <br />aspects of research un cloud modification have been <br />conducted sucessfully, it is still difficult to deter- <br />mine its quantitative effect. Indeed, one of the <br /> <br />Chapter I <br /> <br />purposes of the pilot project is to deteTIlline the exact <br />magnitude of the increase in precipitation on a large <br />areal scale. Following this experiment, it may be <br />possible to isolate the major factors that determine <br />the magnitude of the increase in precipitation. Once <br />precipi tation is induced, the increase in runoff, (llQ) , <br />caused by the increase of precipitation, (LIP), is esti- <br />mated by a statistical relationship between precipita- <br />tion and runoff, (Q = f(P)), often used when forecasting <br />runoff: <br /> <br />llQ = (Q+llQ) - Q = f(P+llP) - f(P) <br /> <br />(1) <br /> <br />Marginal criteria are defined in order to deteTIlline the <br />relative suitability of many potential basins for mini- <br />mum time evaluation, even if the type of statistical <br />test and the design of the experiment are not known [6]. <br />One such criterion is derived from the "two-sample <br />u-test. " <br /> <br />The two-sample u-test is a test of the hypothesis <br />that assumes that the mean of a statistical population <br />(the values of annual runoff for a given basin over <br />many years) has not changed significantly even though <br />there were reasons to suspect it had. As the name <br />implies, the application of the test requires the <br />availability of two samples of data, one sample collec- <br />ted prior to the suspected change and one collected <br />afterward. If the suspected change is real but small, <br />the records of many years may be necessary to determine <br />its significance. If the change is large and the <br />spread of the distribution is narrow, only a few years <br />may be required. <br /> <br />No statistical test is free of assumptions. The <br />two-sample u-test assumes that only the mean of the <br />population may have changed whereas the shape and the <br />spread of the distribution have not. Assuming a normal <br />distribution, the explicit expression [6] for the num- <br />ber of years, N, necessary to guarantee the statistical <br />significance of the observed or expected increase at <br />the 95 percent confidence level is given by: <br /> <br />N <br /> <br />(1.96)2 x~ = <br />(llQ) 2 <br /> <br />(2) <br /> <br />3.84 0 2 <br />Q <br />(llQ) 2 <br /> <br />* M.S. Graduate of Colorado State University, Civil Engineering Department, Fort Collins, Colorado, presently <br />with Planning Division, Chugoku-Shikoku, Nosei kyota, 9-24 Tenjin-cho, Okayama-shi, Japan. <br />Associate Professor, Civil Engineering Department, Colorado State University, Fort Collins, Colorado. <br />***Since the initiation of this study the plans of the Bureau were modified. Currently (45) only one area <br />is considered: the San Juan Mountains region. <br />