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I ,/ J // Reprinted from JOCRNAL OF ApPLIED METEOROLOGY, Vol. 12, No, 2, March 1973, pp, 322-337 ~ American !\'Ieteorological Society Printed in U, S, 1\, m~~ L-c::r- Test of Runoff Increase Due to Precipitation Managemel c for the Colorado River Basin Pilot Project H. J. MOREL-SEYTOUxl AND F. SAHEU2 Dept. of Ci~il Engineering, Colorado State University, Fort Collins (Manuscript received 31 March 1972, in revised form 20 November 1972) ABSTRACT The problem of detection of an increase in runoff due to precipitation management is considered, Classical me~hod~ o~ detection are reviewed and their shortcomings are discussed, The concept of grouping of obser- vatIOns IS mtroduced. The paper answers two fundamental questions: ~) Gi,:,en a region consist~ng o~ ~ basins in which changes are suspected and given that (economic, or finanCial, or other) constramts hmlt to b the number of basins where measurements can be obtained which basins should be selected? ' 2) How should the measurements in individual basins be combined? T,he a~swer,,~re obtained by a constrained optimization procedure. When applied to the Colorado River Basm Pilot Project area the power of the test, expressed in years needed for detection, is increased by a factor of 2, 1. Introduction :1 a. Water resources planning The increasing demand, for water has led men in positions of responsibility to be concerned with the problem of water shortage in particular and of water resources in general (Bureau of Reclamation, 1966; Giln:an et al., 1965; Office of Saline Water, 1963; NatIOnal Academy of Sciences, 1966). The water situation is particularly critical in the Colorado River Basin. The Colorado River system is the largest in the United States that flows mainly through lands with a chronic water deficiency for cultivation of crops (National Academy of Sciences, 1968). The average specific (or unit) yield of the Lower Colorado River Basin is only 0.3 inch, the lowest yield in the United States for a drainage area of this size (National Academy of Sciences, 1968). The Upper Colorado River Basin does not fare much better, 2.2 inches. It outranks only a few basins, the Rio Grande and the Missouri, but it is far below the Mississippi's 10 inches and the Columbia's 16 inches. Population projections and the associated water demands indicate a need for actual importation of approximately 3 million acre-feet annually by the year 2080 (Smith, 1968). Development of the vast oil-shale resources alone would require an additional 1 million acre-feet by the year 2000, assuming a daily oil production of 4 million barrels (National Academy of Sciences, 1968; Smith, 1968). "This amount of water simply is not there now," (Smith, 1 Permanent affiliation: Laboratoires de Mechanique des Fluids des Soli des et des Sols, Universite de Grenoble, Grenoble France. 2 Permanent affiliation: Department of Mechanical Eng'ineering Aria-Mehr University, Tehran, Iran. ' iliic,1,,,",,,~;J:,,., 1968) although "the Colorado Basin is closer than most other basins in the United States to utilizing the last drop of available water for man's needs" (National Academy of Sciences, 1968). b. Precipitation management: An alternative to importation The potential economic and quantitative significance of precipitation management by seeding winter storms is now reasonably well established. Under full-scal~ operations it is estimated that an average additional 1.9 million acre-feet would appear annually in the rivers (Hurley, 1968). In the fall of 1970 The Bureau of Reclamation initiated a 4 or 5 year program of precipi- tation management in the San Juan Mountains area of Colorado. The area is shown in Fig. 1. The program is known as the "Colorado River Basin Pilot Project" (Bureau of Reclamation, 1968). The "target" area is divided in four zones. The original intent of the Bureau of Reclamation, Division of Atmospheric Water Resources :Management, was to seed all zones. Later the decision was made to seed only zones 1 and 2. For this reason, zones 3 and 4 are considered as part of the target area but are nevertheless utilized as "control" zones. The control area refers to the area of Colorado shown in Fig. 2, for which there are currently no weather modification plans for the near future, i.e., for at least the duration of the Colorado River Basin Pilot Project. 2. Evaluation of atmospheric water resources attainments The main difficulty in this evaluation is caused by the natural variability of hydrologic variables which CEP71-72HJMS-fS34a ) ",,:~,;",'_'...i:.::