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. fHifl7::iS . PART V. DITCH DIVERSION RECORDS . Study Period A study period Df 1951 through 1980 was chosen fDr use in this analysis. Ditch diversiDn records, aerial phDtDgraphs Df irrigated lands, and USGS stream- flDW gaging statiDn recDrds thrDughout the San Juan River basin are generally available during the stndy periDd. The availability Df reliable data decreases fDr the years prior tD 1950. . . The 1951 thrDugh 1980 study periDd includes a number Df wet and dry years that are representative of the IDng-term regional climate as evidenced by available gage recDrds in the San Juan River basin and tree-ring studies Df IDng-term precipitatiDn trends. The gaging station with the IDngest periDd Df recDrd in the San Juan River basin in CDloradD is IDcated on the Animas River at DurangD. Annual discharge recDrds are available fDr 79 years during the periDd Df 1897 thrDugh 1984. The Animas River gage can be used as a reference tD cDmpare the 1951 through 1980 study period with the IDnger term periDd of recDrd. A cDmparisDn Df the annual streamflDw in the Animas River is prDvided belDw: . . (Values in Acre-Feet) . Number Df years Mean annual streamflDW Median annual streamflDw Minimum 12 month streamflDw Minimum streamflDw May thrDugh Sept. Maximum 12 mDnth streamflDw 1897-1984 PeriDd 79 604,000 587,000 1897-1950, 1981-1984 PeriDd 49 644,000 682,000 238,000 137,000 1,1~9,000 1951-1980 PeriDd 30 538,000 488,000 213,000 137,000 934,000 . . StreamflDW during 1951 thrDugh 1980 was generally IDwer than during the Dther 49 years fDr which recDrds are available. The dry year flDW in 1977 during the study periDd was Dnly slightly mDre severe than a critical drDuth in the mid 1930's. -7- . .1:; '",,;;;