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OOOS9~ Introduction Setting a realistic goal fDr increasing water yield through vegetatiDn manag.ement requires a careful balance between needs, costs, and resource capabilities. The water user wants tD knDw hDW much water can be produced and the CDSt. The land manager, however, must decide on resource allo- cations based on complex interactions with vari- DUS segments Df the public, and he must adhere tD management guidelines stipulated by CDngress.' Both the water user and the land manager need tD consider hDw the cost Df additional water produc- tiDn cDmpares with the CDSt of achieving more effi- cient delivery and applicatiDn Df water already in the system. Further. the future demand for water and other forest resources and uses can change, and thus affect priorities and availability of treat- able areas. Therefore, no attempt was made in this report tD specify the amount Df additional water that cDuld be produced by management of vegeta- tion and snow I except to show how arbitrary amounts might be generated by applying hYPDtheticaltreatments tD selected portiDns of the Upper and LDwer Basins. The examples chDsen were nDt meant tD suggest an attainable level Df water yield increase, but rather to show the kinds of treatments and how much area might be af- fected, given an augmentation goal. Public acceptance of water yield improvement practices will partly depend on how peDple view the need fDr more water (basically an eCDnDmic issue), and on how they perceive the impacts of water improvement practices on the forest envi- ronment, including the less tangible resources such as wildlife and scenic beauty. FDr water yield improvement projects tD become fully effective would require several decades in commercial forests, where the rDtatiDn age Df tree harvest may vary up to 120 years or more. Type conversiDn in brush lands would becDme Dpera- tional much faster. PilDt applications help to bridge the gap between small watershed ex- perimentation and large-scale action programs. An operatiDnal scale study Df multiresDurce manage- ment in pDnderosa pine is being conducted by the USDA Forest Service Dn the Beaver Creek WDDds Canyon experimental watershed in central Arizona. A similar study is being initiated in the chaparral. and Dthers are being cDnsidered for other vegetatiDn types in both Upper and Lower Basins. 4Provisians 01 the Forest and Rangeland Resources Planning Act of 1974 (88 Stat. 476. et seq), as amended by the National Forest Management Act at 1976 (90 Stat. 2949. et seq.) (16 US.C.1601.1614). Water Resources in the Colorado River Basin The CDlorado River drains nearly 250,000 square miles, or 160 million acres in seven western states before entering the Gulf of CalifDrnia in Mexico. The basin includes virtually all of ArizDna and pDrtions of New MexicD, CDIDradD, Wyoming, Utah, Nevada, and CalifDrnia (fig. 1). The drainage area is divided into Upper and Lower Basins at Lee Ferry, about 10 miles sDuth Df the Utah-Arizona border. The Upper Basin contains about 70 million acres, and the LDwer Basin contains 90 million acres. Lee Ferry is the official CDmpact pDint for apportioning flow from the Upper Basin fDr use by states within both the Upper and Lower Basins. Precipitation averages 15.7 inches annually in the Upper Basin, where it concentrates in the mDuntains (fig. 1): the Lower Basin is drier, with 13 inches (fig. 1). The proportion of precipitation yielded as streamflDw is more than five times greater in the Upper Basin (16% Dr 2.5 inches) than in the Lower Basin (3% or 0.4 inch). Overall, three-fourths of the water yield comes from less than 15% of the land area. Precipitation and streamflow vary greatly from year to year. Annual yields from the Upper Basin at Lee Ferry have varied from 37% tD 163% Df the 83-year mean flow of 14.7 million acre-feet (fig. 2). Yield fluctuates even more in the LDwer Basin. Seasonally, flow is concentrated in a few mDnths Dut of each year when the snow melts. Storage facilities are necessary to fully utilize water resources, especially additional water from vegetation management, because the increases are largest in wet years when flows already are high. Major reservoirs Dn the ColoradD River and its tributaries provide usable stDrage fDr about 32 million acre-feet in each of the Upper and Lower Basins. When full, these reservoirs hDld nearly four times the annual water yield Df the entire Colorado River drainage area. In some headwater areas. however, seasonal deficiencies in water supply may develDp because of inadequate stDrage facilities. Water yield from the Upper Basin averaged 16.8 million acre-feet for the 26 years of records prior to the 1922 Colorado River Compact, which appor- tioned 7.5 milliDn acre-feet for consumptive uses in the Upper Basin, and obligated the Upper Basin to release no less than 75 million acre-feet to the LDwer Basin every 10 years (7.5 million acre-feet per year). A later cDmmitment guaranteed delivery of 1.5 milliDn acre-feet per year to MexicD, with the stipulation that bDth Upper and Lower Basins are tD share equally in meeting this obligatiDn if theren 2